50 questions on this topic of Optics (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.
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Answer: (B) Wavefront
A wavefront is defined as the continuous locus of all adjacent points having the same phase of oscillation. Rays are lines perpendicular to the wavefront pointing along the direction of energy propagation.
Answer: (A) (μ - 1) A
The emerging wavefront is tilted by an angle equal to the deviation produced by the prism. For a thin prism of small apex angle A, the angle of deviation is given by δ = (μ - 1) A.
Answer: (D) Portion of a non-spherical curved wavefront expanding into air
Due to spherical aberration during refraction at a plane interface, rays emerging at different incident angles do not appear to diverge from a single unique virtual point. Consequently, the emerging wavefront in air is curved but non-spherical.
Answer: (C) cos r / cos i
Let AB be the incident wavefront of width w₁ and CD be the refracted wavefront of width w₂ across common boundary interface BC. In triangle ABC, cos i = w₁ / BC, and in triangle BCD, cos r = w₂ / BC. Dividing these gives w₂ / w₁ = cos r / cos i.
Answer: (C) 0
By definition, a wavefront is the locus of all points in a medium that oscillate in the same phase. Therefore, any two points lying on the exact same wavefront have a phase difference of zero.
Answer: (B) cos r / cos i
From the geometry of the refracted beam, the width of the incident beam is w₁ = AB cos i and the width of the refracted beam is w₂ = AB cos r, where AB is the common intercept on the interface. Therefore, the ratio w₂ / w₁ = cos r / cos i.
Answer: (D) A spherical wavefront converging towards the center of curvature
Rays emanating from the center of curvature strike the concave mirror normally and reflect back along their paths. Therefore, the reflected rays converge back to the center of curvature, making the reflected wavefront spherical and converging towards the center of curvature.
Answer: (A) Both Statement I and Statement II are correct
Statement I is true because every point on a wavefront has identical phase by definition, making their phase difference zero. Statement II is also true because rays represent paths orthogonal to wavefronts, and according to wave theory, the optical path and transit time between any two successive wavefronts along different rays are identical.
Answer: (A) Acts as a source of secondary wavelets, whose forward envelope gives the new wavefront
Huygens principle: every point of a wavefront is a source of secondary spherical wavelets; the common forward tangent (envelope) after time t is the new wavefront.
NCERT Class 12
Answer: (A) Both Statement I and Statement II are correct and Statement II explains Statement I
A parallel beam of light consists of parallel rays, which by definition are orthogonal to flat surfaces, i.e., plane wavefronts. A small portion of a spherical wavefront originating from a distant point source has an extremely large radius of curvature (R → ∞), so it approximates a plane wavefront, which corresponds directly to a parallel beam. Hence both statements are true and Statement II explains Statement I.
Answer: (C) t₁ > t₂, causing the lower portion of the wavefront to lag behind the upper portion
Light travels slower in glass (v = c/n) than in air. Since the base has a finite thickness of glass while the apex has negligible thickness, light passing near the base takes a longer time (t₁ > t₂) and lags behind, thereby tilting the emergent wavefront towards the base.
Answer: (A) Diverging spherical
When a plane wavefront strikes a convex mirror, the central part strikes the mirror earlier than the edges and reflects back first. Therefore, the reflected wavefront is convex toward the direction of propagation, appearing to diverge from the virtual focus as a diverging spherical wavefront.