Q100. Why do printed letters below a thick glass slab appear raised?
[RRB Group D, 27 Nov 2025]
(A) Refraction at the slab surfaces
(B) Diffraction of printed ink
(C) Polarisation of reflected light
(D) Absorption by glass
Answer: (A) Refraction at the slab surfaces
Explanation: Light travelling from glass into air bends away from the normal. Observers trace its outgoing rays back along straight lines, causing the letters to appear closer to the upper surface. This optical shift reduces apparent depth.
Q99. Which phenomenon makes light bend around the edges of a small obstruction?
[SSC CGL, 23 Sep 2024]
(A) Reflection
(B) Diffraction
(C) Dispersion
(D) Refraction
Answer: (B) Diffraction
Explanation: Diffraction is the spreading or bending of waves at apertures and around obstacles, especially when their dimensions are comparable with the wavelength. It is evidence of the wave nature of light and differs from refraction at a medium boundary.
Q98. Who formulated the mathematical relationship commonly known as Snell’s law of refraction?
[SSC CGL, 23 Sep 2024]
(A) Isaac Newton
(B) Christiaan Huygens
(C) Willebrord Snellius
(D) Michael Faraday
Answer: (C) Willebrord Snellius
Explanation: Snell’s law relates angles of incidence and refraction through n1 sin i = n2 sin r. The law is named for the Dutch mathematician Willebrord Snellius, although the relationship has an earlier history in several scientific traditions.
Q97. Where does a convex lens form the image of an object placed at its principal focus?
[SSC CGL, 13 Sep 2024]
(A) At the optical centre
(B) At 2F
(C) At infinity
(D) Between F and 2F
Answer: (C) At infinity
Explanation: When the object is at the focal point of a convex lens, refracted rays leave parallel to one another. Parallel rays intersect only at infinity in geometrical optics, so the image is described as highly enlarged and infinitely distant.
Q96. What is the unit of power of a lens?
[SSC CGL, 11 Sep 2024]
(A) Lumen
(B) Candela
(C) Watt
(D) Dioptre
Answer: (D) Dioptre
Explanation: Lens power equals the reciprocal of focal length expressed in metres. Its unit is the dioptre, symbol D, equivalent to one inverse metre. Lumen and candela measure photometric quantities, while watt measures energy transferred per second.
Q95. Who developed the wave theory of light explaining reflection and refraction?
[SSC CHSL, 11 Jul 2024]
(A) Isaac Newton
(B) Thomas Edison
(C) Christiaan Huygens
(D) J. J. Thomson
Answer: (C) Christiaan Huygens
Explanation: Christiaan Huygens developed a wave description in which every point on a wavefront acts as a source of secondary wavelets. Huygens’ principle helps explain the construction of reflected and refracted wavefronts and supports the wave model of light.
Q94. Which statements correctly describe the formation of a primary rainbow?
[UPSC NDA, 21 Apr 2024]
(A) Only reflection, with no refraction
(B) Refraction and one internal reflection inside droplets
(C) Only diffraction of sunlight
(D) Refraction and two internal reflections inside droplets
Answer: (B) Refraction and one internal reflection inside droplets
Explanation: A primary rainbow is produced when sunlight refracts entering a raindrop, reflects once internally, and refracts again upon emerging. Dispersion separates wavelengths into an arc with red on the outside and violet toward the inside.
Q93. Which of these is NOT a component of a compound microscope?
[UPSC NDA, 21 Apr 2024]
(A) Objective lens
(B) Eyepiece lens
(C) Body tube
(D) Retina
Answer: (D) Retina
Explanation: A compound microscope contains an objective lens and an eyepiece lens mounted along an optical tube. The retina is part of the observer’s eye, not the instrument itself. The objective forms an intermediate image magnified by the eyepiece.
Q92. The objective and eyepiece of a standard compound microscope are typically which kind of lenses?
[UPSC NDA, 21 Apr 2024]
(A) Two convex lenses
(B) Two concave lenses
(C) A concave and a plane mirror
(D) Two plane glass plates
Answer: (A) Two convex lenses
Explanation: A conventional compound microscope uses two positive or convex lenses. The objective forms a real, enlarged intermediate image near the eyepiece focal plane, which the eyepiece views as a magnifier to produce a larger virtual image.
Q91. What is the characteristic colour of light emitted by a ruby laser?
[UPPSC RO/ARO, 11 Feb 2024]
(A) Green
(B) Blue
(C) Red
(D) Yellow
Answer: (C) Red
Explanation: A ruby laser normally emits deep red light near 694 nanometres. Chromium ions within the ruby crystal provide the active laser transition. Its laser emission is monochromatic, coherent and highly directional, unlike ordinary broad-spectrum light sources.
