# Light – Reflection and Refraction NCERT 10 SCIENCE SAMPLE PAPER

1. With necessary figure, explain the refraction of light through a rectangular glass slab.

Solution

Consider a rectangular glass slab ABCD having parallel faces AB and CD as shown in above figure. A ray of light EF in air is incident on the glass surface AB at point O. As the ray EO enters from air (rarer medium) to glass (denser medium), the ray bends towards normal and follows the path OO’ inside the glass slab. At point O’, refraction takes place again. As the ray OO’ enters from glass (denser medium) to air (rarer medium), the ray bends away from normal and follow the path O’H outside glass slab. Here, the ray EF is called incident ray, OO’, the refracted ray and O’H, the emergent ray. As per the laws of refraction,  and. The emergent ray O’H is parallel to the incident ray EF. The perpendicular distance O’L between the original path of incident ray and the emergent ray is called the lateral displacement.

2. What is a spherical mirror? What are its two types?

Solution

A highly polished surface which is spherical or curved in shape is known as spherical mirror. OR A mirror whose reflecting surface is a part of a hollow sphere of glass is known as spherical mirror. A spherical mirror whose reflecting surface is curved inwards is concave mirror. A spherical mirror whose reflecting surface is curved outwards is convex mirror.

3. Is mirror formula valid only for the plane mirror?

Solution

No, the mirror formula is valid for plane as well as for all spherical mirrors for all positions of the object.

4. (a) What is meant by magnification in the context of image formation by mirror and lenses? (b) Power of a lens is + 5 D. What is the focal length and nature of the lens?

Solution

(a) The ratio of the size of the image to the size of the object is called as magnification, m = hi/ho. (b) Lens is convex, f(m) = 1/P = 1/5 m = 20 cm

5. Why is convex lens known as converging lens?

Solution

A convex lens is known as converging lens because it converges parallel beam of light passing through it.

6. An object is kept at 70 cm in front of a plane mirror. If the mirror is now moved 35 cm away from the object, how much does the image shift from its previous position?

Solution

Initially, distance of the object from the mirror = 70 cm. Therefore, image is formed at a distance 70 cm from the mirror, behind it. Thus, initial distance between the object and image = 70 + 70 = 140 cm If the mirror is moved 35 cm away from the object, the new distance of the object from the mirror = 70 + 35 = 105 cm. The new image is now at a distance 105 cm from the mirror behind it. Thus, new distance of the image from the object = 105 + 105 = 210 cm Taking the position of the object as the reference point, the distance between the two positions of the image = New distance of image from the object – Initial distance of the image from the object = (210 – 140) cm = 70 cm Thus, the image shifts 70 cm away.

7. What is refraction of light? How is it related to refractive index?

Solution

The phenomenon of a change in the direction of propagation of light when it passes obliquely from one transparent medium to another is called refraction of light. For a given angle of incidence, the extent of refraction of light is different in different media. When the refractive index of the second medium is greater than the refractive index of the first medium, more is the bending of the refracted ray towards the normal. When the refractive index of the first medium is greater than the refractive index of the second medium, lesser is the bending of the ray of light away from the normal.

8. Define power of a lens.

Solution

The power of a lens is defined as the reciprocal of its focal length in metres.

9. Give the three differences between real image and virtual image.

Solution

Real image Virtual image Image can be obtained on a screen. Image cannot be obtained on a screen.   Rays coming from an object actually meet at a point after reflection.  Rays coming from an object only appear to meet at a point when produced backwards after reflection. It is inverted with respect to the object. It is erect with respect to the object.

10. Which lens has greater power, a convex lens of focal length 20 cm or a concave lens of focal length 30 cm?

Solution

The convex lens of smaller focal length (= 20 cm) has greater power since focal length is reciprocal of power.

11. What is difference between the light reflected from a plane mirror and a cardboard?

Solution

The cardboard surface is rough. Thus, irregular or diffuse reflection takes place. A parallel beam of light incident on its surface is reflected in different directions. Mirror has a smooth surface. A parallel beam of light incident on it gets reflected as beam of parallel rays in one direction only. Thus, there is regular reflection of light incident on a plane mirror.

