Q60. For scattering by very small particles, which visible wavelengths are scattered more strongly?
[RRB Group D, 2022]
(A) Red and orange
(B) Violet and blue
(C) All equally regardless of wavelength
(D) Infrared only
Answer: (B) Violet and blue
Explanation: Rayleigh scattering from particles much smaller than light’s wavelength scales approximately with 1/λ⁴. Consequently blue and violet are scattered more strongly than red. Eye sensitivity and absorption help explain why the daylight sky usually looks blue.
Q59. Which optical devices can have a positive focal length under the Cartesian convention?
[RRB Group D, 2022]
(A) Concave lens and concave mirror
(B) Concave lens and plane mirror
(C) Concave mirror and plane mirror
(D) Convex lens and convex mirror
Answer: (D) Convex lens and convex mirror
Explanation: A convex lens has a real focus on the outgoing-light side and therefore a positive focal length. A convex mirror has a virtual focus behind its surface, also positive for mirrors under the usual Cartesian convention.
Q58. A ray passing through the optical centre of a thin lens emerges ________ the original direction.
[RRB Group D, 2022]
(A) At right angles to
(B) Parallel but shifted far away
(C) With its colour changed
(D) Along the same straight line
Answer: (D) Along the same straight line
Explanation: The optical centre is the point at which a paraxial ray passes through a thin lens without appreciable net angular deviation. It is one of the main construction rays used to find image positions in lens diagrams.
Q57. What is the straight line joining the two principal focal points of a thin lens called?
[RRB Group D, 2022]
(A) Principal axis
(B) Optical power
(C) Angle of emergence
(D) Radius of curvature
Answer: (A) Principal axis
Explanation: The principal axis is the reference line passing through the optical centre and the centres of curvature of the lens surfaces. Its two focal points lie on this axis, providing the geometry for standard ray-diagram constructions.
Q56. A spherical mirror forms an image with magnification −1.38. Which description fits the image?
[RRB Group D, 2022]
(A) Virtual, erect and smaller
(B) Virtual, erect and larger
(C) Real, inverted and larger
(D) Real, inverted and smaller
Answer: (C) Real, inverted and larger
Explanation: Mirror magnification m = −v/u includes orientation through its sign. A negative value means the image is inverted and hence real for a single spherical mirror with a real object. Since its magnitude exceeds one, the image is enlarged.
Q55. A lens has power −6.5 dioptres. What is its approximate focal length?
[RRB Group D, 2022]
(A) +15.38 cm
(B) −15.38 cm
(C) −65 cm
(D) +65 cm
Answer: (B) −15.38 cm
Explanation: The focal length is f = 1/P = −1/6.5 metre, approximately −0.1538 metre or −15.38 centimetres. The negative sign shows that the lens is diverging, as expected for a concave lens in air.
Q54. Which mirror produces a virtual and highly diminished image of an object at infinity?
[RRB Group D, 2022]
(A) Plane mirror
(B) Concave mirror
(C) Convex mirror
(D) Parabolic concave mirror
Answer: (C) Convex mirror
Explanation: Parallel rays from a distant object diverge after reflecting from a convex mirror, appearing to originate from its virtual focus. Because the object’s angular extent is tiny, the resulting image is highly diminished and cannot be captured on a screen.
Q53. What is the focal length of a convex lens having power +3 D, approximately?
[RRB Group D, 2022]
(A) 3 m
(B) 0.03 m
(C) −0.33 m
(D) +0.33 m
Answer: (D) +0.33 m
Explanation: Power and focal length are reciprocals when the latter is in metres. Therefore f = 1/3 metre, approximately +0.33 metre. A positive value identifies a converging convex lens, which can form real images for appropriate object distances.
Q52. Where must an object be placed before a concave mirror to obtain a real, inverted image of the same size?
[RRB Group D, 2022]
(A) Between F and C
(B) Between pole and focus
(C) At infinity
(D) At centre of curvature
Answer: (D) At centre of curvature
Explanation: For a concave mirror with the object at C, reflected rays meet at C as well. The real image is inverted and equal in height to the object, since the magnification is −1. This is a standard spherical-mirror image case.
Q51. What sign does the focal length of a convex mirror have in the Cartesian sign convention?
[RRB Group D, 2022]
(A) Positive
(B) Negative
(C) Always zero
(D) Imaginary only
Answer: (A) Positive
Explanation: The convex mirror’s focus is virtual and situated behind its reflecting surface. This position lies along the positive direction of the principal axis under the common mirror convention. Therefore the mirror’s focal length is positive.
