PSEB Solutions for Class 10 Science Chapter 10 Light Reflection and Refraction

PSEB Solutions for Class 10 Science Chapter 10 Light Reflection and Refraction

PSEB 10th Class Science Chapter 10 Light Reflection and Refraction

→ Light is an external physical agent which produces in us the sensation of sight.

→ When light travelling in one medium falls on the surface of another medium, the following three effects are observed:

• A part of the incident light is turned back (or reflected) into the first medium.
• Another part travels through the second medium along a changed path (i.e the fight is refracted).
• The remaining part is absorbed by the second medium.

→ Objects are always placed in front of the mirror to their left, so u (object distance) is always negative.

→ The focal length of the convex mirror is taken as positive (+) and that of the concave mirror as negative (-).

→ Magnification of a concave mirror forming real image is m = v/u and that of convex mirror and also a concave mirror forming virtual image is m = −v/u.

→ All distances measured in the direction of the incident light are taken as positive and distances measured in a direction opposite to the incident ray are taken as negative.

→ The phenomenon of bending of a light ray when it travels from one medium to another medium of different optical densities is called the refraction of light.

→ When a ray of light travels from an optically rarer to an optically denser medium, it bends towards normal.

→ When a ray of light travels from an optically denser to an optically rarer medium, it bends away from normal.

→ The higher the refractive index the more is the denser medium.

→ The ratio of the sine of the angle of incidence (Sin i) to the sine of the angle of refraction (Sin r) is constant, called the refractive index of the medium.

→ The Refractive index of a medium is the ratio of the velocity of light in a vacuum to the velocity of light in a medium.

→ The lens is a portion of the transparent and refracting medium.

→ The relation between the object distance (u), the image distance (v) and the focal length (f) of a lens is called lens formula i.e. 1/f=1/v−1/u

→ The power of a lens is the reciprocal of its focal length. Its unit is dioptre.

→ Mirror: Any smooth and polished surface is called a mirror.

→ Reflection of Light: When light falls on a mirror or a polished surface then without any change in the medium it returns into the same medium along a particular direction. This phenomenon of change of path of light is called reflection.

→ Incident Ray: A ray of light coming from a source of light falling on any surface is called an incident ray.

→ Reflected Ray: After reflection, a ray of light coming back from point of incidence into the same medium is called a reflected ray.

→ Incident Point: The point on the reflecting surface where incident ray strikes is called incident point.

→ Normal: The perpendicular drawn at the point of incidence is called normal.

→ The angle of Incidence: The angle between the incident ray and the normal is called the angle of incidence.

→ The angle of Reflection: The angle between the reflected ray and the normal is called the angle of reflection.

→ Ray of Light: The straight path of light is called a ray of light.

→ Spherical Mirror: If the mirror is a part of some hollow sphere whose one surface is polished and the other surface is reflecting then such mirror is called a spherical mirror.

→ Concave Mirror: A spherical mirror whose reflecting surface is towards the centre of a hollow sphere of which the mirror is a part, is called a concave mirror.

→ Convex Mirror: A spherical mirror whose reflecting surface is away from the centre of a sphere of which the mirror is a part, is called a convex mirror.

→ Centre of Curvature: Centre of curvature of a spherical mirror is the centre of a sphere of which the mirror is a part.

→ Pole: The midpoint of a spherical mirror is called the pole of the mirror.

→ Principal Axis: An imaginary line joining the pole and centre of curvature of a spherical mirror is called the principal axis.

→ The radius of Curvature: It is the radius of a sphere of which the spherical mirror is a part.

→ Aperture: That part of the mirror from where actually reflection takes place is called the aperture of the mirror.

→ Principal Focus: It is the point on the principal axis where the rays of light coming parallel to the principal axis after reflection actually meet or appear to diverge is called the principal focus of the mirror.

→ Focal Length: The distance between pole and principal focus of spherical mirror is called the focal length of the mirror. It is usually denoted by f.

→ Magnification: The magnification of a spherical mirror is the ratio of the length (size) of the image to the length (size) of the object. It is denoted by ‘m’.

→ Refraction of Light: The change of path of light when it goes from one medium to another medium is called refraction of light.

→ Transparent Medium: Mediums like air, water, and glass through which light can pass easily are called transparent mediums.

→ Lens: A portion of transparent and refracting medium bounded by two surfaces is called the lens.

→ Power of Lens: The capacity of a lens to converge or diverge light rays is called the power of the lens. It is denoted by ‘P’.

→ Centres of Curvature of Lens: The centres of spheres of which the spherical surfaces of the lens are part, are called centres of curvature of the lens.