12. Find the focal length of a convex mirror whose radius of curvature is 44 cm.

Solution

13. Give three applications of plane mirror.

Solution

(i) Plane mirrors are used in many optical instruments. (ii) Plane mirrors are used in periscopes and kaleidoscopes. (iii) They are used in dressing rooms to see ourselves.

14. When does Snell’s law fail?

Solution

Snell’s law fails when the light rays are incident normally on the surface of a refracting medium. In this case light passes undeviated from the surface, i.e. no refraction occurs. Refractive index is n = sin i/sin r = 0/0 = 0. But, n is not zero. Thus, Snell’s law fails.

15. How can you identify a plane mirror, a concave mirror and a convex mirror without touching?

Solution

They can be identified by observing the nature of image of our face. Plane mirror – produces virtual, erect image of the same size as our face. Concave mirror – produces virtual, erect and magnified image of our face Convex mirror – produces virtual, erect and diminished image of our face

16. Write any two uses of concave mirror.

Solution

Two uses of concave mirror are as follows: (i) To collect solar radiations in solar heating devices. (ii) As reflectors in torches, head lights, to get powerful beams of light.

17. To see a full length image of a tall building, which type of mirror would you prefer to use?

Solution

Convex mirror. This is because, it is curved outwards, which enables to view a large field area.

18. Name the mirror which always produces an erect and virtual image. How is the size of image related to the size of object?

Solution

Convex mirror always produces erect and virtual images. The images formed are diminished, i.e. the size of the image is shorter than the size of the object.

19. Why does a ray of light bend when it travels from one medium into another?

Solution

When a ray of light travels from one medium to another, its speed changes and this change in speed of light causes a part of wave to travel slower than the other part. Thus, bending of light takes place in refraction.

20. Define reflection of light.

Solution

The phenomenon of bouncing back of light or change in the path of light rays into the same medium after striking a polished surface is called reflection of light.

21. State the sign convention for lenses.

Solution

According to new Cartesian sign convention for lenses i. All the distances are measured from the optical centre of lens. ii. The distances measured in the same direction, as that of incident light is taken as positive and against the direction of incident light are taken as negative. iii. The distances measured upward and perpendicular to the principle axis are taken as positive and downwards and perpendicular to the principal axis is taken as negative.

22. Do the laws of reflection change, when we use a spherical mirror instead of a plane mirror?

Solution

No, the laws of reflection do not change. They are applicable to spherical mirrors also.

23. What are the units of refractive index?

Solution

Refractive index of a medium has no units. This is because, it is the ratio of speeds, and ratio of similar quantities is unitless.

24. What will be the path of ray of light after reflection, when it is incident on (a) focus and (b) centre of curvature of the mirror?

Solution

(a) A ray of light that passes through the focus, after reflection, becomes parallel to the principal axis. (b) A ray of light that passes through the centre of curvature, after reflection, retraces its original path.

25. You are given two mirrors of equal size. One is plane mirror and other is convex mirror. How will you identify them without touching their surface?

Solution

By changing the position of the object if the image size remained same and image position changes equally as that of object distance, then the mirror is a plane mirror. But, by changing the position of the object if the image size is changes and image is always smaller than the size of the object, then the mirror is a convex mirror.

26. A convex lens forms a real image 4 times magnified at a distance of 60 cm from the lens. Calculate the focal length and the power of the lens.

Solution

Magnification (m) = v/u -4 = 60/u So, u = 60/-4 = -15cm 1/f = 1/v – 1/u = 1/60+1/15 = 5/60 Thus, f = 60/5 = 12 cm = 0.12 m P = 1/f = 1/0.12 = 8.33 D

27. An object is placed perpendicular to the principal axis of a convex lens having focal length 10 cm. The distance of an object from the lens is 15 cm. Find the position of an image.

Solution

28. Define the term refractive index of a medium. Can it be less than 1?

Solution

The refractive index of second medium with respect to first medium is defined as the ratio of the sine of angle of incidence in the first medium to the sine of the angle of refraction in the second medium. Refractive index of a medium is always greater than 1 (it cannot be less than 1) because the speed of light in any medium is always less than that in vacuum.

