Q40. Why does the clear daytime sky look blue?
[RRB NTPC, 7 Apr 2021]
(A) Scattering of sunlight
(B) Total reflection at the ground
(C) Interference of clouds only
(D) Dispersion by a glass lens
Answer: (A) Scattering of sunlight
Explanation: Air molecules scatter the shorter visible wavelengths of sunlight more strongly than longer red wavelengths. Scattered blue light reaches the observer from many directions, giving the sky its characteristic colour. This mechanism is called Rayleigh scattering.
Q39. Which equation gives the thin-lens formula under Cartesian sign convention?
[RRB NTPC, 27 Mar 2021]
(A) 1/v + 1/f = 1/u
(B) 1/v − 1/u = 1/f
(C) 1/h + 1/v = 1/f
(D) 1/v + 1/u = 1/f
Answer: (B) 1/v − 1/u = 1/f
Explanation: The thin-lens formula is 1/v − 1/u = 1/f, connecting image distance v, object distance u and focal length f. Proper signs allow use with convex or concave lenses and with real or virtual images.
Q38. Light travels in a ______ through a homogeneous medium.
[RRB NTPC, 12 Mar 2021, Shift 1]
(A) Curved line
(B) Horizontal line only
(C) Straight line
(D) Vertical line only
Answer: (C) Straight line
Explanation: In a uniform transparent medium, a light ray travels approximately along a straight path, described as rectilinear propagation. It explains ordinary sharp shadows and pinhole-camera images. Boundaries, refractive-index gradients and diffraction can alter the path.
Q37. Which lens is suitable for correcting myopia?
[RRB NTPC, 12 Mar 2021, Shift 2]
(A) Compound lens
(B) Convex lens
(C) Goggles without corrective power
(D) Concave lens
Answer: (D) Concave lens
Explanation: Myopia means distant objects focus in front of the retina, often because the eye is too long or too strongly converging. A concave lens spreads incoming light slightly, shifting the final focus back onto the retina.
Q36. What kind of mirror is placed behind the light source in a car headlight?
[RRB NTPC, 2021]
(A) Spherical convex mirror
(B) Cylindrical mirror
(C) Parabolic concave mirror
(D) Plane mirror
Answer: (C) Parabolic concave mirror
Explanation: A parabolic concave reflector converts light from a source near its focus into an approximately parallel beam. This directs more illumination forward along the road, unlike a convex mirror, which spreads reflected light over a wider angle.
Q35. Which of the following phenomena is NOT caused by refraction?
[RRB NTPC, 19 Jan 2021]
(A) A pencil appearing bent in water
(B) Apparent depth of a pool
(C) Twinkling of stars
(D) Red colour of the setting Sun
Answer: (D) Red colour of the setting Sun
Explanation: A red sunset mainly comes from scattering: blue wavelengths are removed from the direct beam over a long atmospheric path. The other effects arise from rays changing direction at boundaries or through layers of varying atmospheric refractive index.
Q34. What is another name for lazy eye?
[SSC CHSL, Oct 2020]
(A) Myopia
(B) Hypermetropia
(C) Astigmatism
(D) Amblyopia
Answer: (D) Amblyopia
Explanation: Amblyopia is a developmental reduction of vision in one eye, sometimes both, when visual processing fails to develop normally during childhood. It is not simply a refractive error, although uncorrected unequal refractive errors can contribute to it.
Q33. A glass lens disappears visually when immersed in a liquid of what refractive index relative to the glass?
[RPSC 2nd Grade Science, 4 Jul 2019]
(A) Equal to that of glass
(B) Much lower
(C) Slightly different
(D) Always zero
Answer: (A) Equal to that of glass
Explanation: When a transparent object and surrounding liquid have identical refractive indices, rays passing through their interface suffer no deviation. The lens becomes difficult to distinguish because its boundaries lose optical contrast, though absorption or imperfections may remain visible.
Q32. Which light-sensitive pigment is found in the rod cells of the human retina?
[SSC CHSL, 3 Jul 2019]
(A) Haemoglobin
(B) Melanin
(C) Chlorophyll
(D) Rhodopsin
Answer: (D) Rhodopsin
Explanation: Rhodopsin is a light-sensitive pigment in retinal rod cells. It is particularly important for vision in dim illumination. Absorbing photons initiates a photochemical signalling cascade; cone cells instead support detailed colour vision in brighter light.
Q31. What physical quantity is measured by a light-year?
[WBCS Prelims, 2019]
(A) Light intensity
(B) Time
(C) Distance
(D) Frequency
Answer: (C) Distance
Explanation: A light-year is the distance light travels in vacuum in one Julian year, roughly 9.46 trillion kilometres. Despite including the word year, it is a length unit used in astronomy to describe large distances between celestial objects.