→ Principal Axis of Lens: The line passing through the centres of curvature of the lens, is called the principal axis of the lens.

→ Optical Centre: A point of the lens through which rays of light go undefeated, is called an optical centre.

→ Principal Focus of Lens: The point on the principal axis of the lens, where rays coming parallel to the principal axis after refraction either actually converge or appear to diverge, is called the principal focus of the lens.

Science Guide for Class 10 PSEB Light Reflection and Refraction InText Questions and Answers

Question 1. Define the principal focus of concave mirror.
Answer:
Principal focus: It is a point on the principal axis of a concave mirror where the rays parallel to principal axis meet after reflection from the mirror.

Question 2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer:
Spherical mirror may be either convex mirror or a concave mirror.

For convex mirror,
Radius of curvature (R) = + 20 cm
Focal length (f) =?

We know, f = R/2
f = 20/2 = 10 cm

For concave mirrior,
Radius of curvature (R) = – 20 cm
Focal Length (f) = ?

We know, f = R/2
∴ f = −20/2 = -10 cm

Question 3. Name a mirror which can give an erect and enlarged image of an object.
Answer:
Concave mirror produces an erect and enlarged image when the object is placed between its pole and principal focus.

Question 4. Why do we prefer a convex mirror as back view mirror in vehicles?
Answer:
Preference of convex mirror as rear view mirror in vehicles. We prefer to have convex mirror as a rear view mirror in vehicles because of the following reasons :

1. It produces an erect image of the object (traffic behind).
2. The size of the image formed is much smaller than the size of the object.
3. It has a wider field of view.

Question 5. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer:
Radius of curvature (R) = + 32 cm [for convex mirror, R is +ve]
Focal length (f) =?

We know, f = R/2

∴ f = 32/2 = 16 cm

Question 6. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?
Answer:
Magnification (m) = – 3 [for real image m is taken as negative]
u = – 10 cm v =? ; [u is always negative]

We know, m = −v/u
-3 = −v/(−10)
∴ v = – 30 cm
Since, the value of v is negative, therefore, the image is formed at 30 cm on the same side (to the left) of the mirror as that of the object.

Question 7. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards normal or away from normal? Why?
Answer:

anw = sin i/sin r
Since anw > 1,
sin i > sin r
or i > r
or r < i
Hence, a ray of light travelling from optically rarer medium (air) to an optically denser medium (water), bends towards normal.

Question 8. Light enters from air to glass having refractive index 1.50. What is speed of light in glass? Speed of light in vacuum is 3 × 10⁸ m s^-1.
Answer:
Refractive index of glass (µ) = 1.50
Speed of light in vacuum (c) = 3 × 10⁸ m s^-1

Question 9. Find out from table 10.3 of the text-book, the medium having highest optical density. Also find the medium with lowest optical density.
Answer:

1. Optical density of a medium depends upon its refractive index. The higher the refractive index, higher the optical density and vice versa.
2. From the data available in the table it is apparent that diamond is having highest refractive index (µ = 2.42) and air is having lowest refractive index (µ = 1,0003).

Question 10. Refractive indices of kerosene, turpentine and water are 1.44,1.47 and 1.33 respectively. In which material does the light travel fastest and why?
Answer:
Refractive indices of kerosene, turpentine and water are 1.44, 1.47 and 1.33 respectively. We know that velocity of light in a medium is inversely proportional to refractive index of the medium.

Therefore, light will travel fastest in water (having least refractive index i.e. µ = 1.33) and slowest in turpentine oil (having maximum refractive index i.e., µ = 1.47.)

Question 11. The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer:

Question 12. Define 1 dioptre of power of a lens.
                            Or
What is power of Lens? Give the commercial unit of power.
Answer:

1. Power of Lens: It is defined as the ability of a lens to bend incident light rays.
2. It is measured by taking reciprocal of focal length measured in metres.
3. The power of a lens is said to be 1 dioptre if its focal length is one metre.

Question 13. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle be placed in front of the convex lens if the image is equal to size of the object? Also, find the power of the lens.
Answer:
Here u =?, f =?, P =?
v = + 50 cm [for real image u is taken as positive]
m = -1 [Real image is always inverted]

Question 14. Find the power of a concave lens of focal length 2 metres.
Answer:
Here f = -2 m [for concave lens f is taken as negative]
P =?
Power of lens P = 1/f
= 1/(−2)
P = – 0.5 D

PSEB 10th Class Science Guide Light Reflection and Refraction Textbook Questions and Answers

Question 1. Which of the following materials cannot be used to make a lens?
(a) Water
(b) Glass
(c) Plastic
(d) Clay.
Answer:
(d) Clay, which is opaque.

