Q130. Which statements about light scattering are incorrect:
(i) it is independent of particle size;
(ii) it is directly proportional to wavelength?
[RRB Group D, 3 Feb 2026]
(A) Only (i)
(B) Both (i) and (ii)
(C) Only (ii)
(D) Neither (i) nor (ii)
Answer: (B) Both (i) and (ii)
Explanation: Scattering depends strongly on particle size relative to the light wavelength. For sufficiently small particles, Rayleigh scattering varies inversely as the fourth power of wavelength, so shorter wavelengths scatter more. Both quoted statements are therefore incorrect.
Q129. Which claims about the Cartesian sign convention for lenses are false:
(i) distances originate at focus;
(iii) heights above axis are negative?
[RRB Group D, 2 Feb 2026]
(A) Both (i) and (iii)
(B) Only (i)
(C) Only (iii)
(D) Neither
Answer: (A) Both (i) and (iii)
Explanation: Lens distances are measured from the optical centre, not a principal focus, so statement (i) is false. Heights above the principal axis count as positive, whereas those below are negative; therefore statement (iii) is also false.
Q128. Where should an object be placed before a convex lens to form a real image of exactly the same size?
[RRB Group D, 9 Jan 2026]
(A) At F
(B) At 2F
(C) Beyond 2F
(D) Between lens and F
Answer: (B) At 2F
Explanation: An object at twice the focal length of a convex lens has its image at 2F on the other side. The lens equation yields equal magnitudes of object and image distances, so the magnification is −1.
Q127. What is meant by one dioptre of lens power?
[RRB Group D, 9 Jan 2026]
(A) A lens with 1 cm focal length
(B) The power of a lens of focal length 1 metre
(C) A focal length of 10 metres
(D) The brightness of a light source
Answer: (B) The power of a lens of focal length 1 metre
Explanation: One dioptre is the optical power of a lens with focal length one metre. Power equals 1/f when f is measured in metres. Positive dioptres signify converging lenses and negative values signify diverging lenses.
Q126. What is the relation between radius of curvature R and focal length f of a spherical mirror?
[RRB Group D, 29 Dec 2025]
(A) R = f
(B) R = f/2
(C) R = 2f
(D) R = 4f
Answer: (C) R = 2f
Explanation: For a small-aperture spherical mirror, the principal focus lies midway between pole and centre of curvature. Therefore f = R/2, or R = 2f, with appropriate Cartesian signs. The relation applies to both concave and convex spherical mirrors.
Q125. Where do rays parallel to the principal axis meet after passing through a convex lens?
[RRB Group D, 27 Dec 2025]
(A) At the principal focus
(B) At twice the focus
(C) At the optical centre
(D) At the first surface
Answer: (A) At the principal focus
Explanation: A convex lens is thicker at its middle and ordinarily acts as a converging lens in air. Rays incident parallel to the principal axis are refracted toward the axis and meet at the far-side principal focus, enabling real image formation.
Q124. Which mirror should be used for shaving to obtain an enlarged upright image of the face?
[RRB Group D, 24 Dec 2025]
(A) Convex mirror
(B) Concave mirror
(C) Plane mirror
(D) Two-way mirror
Answer: (B) Concave mirror
Explanation: When a face is positioned between a concave mirror’s pole and focal point, the reflected rays diverge but appear to meet behind the mirror. The image is virtual, upright and larger, making the mirror useful for shaving.
Q123. Where is the image formed if an object is kept at the focus of a convex lens?
[RRB Group D, 23 Dec 2025]
(A) At its optical centre
(B) At infinity
(C) At twice the focal length
(D) On the same side between F and O
Answer: (B) At infinity
Explanation: Placing an object at the first principal focus of a convex lens produces emerging rays parallel to each other. Because they do not meet at a finite distance, the image is described as real, inverted, enlarged and located at infinity.
Q122. What is the sign of the focal length of a concave lens according to the Cartesian convention?
[RRB Group D, 23 Dec 2025]
(A) Positive
(B) Zero
(C) Sometimes infinite
(D) Negative
Answer: (D) Negative
Explanation: The principal focus of a concave lens lies on the incident-light or object side. Under the standard Cartesian sign convention, distances toward incoming light are negative. Thus a concave lens has negative focal length and negative optical power.
Q121. Which statement correctly defines the power of a thin lens?
[RRB Group D, 21 Dec 2025]
(A) Product of focal length and aperture
(B) Reciprocal of focal length in metres
(C) Distance between optical centre and object
(D) Ratio of image to object size
Answer: (B) Reciprocal of focal length in metres
Explanation: Lens power measures how strongly a lens converges or diverges light. Numerically P = 1/f, where focal length is in metres and power in dioptres. A shorter absolute focal length implies greater refracting ability.
