Ch-9: Light - Reflection and Refraction30 Questions
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1. The focal length of a spherical mirror of radius of curvature 30 cm is:
A. 60 cm
B. 15 cm
C. 30 cm
D. 10 cm
Answer: B. 15 cm
Explanation: Focal length (f) is half of the radius of curvature (R), so f = R / 2 = 30 / 2 = 15 cm.
2. Which type of mirror is used by dentists to inspect an enlarged image of teeth?
A. Convex mirror
B. Plane mirror
C. Concave mirror
D. Cylindrical mirror
Answer: C. Concave mirror
Explanation: When an object is placed between the pole and focus of a concave mirror, an erect and enlarged virtual image is formed.
3. Which spherical mirror always forms a virtual, erect, and diminished image regardless of object position?
A. Concave mirror
B. Convex mirror
C. Plane mirror
D. Parabolic mirror
Answer: B. Convex mirror
Explanation: Convex mirrors always form virtual, erect, and diminished images, providing a wide field of view.
4. Convex mirrors are commonly used as rear-view mirrors in vehicles because:
A. They form real inverted images
B. They form magnified erect images
C. They give an erect, diminished image and a wider field of view
D. They absorb less light
Answer: C. They give an erect, diminished image and a wider field of view
Explanation: Curved outwards, convex mirrors enable drivers to see a much larger area behind them.
5. According to the New Cartesian Sign Convention, the focal length of a concave mirror is always:
A. Positive
B. Negative
C. Zero
D. Infinite
Answer: B. Negative
Explanation: The focus of a concave mirror lies in front of the reflecting surface (to the left of the pole), so its focal length is negative.
6. The mirror formula relating object distance (u), image distance (v), and focal length (f) is:
A. 1/f = 1/v - 1/u
B. 1/f = 1/v + 1/u
C. f = v + u
D. 1/u = 1/v + 1/f
Answer: B. 1/f = 1/v + 1/u
Explanation: The mirror formula is 1/f = 1/v + 1/u, where distances are measured from the pole.
7. Magnification produced by a rear-view mirror fitted in vehicles is:
A. Less than 1
B. Greater than 1
C. Equal to 1
D. Can be more or less than 1
Answer: A. Less than 1
Explanation: Rear-view mirrors are convex mirrors which always form diminished images, so magnification m = h'/h is less than 1.
8. If magnification m has a negative value, it indicates that the image formed is:
A. Real and inverted
B. Virtual and erect
C. Enlarged and erect
D. Virtual and inverted
Answer: A. Real and inverted
Explanation: Negative magnification means the image height is negative (inverted), which corresponds to a real image.
9. The bending of a ray of light when it passes obliquely from one transparent medium to another is called:
A. Reflection
B. Dispersion
C. Refraction
D. Total internal reflection
Answer: C. Refraction
Explanation: Refraction occurs due to the change in the speed of light as it passes from one medium to another.
10. When light travels from a rarer medium to a denser medium obliquely, it bends:
A. Away from the normal
B. Towards the normal
C. Straight without deviation
D. Back into the first medium
Answer: B. Towards the normal
Explanation: Light slows down upon entering an optically denser medium, bending towards the normal line.
11. Snell's law of refraction is represented by which expression?
A. sin i / sin r = constant
B. sin i × sin r = constant
C. cos i / cos r = constant
D. tan i / tan r = constant
Answer: A. sin i / sin r = constant
Explanation: Snell's Law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media.
12. Which of the following transparent media has the highest refractive index?
A. Water
B. Crown Glass
C. Kerosene
D. Diamond
Answer: D. Diamond
Explanation: Diamond has the highest refractive index (~2.42), giving it the maximum optical density and light-bending ability.
13. Absolute refractive index of a medium is defined as the ratio of speed of light in:
A. Vacuum to speed of light in the medium (c/v)
B. Medium to speed of light in vacuum (v/c)
C. Air to speed of light in glass
D. Water to speed of light in diamond
Answer: A. Vacuum to speed of light in the medium (c/v)
Explanation: Refractive index n = c / v, where c is the speed of light in vacuum (~3 × 10^8 m/s) and v is speed in the medium.
14. A spherical lens thicker at the center than at the edges is called a:
A. Concave lens
B. Convex lens
C. Planoconcave lens
D. Cylindrical lens
Answer: B. Convex lens
Explanation: Convex lenses bulge outward at the middle and act as converging lenses.
15. A convex lens forms a real, inverted image of the same size as the object when the object is placed at:
A. Principal Focus (F)
B. Infinity
C. Twice the focal length (2F1)
D. Between Optical Center and F
Answer: C. Twice the focal length (2F1)
Explanation: Placing an object at 2F1 of a convex lens produces a real, inverted image of equal size at 2F2 on the opposite side.