Question 2. The image formed by a concave mirror is observed to be virtual, erect and larger than object. Where should be the position of the object?
(а) between principal focus and the centre of curvature.
(b) at centre of curvature.
(c) beyond centre of curvature.
(d) between the pole of the mirror and its principal focus.
Answer:
(d) between the pole of the mirror and its principal focus.

Question 3. Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(а) at principal focus of the lens.
(b) at twice the focal length of lens.
(c) at infinity.
(d) between optical centre of the lens and its principal focus.
Answer:
(b) at twice the focal length of the lens.

Question 4. A spherical mirror and a thin spherical lens have each a focal length of -15 cm. The mirror and lens are likely to be :
(а) both are concave.
(b) both are convex.
(c) mirror is concave and lens is convex.
(d) mirror is convex but lens is concave.
Answer:
(a) both are concave.

Question 5. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be :
(a) plane only.
(b) concave only.
(c) convex only.
(d) either plane or convex.
Answer:
(d) Either plane or convex.

Question 6. Which of the following lenses would you prefer to use while reading small letters in a dictionary?
(а) A convex lens of focal length 50 cm.
(b) A concave lens of focal length 50 cm.
(c) A convex lens of focal length 5 cm.
(d) A concave lens of focal length 5 cm.
Answer:
(c) A convex lens of focal length 5 cm.

Question 7. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than object? Draw a ray diagram to show the image formation in this case.
Answer:
The image formed in the concave mirror would be erect only when the object is placed between pole and principal focus of the concave mirror. Therefore, range of distance is greater than zero and less than focal length i.e., between 0-15 cm.

This image is erect, enlarged. virtual and behind the mirror.

Question 8. Name the type of mirror used in the following situations :
(a) Head-light of a car.
(b) Side/rear-view mirror of a vehicle.
(c) Solar furnace.
Support your answer with reason.
Answer:
(a) Concave mirrors are used for head-lights of car to give concentrated parallel beam of light.
(b) Side/rear-view mirror of a vehicle is mostly convex mirror because it forms an erect, virtual and diminished image and therefore, gives a wider field of view of the traffic behind.
However, for judging the distance and speed of the rear vehicle, a plane mirror fixed inside the car.
(c) Solar furnaces use concave mirror. It is because parallel sun rays (coming from infinity) after reflection from concave mirror are converged at focus to produce much heat.

Question 9. One half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Explain your observation.
Answer:
If lower half of a convex lens is covered with black paper no doubt it will form complete image but intensity (brightness) of image with half lens will be less as that with complete lens exposed. The nature, size and location of the image will be the same since light from all parts of the object reaches the exposed part of the lens.

Question 10. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of image formed.
Answer:

Here, Length of the object (h) = 5 cm,
Image distance, v =?,
Object distance form the lens (u) = – 25 cm [Object distance is always negative]
h’ = ?; f = + 10cm [For convex lens, f is positive]


Negative sign shows that the image is inverted, real and diminished (3.3 cm) which is formed at 16.7 cm distance on the right side of lens.

Question 11. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw ray diagram.
Answer:
Distance of the image from the lens (V) = – 10 cm [Virtual image]
Focal length of concave lens (f) = – 15 cm [Focal length of concave lens is taken negative];
Distance of the object from the lens (u) = ?


Negative sign shows that the object is placed to the left side of lens.

Question 12. An object is placed at a distance of 10 cm from a convex mirror of focal length of 15 cm. Find the position and nature of the image.
Answer:
Distance of the object from the lens (u) = – 10 cm [u is always negative]
Focal length of the lens (f) = + 15 cm [convex mirror]
Distance of the image from the lens. (v) = ?

Negative sign shows that the image is virtual and erect and is formed towards left of the mirror.

Question 13. The magnification produced by a plane mirror is + 1. What does this mean?
Answer:
It means that image produced by plane mirror is virtual, erect and of the same size as that of the object.

Question 14. An object 5.0 cm of length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.
Answer:
Here height of the object, h = 5.0 cm ;
u = – 20 cm [u is always negative]
Radius of curvature (r) = + 30 cm [convex mirror]
But f = r/2
= 30/2
= + 15 cm

Height of the image h’ = ?

Image distance from the mirror v = 60/7 = 8.56 cm
Thus, the image is formed behind the mirror or to the right of the mirror.

Positive sign (+) indicates that the image is erect and formed above the principal axis.