29. The ray of light traveling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

Solution

When a ray of light travels from air into water obliquely, it bends towards the normal. This is because water is optically denser than air. On entering into water, the speed of light decreases and the light bends towards the normal.

30. Give two characteristics of the image formed by a concave lens.

Solution

Image formed by a concave lens is virtual and diminished.

31. (i) Describe an activity to find the approximate value of focal length of a concave mirror. (ii) What happens to the size of the image of an object when it is moved gradually away from a convex mirror?

Solution

(i) Activity : Hold a concave mirror in your hand and direct it towards the sun. Direct the light reflected by the mirror on to a sheet of paper held close to the mirror. Move the sheet back and forth slowly until a bright, sharp spot of light is seen on the paper. The distance of this image from the position of the mirror gives the approximate value of focal length of the mirror. (ii) Size of the image becomes smaller

32. What do you mean by a focal point of a mirror?

Solution

Focal point is the principal focus of the mirror where a parallel beam of light meets (or appear to meet) after reflection from the mirror.

33. Why is it difficult to shoot a fish swimming in water?

Solution

When we peer through water to observe fish swimming around the pond, they appear to be much closer to the surface than they really are due to the refraction of light rays (as they travel from water into the air).   Light rays reflected from fish are refracted at the surface of the water, but the eyes and brain trace the light rays back into the water as if they had not been refracted but traveled away from the fish in a straight line. This effect creates a ‘virtual’ image of the fish that appears at a shallower depth. Hence, it is difficult to shoot fish swimming in water.

34. Define the term ‘magnification’.

Solution

Magnification is defined as the ratio of height of the image to the height of the object. It tells us the amount by which the object is magnified with respect to the object.

35. State the changes in the position, size and nature of the image of an object when brought form infinity up to a convex lens. Illustrate your answer by drawing diagrams.

Solution

(i) When the object is situated at infinity, the position of image is at F2, it is very much diminished in size and it is real and inverted.  (ii) When the object (AB) is situated beyond 2F1, the position of image (A’B’) is between F2 and 2F2, it is diminished in size and real and inverted.  (iii) When the object (AB) is situated at 2F1, the position of image (A’B’) is at 2F2, it is of same size as the object and real and inverted.  (v)When the object (AB) is situated at F1, the position of image is at infinity; it is very much magnified in size and real and inverted.  (vi) When the object (AB) is situated between lens and F1, the position of image (CD) is on the same side, behind the object; it is magnified in size and virtual and upright.

36. What is meant by lateral displacement?

Solution

The perpendicular distance of separation between the emergent ray and the incident ray is called lateral displacement. It is seen that the emergent ray of light is parallel to the incident ray but displaced from the path of the incident ray after refraction through a glass slab.

37. Define lateral inversion.

Solution

In the image formation by plane mirror, the left side of the object appears to become the right side of the image; and the right side appears to become the left side of image. This change of sides of an object and its mirror image is known as lateral inversion.

38. (a) A concave mirror produces three times enlarged image of an object placed at 10 cm in front of it. Calculate the radius of curvature of the mirror. (b) What is a lens?

Solution

(a) The image formed in front of the concave mirror is real, so m is negative, m = -3, u = -10 cm As m = -v/u or -3 = -v/-10 v = -30 cm By mirror formula, 1/f = 1/v + 1/u = 1/-30 + 1/-10 = -4/30 or f = -30/4 = -7.5 cm  Radius of curvature, R = 2f = 2 x (-7.5) = 15cm (b) A lens is a piece of transparent glass bound by two spherical surfaces.

39. An object is placed perpendicular to the principal axis of concave lens of focal length 30 cm. Find the position of an image when the object is at a distance 20 cm from the lens.

Solution

40. An object is placed at 4 cm distance in front of a concave mirror of radius of curvature 24 cm. Find the position of image by drawing. Is the image magnified?

Solution

The image is 6 cm behind the mirror. Yes the image is magnified.