Ray Optics – Reflection of Light: NEET MCQs with Answers

Explanation: For u = −40 cm, v = 24 cm. For u = −20 cm, v = 60 cm. The image moves from 24 cm to 60 cm, i.e. 36 cm further away from the mirror.

Explanation: Rotating a mirror by θ turns the reflected ray by 2θ. The spot therefore moves y = x(2θ), giving θ = y/2x.

Explanation: Ray optics treats light as travelling in straight lines and ignores diffraction. This approximation holds only when the obstacles and apertures involved are much larger than the wavelength of light; when the dimensions become comparable to the wavelength, wave optics must be used instead.

Explanation: For two mirrors inclined at angle θ, if 360°/θ is an even number the number of images is (360°/θ) − 1. Here 360°/60° = 6, which is even, so the number of images is 6 − 1 = 5.

Explanation: By the laws of reflection, a plane mirror need only be half the height of the object for a full-length view, and this result is independent of how far away the person stands. Hence the minimum length is 6/2 = 3 ft.

Explanation: Parallel light converges to the focus of the lens, a distance f₂ beyond it. For the beam to return to infinity, the mirror must send the light back as a parallel beam, which requires the rays to strike the mirror normally — that is, the convergence point must coincide with the mirror's centre of curvature, at 2f₁ from the mirror. Hence d = 2f₁ + f₂.

Explanation: A negative magnification means a real, inverted image, which only a concave mirror can produce — so A and B are both 'concave mirror, real image'. A positive magnification greater than 1 means an enlarged virtual image, again possible only with a concave mirror (object inside the focus) — so C is 'concave mirror, virtual image'. A positive magnification less than 1 means a diminished virtual image, the signature of a convex mirror — so D is 'convex mirror, virtual image'.

Explanation: When a mirror is rotated by θ, the reflected ray turns through 2θ. The spot moves along the scale by y at a distance x, so tan 2θ ≈ 2θ = y/x, giving θ = y/2x.

Explanation: For u₁ = −40 cm and f = −15 cm, 1/v₁ = −1/15 + 1/40 = −5/120, so v₁ = −24 cm. For u₂ = −20 cm, 1/v₂ = −1/15 + 1/20 = −1/60, so v₂ = −60 cm. The image moves from 24 cm to 60 cm, a displacement of 36 cm directed away from the mirror.

Explanation: Using the mirror formula 1/v + 1/u = 1/f with u = −1.5f and f = −f: 1/v = −1/f + 1/(1.5f) = (−1.5 + 1)/(1.5f) = −1/(3f). Hence v = −3f, i.e. a real image formed 3f in front of the mirror.

Explanation: The magnification of a mirror is m = −v/u, and an erect image needs m to be positive, so u and v must have opposite signs. The object and image then lie on opposite sides of the mirror, which makes one of them real and the other virtual. A real object with a virtual image and a virtual object with a real image both satisfy this, whereas two real or two virtual points lie on the same side and give an inverted image.

Explanation: The pole, the centre of curvature and the principal axis are fixed by the geometry of the mirror alone and would be the same if no light fell on it. The focus is the point where rays parallel to the axis meet after reflection. Rays far from the axis cross nearer the pole than paraxial rays do, so a single focus exists only for paraxial rays.

Explanation: The plane mirror at 45° turns the parallel beam through 90°, so it remains a parallel beam, now travelling along +y. This direction is parallel to the principal axis of the concave mirror, so the reflected rays converge at its focus. The focus lies 12 cm from the pole along the axis, on the side the mirror faces, at y = 30 − 12 = 18 cm with x = 15 cm.

Explanation: The magnification is 0.4/1.6 = 1/4. For a convex mirror with object distance u (taken as a positive size), the mirror formula gives 1/v = 1/f + 1/u, and the magnification is v/u = f/(f + u). Setting 2/(2 + u) = 1/4 gives u = 6 m, and the image then lies 1.5 m behind the mirror.

Explanation: An erect image of a real object is virtual, and a size larger than the object is needed here. A concave mirror does this when the object lies between the pole and the focus, and a convex lens does it when the object lies within its focal length. A convex mirror and a concave lens always give a diminished image of a real object.