Question 15 . An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should the screen be placed, so that a sharp focussed image can be obtained? Find the size and the nature of the image.
Answer:
Here h = 7.0 cm ; u = – 27 cm [u is always negative]
f = – 18 cm [concave mirror]
v =?; h’ =?

Using mirror formula ;

∴ Image distance from the mirror v = – 54 cm
i. e., at 54 cm on the same side of the mirror as that of the object.

Image is of size 14 cm i.e. magnified. Negative sign (-) shows that the image is real and inverted.

Question 16. Find the focal length of a lens of power – 2.0 D. What type of lens is this?
Answer:
Here, Power of lens (P) = – 2.0 D
Focal length of lens (f) =?
We have f = 1/P
= 1/−2
= – 0.5 m
∴ f = – 0.5 m = – 50 cm
Negative sign indicates that the lens is concave. Therefore, it is a divergent lens.

Question 17. A doctor has prescribed a corrective lens of power + 1.5 D. Find the focal length of lens. Is prescribed lens diverging or converging?
Answer:
Here power of lens (P) = + 1.5 D
Focal length of lens (f) =?
We know f = 1/P
= 1/1.5
∴ f = 0.67 m = 67 cm
The positive sign (+) shows that the lens is a convex lens which is diverging in nature.

PSEB 10th Class Science Important Questions Chapter 10 Light Reflection and Refraction

Multiple Choice Questions :

Question 1. The power of a lens is 2 Dioptre. Its focal length will be ________

(A) 20cm
(B)4Ocm
(C) 10cm
(D)5Ocm.
Answer:
(D) 50 cm

Question 2. ________ form.s virtual and same size of image of an object.
(A) Concave mirror
(B) Convex mirror
(C) Plane mirror
(D) None of these.
Answer:
(C) Plane mirror

Question 3. The image of an object formed by a convex mirror is always ______
(A) Real, inverted and smaller than the object
(B) Virtual, inverted and smaller than the object
(C) Virtual, erect and smaller than the object
(D) Virtual, erect and bigger than the object.
Answer:
(C) Virtual, erect and smaller than the object

Question 4. ________ is used in motor vehicles to take rearview.
(A) Concave mirror
(B) Plane mirror
(C) Convex mirror
(D) Any spherical mirror
Answer:
(C) Convex mirror

Question 5. No matter how far you stand from a mirror, your image appear erect. The mirror is likely to be:
(A) Plane
(B) Concave
(C) Concavo-convex
(D) Convex.
Answer:
(A) Plane

Question 6. The refractive index of diamond is:
(A) 2.42
(B) 2.43
(C) 2.45
(D) 2.40.
Answer:
(A) 2.42

Question 7. A ray of light travelling in a glass merges into air, it will bend :
(A) Towards the normal
(B) Away from the normal
(C) Goes along the normal
(D) All of these.
Answer:
(B) Away from the normal.

Question 8. Where will the real and inverted image of an object placed at the centre of curvature of a concave mirror will be formed?
(A) At F
(B) At C
(C) Between C and F
(D) At infinity
Answer:
(B) At C

Question 9. The mirror used by dentist is ________
(A) Convex mirror
(B) Concave mirror
(C) Plane mirror
(D) None of the above.
Answer:
(B) Concave mirror

Question 10. The mirror used for getting real and enlarged image is ________
(A) Convex mirror
(B) Concave mirror
(C) Plane mirror
(D) None of these.
Answer:
(B) Concave mirror

Question 11. Parallel rays incident on a mirror, after reflection converge at a point, then the mirror will be _________
(A) Plane
(B) Concave
(C) Convex
(D) None of these.
Answer:
(B) Concave

Question 12. The Unit of power of lens is ____________
(A) Coulomb
(B) Watt
(C) Joule
(D) Dioptre
Answer:
(D) Dioptre

Question 13. Power of a lens is – 5D, its focal length is :
(A) 20 cm
(B) – 20 cm
(C) – 0.2 m
(D) 5 cm.
Answer:
(C) – 0.2 m

Question 14. Which of the following lenses would you prefer to use while reading small letters found in dictionary?
(A) A convex lens of focal length 50 cm
(B) A concave lens of focal length 50 cm
(C) A convex lens of focal length 5 cm
(D) A concave lens of focal length 5 cm
Answer:
(C)A convex lens of focal length 5 cm

Question 15. Shaving mirrors are ________
(A) Convex mirrors
(B) Plane mirrors
(C) Concave mirrors
(D) Parabolic mirrors.
Answer:
(C) Concave mirrors

Question 16. Which of the following is the property of light?
(A) Reflection
(B) Refraction
(C) Rectilinear propagation
(D) All of these.
Answer:
(D) All of these.

