10. LIGHT REFLECTION & REFRACTION
Questions:
1. What is virtual image?
Ans: If the rays of light do not actually meet after reflection or refraction, but appear to meet when produced backwards, then that point constitutes virtual image.
2. What makes things visible?
Ans: In a lighted room, when light falls on an object, it gets either partially or completely reflected. This reflected light when received by our eyes, enables us to see things.
3. Why are convex mirrors preferred over plane mirrors as rear view mirrors?
Ans: The field of view of convex mirror is wider in comparison to plane mirror. The convex mirror is preferred as a rear view mirror as it enables the driver to view much larger area than with a plane mirror.
4. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Ans:
Hence, the focal length of the given convex mirror is 16cm.
5. List four specific characteristics of the images of the objects formed by convex mirrors.
Ans: Four specific characteristics of the image formed by convex mirror:
(i) It is always formed behind the mirror, between the pole and its focus.
(ii) The image is always virtual and erect.
(iii) The size of image is always smaller than the object.
(iv) Magnification is always positive.
6. Differentiate a real image from a virtual image giving two points of difference.
Ans: Difference between Real Image and Virtual Image. Real Image Virtual Image
7. The linear magnification produced by a spherical mirror is +3. Analyse this value and state the (i) type of mirror and (ii) position of the object with respect to the pole of the mirror. Draw ray diagram to show the formation of image in this case.
Ans: (i) Concave mirror
(ii) Between the pole and focus.
8. Name the type of mirrors used in the design of solar furnaces. Explain how high temperature is achieved by this device.
Ans: (i) Concave Mirrors/ Converging Mirrors. (ii) When a solar furnace is placed at the focus of a large concave mirror/reflector, it focuses a parallel beam of light on the furnace, consequently a high temperature is achieved after some time.
9. Draw a ray diagram to show the path of the reflected ray corresponding to an incident ray which is directed parallel to the principal axis of a concave mirror. Mark on it the angle of incidence and the angle of reflection.
Ans:
10. An object is placed at a distance of 20 cm in front of convex mirror of radius of
curvature 30 cm. Find the position and nature of the image.
Ans:
11. (i) Name the spherical mirror used as:
(a) Shaving mirror
(b) Rear view mirror in vehicles
(c) Reflector in search-lights.
(ii) Write any three differences between a real and a virtual image.
Ans: (i) (a) Shaving mirror: Concave mirror
(b) Rear view mirror: Convex mirror
(c) Reflector in search-lights: Concave mirror
(ii) (a) Real image can be obtained on screen but virtual image cannot be obtained.
(b) Reflected / Refracted rays actually meet where real image is formed while for virtual image they only appear to meet.
(c) Real image is always inverted while virtual image is always erect.
12. The image of an object formed by a mirror is real, inverted and is of magnification. If the image is at a distance of 40 cm from the mirror, where is the object placed, where would the image be if the object is moved 20cm towards the mirror? State reason and also draw ray diagram for the new position of the object to justify your answer.
Ans: Object position: At C (Centre of curvature) Object distance = 40 cm Position of the image − at infinity Reason: Focal length of the mirror = 20 cm If the object is moved 20 cm towards the mirror then its new position would be at the focus of the mirror.
13. A student wants to project the image of a candle flame on a screen 60cm in front of a mirror by keeping the flame at a distance of 15 cm from its pole. (i) Write the type of mirror he should use. (ii) Find the linear magnification of the image produced. (iii) What is the distance between the object and its image? (iv) Draw a ray diagram to show the image formation in this case.
Ans: (i) He should use a concave mirror, as it forms a real image on the same side of the mirror.
(ii) Object distance,
u = −15 cm Image distance,
v = − 60 cm Magnification, m = − /
u = − (−60) / ( −15) = −4cm,
The minus sign in magnification shows that the image formed is real and inverted.
(iii) The image is formed at a distance of 45 cm from the object.
(iv) In this case, the image is formed beyond the centre of curvature. This image is real inverted and enla
14. It is desired to obtain an erect image of an object, using concave mirror of focal length of 12 cm. (i) What should be the range of distance of an object placed in front of the mirror? (ii) Will the image be smaller or larger than the object. Draw ray diagram to show the formations of image in this case. (iii) Where will the image of this object be, if it is placed 24 cm in front of the mirror? Draw ray diagram for this situation also to justify your answer. Show the positions of pole, principal focus and the centre of curvature in the above ray diagrams.
Ans: (i) Range of distance − between 0 cm − < 12 cm
(ii) Larger than the object
(iii) Image is at 24 cm in front of the mirror
15. (i) An object is placed at a distance of 60 cm from a convex mirror where the magnification produced is 1 2
Where should the object be placed to get a magnification of 1 3 ? (ii) A small electric lamp is placed at the focus of a convex lens. State the nature of beam of light produced by the lens. Draw a diagram to show this.
Ans:
(ii) When a small electric lamp is placed at the focus of a convex lens, a parallel beam of light is produced by the lens. Ray diagram:
16. Red coloured light is used in traffic signals to indicate the vehicles to stop, because compared to other colours red light [SSLC July, 2018]
(a) Has high frequency
(b) Has less wavelength
(c) Scafters more
(d) Scafters less
Ans: (d) scatters less
17. Observe the figure. The correct figure indicating the direction of the light ray FG after refraction is [SSLC July, 2018]
Ans: (d)
18. The refractive index of diamond is 2.42. What is the meaning of this statement? [SSLC July, 2018]
Ans: The refractive index of diamond 242 suggests that the speed of light in diamond will reduce by a factor 2.42 as compared to its speed in air.
