Q50. Which two kinds of mirrors always form a virtual, upright image of a real object?
[RRB Group D, 2022]
(A) Concave and convex
(B) Plane and concave
(C) Plane and convex
(D) Concave and parabolic
Answer: (C) Plane and convex
Explanation: A plane mirror produces a virtual upright image of unchanged size, while a convex mirror produces a virtual upright reduced image. A concave mirror can instead generate real, inverted images when its object lies beyond the focal point.
Q49. What is the SI unit of focal length?
[RRB Group D, 2022]
(A) Metre
(B) Dioptre
(C) Candela
(D) Radian
Answer: (A) Metre
Explanation: Focal length is a distance measured between an optical centre or mirror pole and its principal focus. Therefore its SI unit is the metre. Dioptre, in contrast, expresses inverse metres and measures optical power.
Q48. What is the focal length of a concave lens whose power is −5 D?
[RRB Group D, 2022]
(A) +20 cm
(B) −5 cm
(C) +5 cm
(D) −20 cm
Answer: (D) −20 cm
Explanation: The thin-lens formula gives f = 1/P. For −5 D, f = −0.2 metre = −20 centimetres. Negative focal length is characteristic of a diverging lens, with its virtual focus on the incident-light side.
Q47. An object placed very far from a concave mirror forms an image where?
[RRB Group D, 2022]
(A) At centre of curvature
(B) Behind the mirror
(C) At the principal focus
(D) At twice its focal length
Answer: (C) At the principal focus
Explanation: Incident rays from a very distant object are nearly parallel to a concave mirror’s principal axis. Reflection makes them converge at the principal focus, yielding a real, inverted and extremely small image near F.
Q46. Splitting white light into component colours on passing through a prism is due to ________.
[WBCS Mains 2021, held May 2022]
(A) Reflection
(B) Refraction only
(C) Transmission
(D) Dispersion
Answer: (D) Dispersion
Explanation: Dispersion means wavelength-dependent deviation, because the refractive index of glass changes with colour. Violet typically bends more than red, allowing a prism to resolve a single beam of white light into a coloured spectrum.
Q45. Who among the following made the first compound microscope?
[SSC MTS, 2021]
(A) William Gilbert
(B) Zacharias Janssen
(C) John Harington
(D) Galileo Galilei
Answer: (B) Zacharias Janssen
Explanation: Zacharias Janssen, often working in accounts with his father Hans, is traditionally associated with early compound microscopes. Such instruments use an objective and eyepiece to magnify small objects in successive stages, unlike a single-lens magnifier.
Q44. People who wear bifocal lenses commonly have which age-related eye condition?
[SSC MTS, 14 Oct 2021]
(A) Hypermetropia only
(B) Myopia only
(C) Presbyopia
(D) Cataract
Answer: (C) Presbyopia
Explanation: Presbyopia involves reduced near focusing caused by age-related stiffening of the crystalline lens. Bifocal spectacles combine portions of different optical powers, typically aiding distant and close vision without changing glasses each time.
Q43. What is the coloured ring behind the cornea that changes the size of the pupil?
[SSC MTS, 8 Oct 2021]
(A) Retina
(B) Iris
(C) Fovea
(D) Choroid
Answer: (B) Iris
Explanation: The iris is the coloured muscular diaphragm of the eye. It contracts or relaxes to vary pupil size and thereby controls the amount of light entering. The retina is the light-sensitive layer where images are formed.
Q42. The spectrum of colours on a compact disc (CD) is mainly due to ______.
[WBCS Prelims, 22 Aug 2021]
(A) Refraction
(B) Reflection
(C) Diffraction
(D) Polarisation
Answer: (C) Diffraction
Explanation: Closely spaced tracks on a compact disc behave as a diffraction grating. Light reflected from the tracks interferes, with each wavelength reinforced in different directions. Thus shifting the disc or the viewing angle reveals bands of different colours.
Q41. Which electromagnetic radiation is used in most television remote controls?
[RRB NTPC, 31 Jul 2021]
(A) X-rays
(B) Ultraviolet
(C) Gamma rays
(D) Infrared
Answer: (D) Infrared
Explanation: Common television remotes encode signals as pulses of infrared light from an LED. An infrared receiver in the television detects those pulses and decodes commands. Infrared wavelengths are longer than visible red light and are invisible to human eyes.