Question 17. Twinkling of stars is due to atmospheric :
(A) Reflection of light
(B) Dispersion of light
(C) Interference of light
(D) Refraction of light.
Answer:
(D) Refraction of light.

Question 18. Defect of vision that cannot be corrected by spectacles is :
(A) Myopia
(B) Presbyopia
(C) Cataract
(D) Hypermetropia.
Answer:
(C) Cataract

Question 19. Which one of the following material cannot be used to make a lens?
(A) Clay
(B) Glass
(C) Water
(D) Plastic.
Answer:
(A) Clay

Question 20. Where should object be placed in front of a convex lens to get real image of the size of object?
(A) At principal focus of lens
(B) At twice the focal length
(C) At infinity
(D) Between optical centre of lens and its principal focus. –
Answer:
(B) At twice the focal length

Question 21. A spherical mirror and a thin spherical lens have each a focal length -15 cm. The mirror and lens are likely to be _____
(A) Both are convex
(B) Both are concave
(C) Mirror is concave but lens is convex
(D) Mirror is convex and lens is concave.
Answer:
(B) Both are concave

Very Short Answer Type Questions

Question 1. Which mirror has a widest field of view?
Answer:
Convex mirror.

Question 2. Define a spherical mirror.
Answer:
Spherical Mirror. A spherical mirror is a part of hollow sphere whose one surface is polished and the other is reflecting.

Question 3. What is concave mirror?
Answer:
Concave Mirror. Concave mirror is one which is silvered from outer side and reflection takes place at inner surface.

Question 4. What is convex mirror?
Answer:
Convex Mirror. Convex mirror is one which is silvered from inner side and reflection takes place from outer convex side.

Question 5. Define the pole of a mirror.
Answer:
Pole. Pole is the mid-point of mirror.

Question 6. Define principal focus of a mirror.
Answer:
Principal focus. It is a point on the principal axis at which the ray of light incident in a direction parallel to its axis actually meet or converge or appear to diverge after reflection from the mirror.

Question 7. Define the focal length of mirror.
Answer:
Focal Length. It is the distance between pole and principal focus of the mirror.

Question 8. What is light?
Answer:
Light. It is a form of energy which causes in us sensation of sight.

Question 9. What is a real image?
Answer:
Image formed on a screen is called real image.

Question 10. What is refraction of light?
Answer:
The phenomenon of change of path of light from one transparent medium to another is called refraction of light.

Question 11. Define refractive index?
Answer:
It is the ratio of sine of the angle of incidence to the sine of the angle of refraction.

Question 12. What is a lens?
Answer:
Lens. It is a portion of a transparent medium which is bounded by two surfaces.

Question 13. Define power of a lens.
Answer:
Power of lens. It is the ability of a lens to bend the rays.

Question 14. Define a ‘dioptre’.
Answer:
Dioptre. It is the power of a lens whose focal length is 1 m.

Question 15. Define focal length of a lens.
Answer:
Focal length of Lens. The distance between optical centre and principal focus is called focal length.

Question 16. What is the nature of the mirror having focal length – 15 cm?
Answer:
It is a concave mirror haying focal length – 15 cm and radius of curvature – 30 cm.

Question 17. A mirror has magnification 0.4, what type of the mirror is and what type of the image is formed?
Answer:
It is a convex mirror since the magnification is positive as well as less than one. Image is diminished and erect.

Question 18. What is curvature of a mirror? What is its value for plane mirror?
Answer:
Curvature is reciprocal of focal length.
Radius of curvature of a plane mirror is infinity and hence its curvature is = 1/∞ = zero.

Question 19. A ray strikes the mirror normally, what is the angle of incidence?
Answer:
Zero degree. Angle of incidence is the angle which the incident ray makes with the normal.

Short Answer Type Questions

Question 1. Define the terms :
(i) Centre of Curvature
Answer:
Centre of Curvature: It is the centre of that hollow sphere of which the mirror is a part. In the above figure (a) C is a centre of curvature of concave mirror and in figure (b) C is centre of curvature of convex mirror.

(ii) Pole
Answer:
Pole. The mid point of spherical mirror is called its pole or vertex. In the fig.l0.14.(a) and fig.10.14.(6) given below P is pole.

(iii) Principal Focus
Answer:
Principal Focus: The principal focus of the mirror is a point on the principal axis where parallel rays of light after reflection from the mirror actually meet at a point or appear to diverge.

