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ASSIGNMENT 1 Light-Reflection and Refraction.

1. . Find the size, nature and position of image formed when an object of size 1 cm is placed at a
distance of 15 cm from a concave mirror of focal length 10 cm. Ans. v = -30 cm, Real and inverted,
size of the image is -2 cm.
2. The image of an object formed by a mirror is real, inverted and is of magnification -1. 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 20 cm towards the mirror? State reason and also draw ray diagram for the new
position of the object to justify your answer. Ans. new position would be at the focus of the mirror.
3. An object 2 cm high is placed at a distance of 16 cm from a concave mirror which Produces a real
image 3 cm high.
(i) Find the position of the image.
(ii) What is the focal length of mirror ? Ans. (i) v = -24cm (ii)Focal length is -9.6 cm
4. A Concave mirror of focal length 10 cm is placed at a distance of 35 cm from a wall. How far from the
wall an object be placed so that its image formed by the mirror falls on the wall ? Ans. The distance
of the object from the wall = 21 cm
5. A 2.0 cm tall object is placed perpendicular to the principal axis of a concave mirror of the focal
length 10 cm. The distance of the object from the mirror is 15 cm. Find the nature, position and size
of the image formed. Represent the situation with the help of a ray diagram Ans. V = -30 cm, H’ = -4
cm, image formed is real, inverted and enlarged.
6. A student has focused the image of a candle flame on a white screen using a concave mirror. The
situation is as given below: Length of the flame = 1.5 cm Focal length of the mirror = 12 cm
Distance of flame from the mirror = 18 cm. If the flame is perpendicular to the principal axis of
the mirror, then calculate the following ;
(a) Distance of the image from the mirror
(b) Length of the image

If the distance between the mirror and the flame is reduced to 10 cm, then what would be observed on
the screen? Draw ray diagram to justify your answer for this situation. Ans. So v = -36 cm, H’ = - 3 cm
(Magnified, inverted image)

