Q90. In optics, which term refers to the diaphragm opening that limits the propagation of light through a lens?
[SSC MTS, 13 Sep 2023]
(A) Collimator
(B) Aperture
(C) Apostilb
(D) Meniscus
Answer: (B) Aperture
Explanation: The aperture is the effective opening admitting light into an optical system. Its diameter governs illumination and influences depth of field and diffraction blur. In cameras, the f-number is focal length divided by the aperture diameter.
Q89. A concave mirror has radius of curvature 50 cm. At what object distance will the reflected image be at infinity?
[UPSC CDS, Sep 2023]
(A) 25 cm
(B) 50 cm
(C) 100 cm
(D) 12.5 cm
Answer: (A) 25 cm
Explanation: The principal focus of a spherical mirror lies halfway between its pole and centre of curvature. Therefore, R = 50 cm gives focal length magnitude 25 cm. Placing the object at that focus produces parallel reflected rays and an image at infinity.
Q88. In a microscope, which objective lens is usually selected first while using the coarse adjustment knob?
[SSC CHSL, 17 Aug 2023]
(A) Oil immersion objective
(B) Low-power objective
(C) Highest-power objective
(D) No objective
Answer: (B) Low-power objective
Explanation: The low-power objective provides a comparatively wide field of view and adequate working distance, making initial focusing safer and easier. Coarse adjustment moves the microscope stage substantially; high-power focusing should generally use the fine-adjustment knob.
Q87. When entering dim surroundings after bright light, which part of the eye changes the pupil size?
[SSC CGL, 25 Jul 2023]
(A) Retina
(B) Iris
(C) Optic nerve
(D) Cornea
Answer: (B) Iris
Explanation: The iris contains muscles that regulate pupil diameter. In bright light it constricts the pupil, whereas in dim surroundings the pupil gradually dilates to admit more light. Rod adaptation also contributes to improved dark sensitivity.
Q86. What is generally formed behind an opaque object when it blocks light?
[SSC MTS, 15 May 2023]
(A) A rainbow
(B) A shadow
(C) A real image in air
(D) A spectrum
Answer: (B) A shadow
Explanation: An opaque object prevents light rays from travelling directly through it, leaving a region of reduced illumination behind. The resulting shadow demonstrates the approximately straight-line propagation of light, with penumbra and umbra depending on source size.
Q85. The SI unit of power of a lens is ______.
[SSC MTS, 10 May 2023]
(A) Candela
(B) Mole
(C) Watt
(D) Dioptre
Answer: (D) Dioptre
Explanation: The dioptre measures reciprocal focal length, with one dioptre equal to one inverse metre. A lens of focal length 0.5 metre has power 2 D. Candela, mole and watt correspond to different physical quantities.
Q84. An image that cannot be captured directly on a screen is called a ______.
[SSC MTS, 9 May 2023]
(A) Virtual image
(B) Real image
(C) Vector image
(D) Spectral image
Answer: (A) Virtual image
Explanation: A virtual image appears where backward extensions of rays meet, even though the actual rays do not intersect there. Because no rays physically converge at its location, a screen cannot directly capture it. Plane mirrors commonly produce virtual images.
Q83. Why is blue light deviated more strongly than red light when passing through a glass prism?
[UPSC NDA, Apr 2023]
(A) Blue has a longer wavelength
(B) Blue travels faster through glass
(C) Their refractive indices are identical
(D) Glass has a larger refractive index for blue than red
Answer: (D) Glass has a larger refractive index for blue than red
Explanation: Ordinary glass exhibits normal dispersion: its refractive index is greater for shorter visible wavelengths. Hence blue light travels more slowly than red within the glass and bends more at prism surfaces, yielding a greater angular deviation.
Q82. Which statements are correct about lens power:
(i) a convex lens has positive power;
(ii) a concave lens has negative power?
[SSC CHSL, 17 Mar 2023]
(A) Both (i) and (ii)
(B) Only (i)
(C) Only (ii)
(D) Neither
Answer: (A) Both (i) and (ii)
Explanation: Lens power P = 1/f carries the same sign as focal length. A convex lens in air converges parallel rays and has positive focal length, whereas a concave lens diverges them and has negative focal length. Both statements are correct.
Q81. A convex mirror always forms an image that is ______.
[SSC CHSL, 16 Mar 2023]
(A) Real and inverted
(B) Real and magnified
(C) Virtual and erect
(D) Virtual and inverted
Answer: (C) Virtual and erect
Explanation: The convex surface reflects rays away from the axis, producing diverging reflected rays. Their extensions meet behind the mirror and create an erect virtual image, always smaller than a real object. This makes convex mirrors suitable for vehicle rear-view applications.