(iv) Focal length.
Answer:
Focal Length: The distance between pole and focus is called focal length of the mirror. It is denoted by ‘f. In fig. 10.14. PF is focal length. In SI system the unit of focal length is metre (m).

Question 2. What are New Cartesian Sign Conventions used for reflection in spherical mirror? What is the mirror formula?
Answer:
New Cartesian Sign Conventions for Spherical Mirrors.
The modern cartesian sign conventions are :

• All the rays are drawn from left to right.
• All distances are measured from the pole of the spherical mirror.
• The distances measured in the direction of the incident light are taken as positive and those measured in a direction opposite to the direction of incident light are taken as negative.
• The heights measured upwards and perpendicular to the principal axis of the mirror are taken as positive and vice versa.

Since the object is always placed on the left of the mirror therfore, u is always negative.

Question 3. Write uses of spherical mirrors.
                            Or
What are the uses of concave and convex mirrors?
Answer:
Uses of Spherical Mirrors. Spherical mirrors are of two types

1. Concave mirror
2. Convex mirror.

1. Uses of Concave Mirror:

• Large sized concave mirrors are used in solar cookers to focus solar energy on an object to be heated. Due to a limited reserve of oil and gas, the importance of solar cookers can no longer be ignored.
• Concave mirrors are used as reflectors in car head-lights, search-lights, torches and table lamps.
• Concave mirrors can be used as a shaving mirror or make up mirror. When the concave mirror is placed at a distance less than its focal length from the face, a magnified erect image is produced. Hence any cut or left over hair or minor flaw in the make up can easily be observed with the help of a concave mirror.
• Concave mirrors are used by E.N.T. doctors to concentrate hght on ear, nose, throat, etc.

2. Uses of Convex Mirror:

• Convex mirrors are used in automobiles (e.g., scooters, cars, trucks, buses, etc.) to see bulk of rear traffic. The advantage of using a convex mirror is that it gives a very larger field of view of the rear traffic than a plane mirror.
• The disadvantage of convex mirror is that since the image is always formed between P and F, we cannot accurately judge the distance and speed of the vehicles approaching from behind.
• Usually two mirrors, one plane and the other convex are used to see rear traffic whereas convex mirror on side of vehicle gives wider field of view, plane mirror in front of driver helps us in judging the distance and speed of the vehicles coming behind.

Question 4. What is light? Give the nature of light.
Answer:
Light. Light is a form of energy which produces in us the sensation of sight. Light itself is not seen but it helps us to see the surrounding objects.

Nature of light. Light has a dual nature, a wave nature and a particle nature. To explain the phenomenon it was thought of as a wave but when it could not explain the treatment of interaction of light with matter then light was believed to have particle nature. The confusion about true nature of light was later explained when Model Quantum Theory emerged according to which “light is neither a wave nor a particle”. Modern Quantum Theory reconciles the particle properties of light with the wave nature

i. e. light is emitted as a particle but travels in the form of wave.

Question 5. Write the characteristics of light.
Answer:

1. It does not require medium for its propagation.
2. It is in the form of electro-magnetic waves.
3. Its speed depends upon the nature of medium through which it passes.

Question 6. What are the various artificial sources of light? Give examples.
Answer:
Artificial Sources of Light: The main artificial sources of light are : fire, electricity gas and some chemical reactions.

Question 7. What is Reflector?
Answer:
Reflector. Any smooth and polished surface that returns light into the same medium from which it is coming, is called reflector.

Question 8. What is reflection of light? State the laws of reflection of light.
Answer:
Reflection of Light. When rays of light fall on a smooth and polished surface (incident) then they come back into the same medium in a particular direction. This phenomenon of coming back of light is known as reflection of light.

Laws of Reflection. There are two laws of reflection :

1. The angle of incidence is equal to the angle of reflection i.e. ∠i = ∠r.
2. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
3. In fig. AB is a reflecting surface (plane mirror), PQ is incident ray. QR is reflected ray and QN is normal at the point of incidence Q.

Figure shows that incident ray, reflected ray and the normal all lie in the same plane of paper.

Question 9. What is the angle of incidence when incident ray falls normal to the mirror?
Answer:
When a ray of light is incident normal to the mirror then the angle of incidence is zero (i.e. ∠i= 0°).

Question 10. What is the angle of reflection when a ray of light is incident normally to the mirror?
Answer:
When a ray of light is incident to the mirror normally (∠i = 0°) then after reflection it returns along the same path. In that case, angle of reflection will also be zero (∠r = 0°).

Question 11. A ray of light falling normal to the mirror returns along the same path. Why?
Answer:
A ray of light falling normal to any mirror returns along the same path. In that case ∠i = 0° and according to the law of reflection ∠i = ∠r. So ∠r will be equal to zero and the reflected ray would be along the same path.