16. Where should an object be placed in front of a convex lens to get a virtual and magnified image?
A. At 2F1
B. At Focus F1
C. Between Optical Center (O) and Focus (F1)
D. Beyond 2F1
Answer: C. Between Optical Center (O) and Focus (F1)
Explanation: Positioned between optical center and principal focus, the convex lens functions as a magnifying glass forming a virtual erect enlarged image.
17. Which lens always forms a virtual, erect, and diminished image regardless of object position?
A. Convex lens
B. Concave lens
C. Bifocal lens
D. Cylindrical lens
Answer: B. Concave lens
Explanation: Concave lenses diverge light rays and always form virtual, erect, and smaller images on the same side as the object.
18. The lens formula relating object distance (u), image distance (v), and focal length (f) is:
A. 1/f = 1/v + 1/u
B. 1/f = 1/v - 1/u
C. 1/f = 1/u - 1/v
D. f = v - u
Answer: B. 1/f = 1/v - 1/u
Explanation: The lens formula is 1/f = 1/v - 1/u, using proper Cartesian sign conventions for u, v, and f.
19. Magnification produced by a lens (m) in terms of image distance (v) and object distance (u) is:
A. m = -v / u
B. m = v / u
C. m = u / v
D. m = -u / v
Answer: B. m = v / u
Explanation: For spherical lenses, magnification m = height of image / height of object = v / u.
20. The power of a lens (P) is related to its focal length (f in meters) by the formula:
A. P = f
B. P = 1 / f
C. P = 100 × f
D. P = f^2
Answer: B. P = 1 / f
Explanation: Power of a lens is the reciprocal of its focal length expressed in meters (P = 1/f).
21. The SI unit of power of a lens is:
A. Meter
B. Watt
C. Dioptre (D)
D. Joule
Answer: C. Dioptre (D)
Explanation: 1 Dioptre (1 D) is the power of a lens whose focal length is 1 meter (1 D = 1 m^-1).
22. What is the power of a convex lens having a focal length of +50 cm (+0.5 m)?
A. +0.5 D
B. +2.0 D
C. -2.0 D
D. +5.0 D
Answer: B. +2.0 D
Explanation: P = 1 / f(m) = 1 / 0.5 = +2.0 D.
23. Power of a concave lens is always:
A. Positive
B. Negative
C. Zero
D. Variable
Answer: B. Negative
Explanation: Focal length of a concave lens is negative, hence its power is always negative.
24. The optical point inside a lens through which a ray of light passes without suffering any deviation is:
A. Focus
B. Center of curvature
C. Optical center
D. Pole
Answer: C. Optical center
Explanation: Light rays directed through the optical center (O) of a thin lens pass undeviated along their original direction.
25. An object placed at infinity in front of a concave mirror produces an image located at:
A. Center of curvature (C)
B. Focus (F)
C. Between C and F
D. Behind the mirror
Answer: B. Focus (F)
Explanation: Parallel rays from infinity converge at the principal focus F, creating a point-sized real image.
26. To produce a large amount of heat in solar furnaces, which mirror is used to concentrate sunlight?
A. Plane mirror
B. Convex mirror
C. Concave mirror
D. Cylindrical mirror
Answer: C. Concave mirror
Explanation: Large concave mirrors focus incoming parallel solar rays onto a single focal spot to reach high temperatures.
27. If the refractive index of glass with respect to air is 1.5, what is the speed of light in glass?
A. 3.0 × 10^8 m/s
B. 2.0 × 10^8 m/s
C. 1.5 × 10^8 m/s
D. 2.25 × 10^8 m/s
Answer: B. 2.0 × 10^8 m/s
Explanation: v = c / n = (3 × 10^8) / 1.5 = 2.0 × 10^8 m/s.
28. Two thin lenses of power +3 D and -1 D are placed in contact. The net power of the combination is:
A. +4 D
B. +2 D
C. -3 D
D. +3 D
Answer: B. +2 D
Explanation: Total power P = P1 + P2 = (+3 D) + (-1 D) = +2 D.
29. When light enters a rectangular glass slab obliquely and emerges out, the emergent ray is:
A. Perpendicular to the incident ray
B. Parallel to the incident ray but laterally shifted
C. Refracted back into the glass
D. Converged to a single focus
Answer: B. Parallel to the incident ray but laterally shifted
Explanation: Refraction at parallel faces causes equal and opposite bending, so the emergent ray is parallel to the incident ray with a lateral displacement.
30. A real image formed by a mirror or lens is always:
A. Erect
B. Inverted
C. Magnified
D. Diminished
Answer: B. Inverted
Explanation: Real images are formed by actual intersection of light rays and are always inverted relative to the object.
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