19. Why does light change its path as the medium changes during the transit?
Ans: Speed of light is different in different media. As the medium changes, the light has to choose a path of minimum time. Hence, the direction of the light changes. This phenomenon is known as refraction of light.
20. State two laws of refraction. [NCERT Exemplar]
Ans: (i) The incident ray, the refracted ray and the normal all lie in the same plane. (ii) The ratio of sine of angle of incidence in the first medium to the sine of angle of refraction in the second medium is a constant and is known as refractive index of the second medium with respect to the first medium.
21. The refractive indices of glass and water with respect to air are 3/2 and 4/3 respectively If speed of light in glass is 2 x 108 m/s, find the speed of light in water.
Ans:
22. State the laws of refraction of light. If the speed of light in vacuum is 3 x 108 m/s, find the absolute refractive index of a medium in which light travels with a speed of 1.4 x 108 m /s. [SSLC July, 2018]
Ans: (i) There are two laws of refraction: (a) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. This is known as Snell’s law. Mathematically, it can be expressed as: sin / sin r = n12 Here, n12 is the relative refractive index of medium 1 with respect to medium 2. (b) The incident ray, the refracted ray and the normal to the interface of two media at the point of incidence lie on the same plane
23. (a) An object is kept at a distance of 18 cm, 20 cm, 22 cm and 30 cm, from a lens of power + 5D. (i) In which case or cases would you get a magnified image? (ii) Which of the magnified image can we get on a screen? (b) List two widely used applications of a convex lens.
Ans: (a) (i) P = 1 , = 100 5
= 20 cm Object at 18cm, 22cm, and 30 cm, image can be magnified. (ii) At 22cm and 30cm, image can be obtained on a screen. (b) Film projectors and telescopes.
24. (a) Define focal length of a divergent lens.
(b) A divergent lens of focal length 30 cm forms the image of an object of size 6 cm on the same side as the object at a distance of 15 cm from its optical centre. Use lens formula to determine the distance of the object from the lens and the size of the image formed.
(a) The distance between the principal focus and the optical centres of the concave lens or diverging lens is called the focal length of a diverging lens. (b) = −30 cm, ℎ = 6 cm , = −15 cm, u = ? , ℎ = ?
Ans:
25. (i) Two convex lenses A and B have powers P1 and P2, respectively and P2 is greater than P1. Draw a ray diagram for each lens to show which one will be more converging. Give reason for your answer. (ii) A 2.0 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm. The distance of the object from the lens is 15 cm. Find the nature, position and size of the image. Also find its magnification.
Ans:
Text Book Exericses:
1. Which one of the following materials cannot be used to make a lens?
(a) Water (b) Glass (c) Plastic (d) Clay
Ans: (d) Clay
2. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature
(b) At the centre of curvature
(c) Beyond the centre of curvature
(d) Between the pole of the mirror and its principal focus.
Ans: (d) Between the pole of the mirror and its principal focus.
3. Where should be an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.
Ans: (b) At twice the focal length
4. A spherical mirror and a thin spherical lens have each of a focal length of —15 cm. The mirror and the lens are likely to be
(a) Both concave
(b) Both convex
(c) The mirror is concave and the lens is convex. (d) The mirror is convex, but the lens is concave.
Ans: (a) Both concave
5. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
(a) Plane
(b) Only concave
(c) Convex
(d) Either plane or convex
Ans: (d) Concave
6. Which of the following lenses would you prefer to use while reading small letters found in a 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.
Ans: (c) A convex lens of focal length 5 cm.
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 the object? Draw a ray diagram to show the image formation in this case.
Ans: Object must be placed between pole and focus of the mirror. So, range of distance object from the mirror is less than 15 cm from the pole of the mirror. Image wifi be virtual, erect and larger than the size of the object.
8. Name the type of mirror used in the following situations.
(a) Headlights of a car
(b) Side/rear-view mirror of a vehicle
(c) Solar furnace. Support your answer with reason.
Ans: (i) A convex mirror always forms an erect, virtual and diminished image of an object placed anywhere in front of it. Thus, convex mirrors enable the driver to view much larger traffic behind him that would not be possible with a plane mirror.
(ii) A concave mirror is used as a shaving or make-up mirror because it forms an erect and enlarged image of the face when it is held closer to the face.
9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally?
Ans: When one half of a convex lens is covered with a black paper, the lens will produce a complete image of the object but the intensity of the image is reduced because rays from the top portion of the lens only are refracted and forms the image.
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 nature of the image formed.
Ans:
Image is real and inverted.
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 the ray diagram.
Ans:
12. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.
Ans:
25 Image is virtual erect, small in size and behind the mirror.
13. The magnification produced by a plane mirror is +1. What does this mean?
Ans: It indicates that the size of image is equal to size of object and image formed is virtual and erect.
14. An object 5.0 cm in 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.
Ans:
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 a screen be placed, so that a sharp focused image can be obtained? Find the size and the nature of the image.
Ans:
16. Find the focal length of a lens of power − 2.0 D. What type of lens of this?
Ans:
17. A doctor has prescribed a corrective lens of power + 1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
Ans:
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