Question 12. Define the terms :
(i) Spherical mirror
Answer:
Spherical Mirror: If the mirror is a part of some hollow sphere whose one surface is polished and the other surface is reflecting then such a mirror is called spherical mirror.

Spherical mirrors are of two types :
(a) Concave mirror
(b) Convex mirror.

(ii)Concave mirror
Answer:
Concave Mirror: A spherical mirror whose reflecting surface is towards the centre of that sphere of which the mirror is a part is called concave mirror. The outer surface of concave is polished and reflection occurs from the inner surface.

Concave mirror

(iii) Convex mirror
Answer:
Convex Mirror: A spherical mirror whose reflecting surface is away from the centre of that sphere of which the mirror is a part, is called convex mirror.
The inner surface of convex mirror is polished and reflection takes place from the outer surface.

Convex Mirror

(iv) Aperture.
Answer:
Aperture: That part of the mirror from which reflection takes place is called aperture of the mirror.
In figure (a) and (b) the length M1M2 is aperture

Aperture and Centre of Curvature

Question 13. When the image formed by a concave mirror is at infinity then what is the position of the object?
Answer:
When the object is placed at focus of a concave mirror then the image of the object is formed at infinity. This image is real, big in size and is bigger then the size of the object. In this position the rays of light coming from the object after suffering reflection from the mirror become parallel.

Reflected rays meet at Infinity.

Question 14. Where should an object be placed so that its real and same size image is formed?
Answer:
To get real image of saine size being formed by a concave mirror, the object should be place at its centre of curvature. In this case the image will also be formed at the same position i.e at centre of curvature. This image will be real, inverted and of the same size of the object.

Question 15. When is virtual and magnified image of an object formed in a concave mirror. Show with the help of a diagram.
Answer:
When an object is placed between pole and focus of a concave mirror then in that case a virtual, erect and magnified image is formed with respect to the size of the object.

Question 16. Which mirror is shown in the figure? Where is object placed with respect to the mirror? Write one characteristic of the image formed.

Answer:
(a) Concave Mirror.
(b) Object is placed between Focus and Pole of the mirror.
(c) Characteristic of Image formed. The image is virtual, erect and magnified and is formed behind the mirror.

Question 17. Which mirror is shown in the diagram? Where is object placed in relation to the mirror? Write characteristics of the image formed :

Answer:
In the diagram concave mirror is shown.
The object AB is placed beyond centre of curvature of the mirror i.e. it is at a distance more than radius of curvature (r = CP) from the pole of the mirror.

Characteristics of image formed
The image ¡VB’ of the object AB is :

1. Real and inverted
2. The image is smaller in size as compared to the size of the object.

Question 18. Which mirror is used as a shaving mirror and why? Explain its working with the help of a ray diagram.
Answer:
Concave mirror is used as a shaving mirror because we place our face close to the mirror (in between Pole and Focus), the image formed is erect and magnified. This makes the fine hairs to appear quite big so that we can shave properly.

Question 19. Which mirror always forms virtual, erect and smaller image?
Answer:
Convex mirror always forms a virtual, erect and reduced image of whatever may be the position of the object. The image is formed behind the mirror as is shown in figure.

Formation of Image in a Convex Mirror

Question 20. Which mirror has wider filed of view?
Answer:
In a convex mirror the image formed is always virtual and smaller in size as compared to the size of the object. This image is formed behind the mirror. On moving the mirror away, the images of objects lying over the large area can be seen in the mirror. Therefore, its area of view becomes wide.

Question 21. Which mirror is preferred as a driver’s mirror and why? Draw a ray diagram.
Answer:
Convex mirror is preferred as a driver’s mirror because the image formed by convex mirror is erect and smaller in size. Therefore, it enables the driver to see traffic coming behind over large area.

A convex mirror as a driver’s mirror

Question 22. Where should an object be placed with respect to a concave mirror to get real and enlarged image? Show with the help of ray diagram.
Answer:

Object between F and C. Image ¿s beyond C. It is real, inverted and enlarged.

Long Answer Type Questions

Question 1. What is refraction of light? with the help of diagram explain refraction of light by rectangular glass slab and show that emergent ray and incident ray are parallel.
Answer:
Refraction of Light. When a ray of light enters from one medium to another medium then it changes its path at the meeting plane of two media. This phenomenon of light is called refraction of light.

Refraction by glass slab.

