Physics Questions and Answers – Reflection of Light by Spherical Mirrors

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This set of Physics Question Papers for Class 12 focuses on “Reflection of Light by Spherical Mirrors”.

1. A concave mirror is held in water. What should be the change in the focal length of the mirror?
a) Halved
b) Doubled
c) Remains the same
d) Increases exponentially
View Answer

Answer: c
Explanation: No change. The focal length of a concave mirror does not depend on the nature of the medium. Therefore, the focal length of the mirror, even after holding it in water, will remain the same.
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2. Two concave mirrors have the same focal length but the aperture of one is larger than that of the other. Which mirror forms the sharper image and why?
a) Plane
b) Concave
c) Convex
d) Prism
View Answer

Answer: b
Explanation: The concave mirror with a smaller aperture forms the sharper image because it is free from spherical aberration. So, even though both have the same focal length, the change of aperture affects the formation of the image.

3. A man stands in front of a mirror of special shape. He finds that his image has a very small head, a fat body, and legs of normal size. What can we say about the shapes of the three parts of the mirror?
a) Convex, Concave, Plane
b) The plane, Concave, Convex
c) Concave, Convex, Plane
d) Convex, Plane, Concave
View Answer

Answer: a
Explanation: The upper part of the mirror is convex. The middle part of the mirror is concave. The lower part of the mirror is plane. The image formed by a plane mirror will not have any changes, whereas that will not be the case for concave and convex mirrors.
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4. Can the relative refractive index of a medium w.r.t. another medium be less than unity?
a) Insufficient data
b) Indefinite
c) No
d) Yes
View Answer

Answer: d
Explanation: Yes. The refractive index of a medium concerning another medium can be less than unity. For example, the relative refractive index of water concerning glass is less than unity.

5. Which of the following causes refraction of light?
a) Change in the density of light from one medium to another
b) Change in viscosity of light from one medium to another
c) Change in the speed of light from one medium to another
d) Change in direction of light from one medium to another
View Answer

Answer: c
Explanation: Light travels at different speeds in a different medium. The bending or refraction of light occurs due to the change in the speed of light as it passes from one medium to another. Other statements are not true regarding the refraction of light.
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6. Frequency and wavelength change when light passes from a rarer to a denser medium.
a) True
b) False
View Answer

Answer: b
Explanation: When light passes from a rarer to a denser medium, the wavelength of light changes but the frequency remains unchanged. Therefore, both frequency and wavelength do not change when light passes from a rarer to a denser medium.

7. The magnification of ‘X’ is more than unity. Identify X.
a) Convex mirror
b) Concave mirror
c) Plane mirror
d) Prism
View Answer

Answer: b
Explanation: If the magnification of the mirror is one, then it is a plane mirror. If magnification is more than one, the mirror is concave. If magnification is less than one, then the mirror is convex. So the X required in this question is a concave mirror.
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8. What happens to the shining of diamond if it is dipped in a transparent oil?
a) Reduces
b) Increases
c) Remains the same
d) Becomes colorless
View Answer

Answer: a
Explanation: As the critical angle for diamond – oil interface is greater than that for the diamond – air interface, the shining of diamond reduces when it is dipped in transparent oil.

9. Calculate its focal length if the radii of curvature of a double convex lens are 10 cm, and 20 cm and its refractive index is 1.75.
a) 50 cm
b) 13 cm
c) 8.9 cm
d) 8 cm
View Answer

Answer: c
Explanation: \(\frac {1}{f}\) = (μ-1)\( [ \frac {1}{R_1} – \frac {1}{R_2} ] \)
\(\frac {1}{f}\) = (1.75 – 1)\( [ \frac {1}{10} – \frac {1}{(-20)} ] \)
\(\frac {1}{f} = \frac {9}{80}\)
f = +8.89 cm ≈ 8.9 cm.
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10. If the radius of curvature of each face of the biconcave lens, made of glass of refractive index 1.25 is 50 cm, then what is the focal length of the lens in air?
a) 30 cm
b) -30 cm
c) 5 cm
d) -5 cm
View Answer

Answer: b
Explanation: \(\frac {1}{f}\) = (μ-1)\( [ \frac {1}{R_1} – \frac {1}{R_2} ] \)
\(\frac {1}{f}\) = (1.25 – 1)\( [ \frac {1}{-50} – \frac {1}{50} ] \)
\(\frac {1}{f} = \frac {-1}{100}\)
f = -100 cm
Therefore, the focal length of the lens in air is -100 cm.

Sanfoundry Global Education & Learning Series – Physics – Class 12.

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