7. A 2 cm high object is placed at a distance of 32 cm from a concave mirror. The image is real, inverted
and 3 cm in size. Find the focal length of the mirror and the position where the image is formed? Ans
v = -48 cm, f= -19.2 cm
8. A convex mirror used for rear view on an automobile has a radius of curvature of 3.0 m. If a bus is
located at 5.0 m from the mirror, find the position, nature and size of the image. Ans. v = 1.15 m
approx., Magnification, m= +0.23, The image is virtual, erect and smaller in size
9. Size of image of an object formed by a mirror having a focal length of 20 cm, is observed to be
reduced to 1/3rd of its size. At what distance the object has been placed from the mirror? What is the
−80
nature of the image and the mirror? Ans. Case 1; v = cm, Image is real and inverted. Case 2: when
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40
mirror is convex, v = cm Image is virtual and erect.
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10. A truck uses a convex mirror as view finder whose radius of curvature is 2.0 m. A maruti car is
coming behind the truck at a distance of 10 m. What will be the position of the image of the car and
size of the image of the car when observed by the driver of the truck through the convex mirror?
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Ans. v = 0.9 m, size of the image of the car will be a fraction of 11 the actual size of the car through the
convex mirror.
11. If the angle of incidence (i) for a light ray in air be 45 0 and the angle of refraction (r) in glass be 30 0,
find the refractive index of glass with respect to air. Ans. Refractive Index of glass, μ = √2
12. Refractive index of water with respect to air is 1.33. What is the value of refractive index of air with
respect to water? Ans. wμa = 0.75
13. The absolute refractive indices of glass and water are 4/3 and 3/2 respectively. If the speed of
light in glass is 2 × 108 m/s, calculate the speed of light in (i)vacuum (ii) water. Ans. (i)Speed of
light is vacuum = 2.67 ×108 m/s (ii) Speed of light in water = 1.73 × 108 m/s
14. Refractive index of water with respect to air is 4/3 and the glass is 3/2. What is the refractive index
of glass with respect to water? Ans. Refractive index of glass with respect to water is 9/8.
15. The refractive indices of glass and water with respect to air are 3/2 and 4/3 respectively. If the
speed of light in glass is 2×108 m/s, find the speed of light in water.Ans. Speed of light in water =
2.25×108 m/s
16. Refractive indices of water and benzene with respect to air are 1.33 and 1.50 respectively.
Calculate the refractive index of benzene with respect to water. Ans. The refractive index of
benzene with respect to water is 1.13
17. Refractive index of diamond with respect to glass is 1.6 and absolute refractive index of glass is
1.5. Find out the absolute refractive index of diamond. Ans. The absolute refractive index of
diamond is 2.4
18. The absolute are refractive indices of two media ‘A’ and ‘B’ are 2.0 and 1.5 respectively. If the
speed of light in medium ‘B’ is 2×108 m/s. calculate the speed of light in(i)vacuum (ii)medium ‘A’
Ans. (i)The speed of light in vacuum is 3×108 m/s (ii) The speed of light in medium ‘A’ is 1.5×108
m/s
19. The image of a candle flame formed by a lens is obtained on a screen placed on the other side of
the lens. If the image is three times the size of the flame and the distance between lens and image
is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the
image at a distance of 80 cm from the lens? Ans. The distance of the candle is – 80/3. Image is real
and inverted
20. An object 3 cm high is placed 20 cm from convex lens of focal length 12 cm. Find the nature,
position and height of the image. Ans. The image is 30 cm from the convex lens, It is real, inverted
and 4.5 cm high.
21. An object 1 m tall is placed on the principal axis of a convex lens and its 40 cm tall image is formed
on the screen placed at a distance of 70 cm from the object. What is the focal length of the lens?
Ans. Focal length of the lens is 14.3 cm
22. A 5cm tall object is placed on the principal axis of a convex lens of focal length 50 cm at a distance
of 40 cm from it. Find the nature, position and size of the image. Ans. v = -200cm, the image is
virtual. Magnification m = +5, h’ = +25cm
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23. A student focused the image of a candle flame on a white screen using a convex lens. He noted
down the position of the candle, screen and the lens as under Position of candle = 12.0 cm
Position of convex lens = 50.0 cm, Position of the screen = 88 cm.(1)What is the focal length of
the convex lens? (2)Where will the image be formed if he shifts the candle towards the lens at a
position of 31.0 cm (iii)What will be the nature of the image formed if he further shifts the candle
towards the lens (iv) Draw a ray diagram to show the formation of the image in case (iii)as said
above. Ans. (i) focal length of the convex lens is 19 cm (ii) The image will be formed at infinity
(iii)virtual,erect and magnified image is formed on the same side as the object.
24. An object 2 cm high is placed at a distance of 64 cm from a white screen. On placing a convex lens
at a distance of 32 cm from the object it is found that a distinct image of the object is formed on
the screen. What is the focal length of the convex lens and size of the image formed on the screen?
Ans. Focal length, f = 16 cm, Size of the image = -2 cm
25. A real image, 4/5 size of the object is formed 18 cm from a lens. Calculate the focal length of the
lens. Ans. Focal length of the lens, f = 10 cm
26. A object kept at a distance of 60 cm from a lens gives a virtual image at a distance of 20 cm over
the same side of the lens. What is the focal length of the lens? Is the lens converging or diverging?
Ans. The focal length of the lens is 30 cm
27. ‘’A convex lens can form a magnified erect as well as magnified inverted image of an object placed
in front of it.’’ Draw ray diagram to justify this statement stating the position of the object with
respect to the lens in each case. Ans. The image is formed at a distance of 6.67 cm on the same side
as that of the object.
28. At what distance from a concave lens of focal length 20 cm and 6 cm tall object be placed so as to
obtain its image at 15 cm from the lens? Also calculate the size of the image Q Ans. Object is placed
60 cm from the lens, Height of an image is +1.5 cm diminished erect
29. A convex lens has a focal length of 10 cm. What is its power?Ans. The power P =+10 dioptre (or
+10D)
30. A student uses a lens of focal length 50 cm and another of -50 cm. What is the nature of the lens
and its power used by each of them? Ans. Case 1: P= 2D, Case 2: -2D
31. Two thin lenses of focal lengths +10 cm and -5cm are kept in contact. What is the focal length and
power of the combination? Ans. Focal length, f = -10cm,Power of the combination = -10 D
32. Which one of the two- glass and water is optically denser and why?
33. A concave mirror of focal length f produces an image n times the size of the object. What would
be the object distance for which the image is real?
34. If x,y and z denote the object distance, image distance and the radius of curvature respectively of a
spherical mirror, which one of the following is the correct relation between them?
𝒙𝒚
a) Z = 2 𝒙+𝒚
𝒙+𝒚
b) Z = 2 𝒙𝒚
𝒙𝒚
c) Z = 2 𝒙−𝒚
𝒙−𝒚
d) Z = 2 𝒙𝒚
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