Refraction by rectangular glass slab

A rectangular glass slab PQRS is placed in air. When? ray of ligxu, in air enters glass slab then AO, OO’ and O’B are incident ray, refracted ray and emergent ray respectively

Therefore, when refraction of light takes place in a rectangular glass slab then emergent ray and incident ray are equal.

Question 2. Give the nature, position and size of the image formed by a convex lens when the object lies at 2F and between focus F and optical centre O.
Answer:
1. Object at 2F. A ray AD from point A of the object going parallel to principal axis, passes through F on the other side of the lens goes towards DA’. Another ray from A through optical centre O goes towards OA’. The two meet at A’ forming image of A at A’. Image of object AB is formed at 2F on the other side of the lens. Image is real, inverted and of the same size as that of object [Figure (a)].

2. Object between F and O. When an object is placed between principal focus and optical centre of the lens, virtual, erect, magnified image is formed on the same side of the lens [Figure.(b)].

(b) Object between F and O, a virtual. erect, magnified image is formed on the same side.

Question 3. (a) Form the image in case an object is moved from infinity to the concave mirror.
(b) Describe with the help of diagram the nature, size and position of the image formed when an object placed at centre of curvature of a concave mirror.
Answer:
(a) Images formed by concave mirror when it is moved from infinity to the mirror
1. Object is at Infinity: Two cases arise :

1. When mirror is in parallel plane to the object: In such a case, rays from infinity come parallel to principal axis. After reflection they pass through principal focus F. Image is extremely small, real, inverted and it is formed at the principal focus [Figure (a)].
2. When the rays strike the mirror obliquely. The ray AB passing through F after reflection goes parallel to principal axis towards BA’. Another ray DE through C striking normally to the mirror at E is reflected back. The two rays form an image at A’. Image is real, inverted, extremely diminished and is formed at F [Figure (b)].

2. Object beyond C. A ray of light AD from A parallel to principal axis after reflection passes through F.
Another ray from A passes through C, and is reflected back along the same path, forming real, diminished, inverted image A’B’, between F and C. [Figure (c)].

(c) Object beyoid C. a real, inverted, diminished image hetiveen F and C. inverted at C and is of saine size as that of object.

3. Object at centre of curvature, C i.e., at 2f. A ray AD from A parallel to principal axis after reflection from mirror passes through F.
Another ray AD’ from A through F. goes, parallel to principal axis i.e., towards, D A forming real, inverted image of AB at C i.e., at 2f. The image is of the same size as the object [Figure (d)].

(d) Object at C. Image is also at C. It is real.

4. Object between F and C between [f and 2f]
A ray AD from object going parallel to principal axis is reflected towards F. Another ray AE as of coming through C is reflected back forming image of A at A’. Similarly image of B is formed at B’. Image is real, inverted, enlarged and formed beyond C (2f) i.e., as shown in Figure (e).

(e) Object between F and C. Image is beyond C. It is real, inverted and enlarged.

5. Object at F. A ray AD parallel to principal axis passes through F. Another ray AE strikes the mirror normally at E is reflected back as it appears to pass through C. This reflected ray becomes parallel to reflected ray DY and, therefore, forms an image of object AB at infinity. The image is very much enlarged and is real and inverted [Figure (f)].

(f). Object at F. Real, inverted, extremely enlarged image is formed at infinity.

6. Object between F and P (Principal Focus and Pole).
A ray AD from A goes parallel to principal axis after reflection passes through F. Another ray AE striking the mirror normally through C is reflected back. These to reflected rays do not actually meet but meet when produced backward, therefore, they form virtual image of the object behind the mirror. The image is erect and enlarged and formed behind the mirror [Figure (g)].

(g). Object between F and P. An erect, enlarged, virtual image is formed behind the mirror.
(b) Please see Figure(d) and its description given on previous page.

Question 4. Draw the ray diagrams and find position, nature and size of image formed by a convex lens, when object is placed:
(i) Between F and 2F
Answer:
Object between F and 2F. When the object is between F and 2F, its real. inverted, magnified image is formed on the other side of the convex lens as shown in Figure (a).

(a) Object bet weer F and 2F real, inverted, magnified image is formed beyond 2F on the other side of lens.

(ii) Beyond 2F
Answer:
Object beyond 2F’. When the object is beyond 2F, a real, inverted, diminished image is formed between F’ and 2F [Fig (b)].

(b) Object beyond 2F, real, inverted, diminished image between Fund 2F

(iii) At F.
Answer:
Object at F. When object is placed at principal focus, a real, inverted and highly magnified image is formed at infinity [Fig. (c).]

(c) Object at F, highly magnified, real, inverted image is formed at infinity.

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