Optics — NEET Physics MCQs with answers

Optics is a Class 12 Physics chapter in the NEET (UG) syllabus. NEET720 has 1,050 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

Reflection and spherical mirrors · Refraction and total internal reflection · Reflection and Refraction · Lenses and lens maker's formula · Prism and dispersion · Optical instruments · Huygens principle · Interference and Young's double slit · Diffraction · Reflection at spherical mirrors · Polarisation · Power of a lens and combinations · Combination of a lens and a mirror · Reflection of Light · Refraction and TIR · Refraction at spherical surface · Refraction through prism · Dispersion by prism · Refraction and prism · Lens maker's equation · Power of a lens · Combination of lenses · Dispersion of light · Human eye and defects of vision · Refraction at Spherical Surfaces · Refraction of light · Plane Mirrors · Refraction through lenses · Young's Double Slit · Refraction through a prism · Polarization · Dispersion · Scattering of light · Optics · Ray Optics · Mirrors + Lenses · Refraction + Prism · Lens + Optical Instruments · YDSE + Huygens' Principle · Polarisation + Refraction

8 free Optics practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · easy · Huygens principle

   What is a wavefront, and how is it oriented relative to the direction of propagation of light?

   • A.The locus of all points vibrating in the same phase; it is always perpendicular to the direction of ray propagation
   • B.The path traced by a single point on the wave; it is always parallel to the direction of propagation
   • C.The locus of points with the same amplitude only, unrelated to phase; orientation can be arbitrary
   • D.The locus of points vibrating in the same phase; it is always parallel to the direction of ray propagation
   Show answer and explanation

   Answer: A. The locus of all points vibrating in the same phase; it is always perpendicular to the direction of ray propagation

   A wavefront is the surface joining all points that are in the same phase of vibration; by the geometry of wave propagation, it is always perpendicular to the rays (direction of energy/wave travel) at every point.

   As a wave spreads out from a source, points located at the same distance (along the wave's travel) from the source vibrate in phase with each other; the surface joining all such points is called the wavefront. Since rays represent the direction of propagation (the direction of energy flow, perpendicular to the local wavefront surface at every point), a wavefront is always oriented perpendicular to the rays passing through it — for example, spherical wavefronts around a point source have radial rays perpendicular to each spherical shell.

   Common mistake: Confusing a wavefront (a surface of constant phase) with the path of a ray, or reversing the perpendicularity rule.

2. Question 2 · medium · Huygens principle

   A plane wavefront is incident normally on a thin convex (converging) lens. Using Huygens' principle, what is the shape of the wavefront just after it emerges from the lens?

   • A.A plane wavefront, unchanged in shape
   • B.A spherical wavefront converging toward the focal point of the lens
   • C.A spherical wavefront diverging away from the lens as if from a point source at the lens
   • D.A cylindrical wavefront aligned along the lens axis
   Show answer and explanation

   Answer: B. A spherical wavefront converging toward the focal point of the lens

   A convex lens delays the central part of an incident plane wavefront less than the edges are delayed relatively (the lens is thicker at the centre, but effectively speeds convergence toward the focus); the emergent wavefront becomes spherical, curving inward and converging to the focal point.

   For a converging lens to bring parallel rays to a common focus, all points of the wavefront must arrive at the focal point in phase simultaneously. This requires the emergent wavefront's shape to be spherical, centred on the focal point, with the wavefront's curvature increasing (radius decreasing) as it approaches the focus. Physically, the glass at the centre of a convex lens is thicker, and light travels slower in glass; but the geometry of the lens is such that the optical path length from the incident plane wavefront to the focus is the same for every ray (Fermat's principle), which is exactly the condition for the emergent wavefront to be a sphere converging on the focus.

   Common mistake: Assuming the wavefront shape stays plane just because the incident wavefront was plane, ignoring the lens's converging action.

3. Question 3 · easy · Huygens principle

   A point source of light emits waves into an isotropic homogeneous medium. What is the shape of the wavefronts, and how does their radius of curvature change with distance from the source?

   • A.Plane wavefronts at all distances from a point source
   • B.Spherical wavefronts, with radius of curvature increasing with distance from the source
   • C.Spherical wavefronts, with radius of curvature decreasing with distance from the source
   • D.Cylindrical wavefronts regardless of source shape
   Show answer and explanation

   Answer: B. Spherical wavefronts, with radius of curvature increasing with distance from the source

   A point source radiates spherical wavefronts; far from the source these approximate plane wavefronts because the radius of curvature becomes very large.

   A point source emits energy equally in all directions in a homogeneous isotropic medium, so at any given instant all points at the same distance from the source have experienced the same phase of oscillation — these form a sphere centered on the source. As the wave travels outward, this sphere's radius increases, so at very large distances a small patch of the wavefront looks essentially flat (a plane wave approximation), because the radius of curvature has grown very large, not because the wavefront literally becomes planar at any finite distance.

   Common mistake: Believing wavefronts become plane at some finite distance, or that curvature increases (rather than decreases in the sense of larger radius) with distance.

4. Question 4 · easy · Huygens principle

   A plane wavefront falls on a converging (convex) lens, and the emergent wavefront becomes spherical, converging toward the focal point. Which key idea of Huygens' construction explains why the central part of the wavefront is delayed relative to the edges as it crosses the lens?

   • A.Light travels faster through thicker glass, so the centre arrives earlier
   • B.The lens bends light only at its very centre, leaving edge rays completely unaffected
   • C.The lens is thicker at the centre than at the edges, so light travelling through the (slower) glass at the centre takes longer, delaying the central wavelets relative to the edge wavelets and curving the emergent wavefront inward
   • D.The wavefront shape change is due to diffraction at the lens edges only, unrelated to path length differences through glass
   Show answer and explanation

   Answer: C. The lens is thicker at the centre than at the edges, so light travelling through the (slower) glass at the centre takes longer, delaying the central wavelets relative to the edge wavelets and curving the emergent wavefront inward

   Light travels more slowly in glass than air; since a convex lens is thickest at the centre, central wavelets are delayed relative to edge wavelets, bending the emergent wavefront into a converging spherical shape.

   Using Huygens' construction, each point on the incident plane wavefront becomes a source of secondary wavelets. Because a convex lens is thicker at its centre, light travelling along the central axis spends more time inside the (slower) glass than light passing near the thin edges. This extra optical path length delays the central wavelets relative to the edge wavelets. When the emergent wavelets are combined, the delayed centre and advanced edges combine into a spherical wavefront that curves inward, converging toward the focal point — exactly matching the observed focusing action of a convex lens.

   Common mistake: Believing light travels faster through denser media, or attributing the effect to diffraction rather than differential transit time.

5. Question 5 · medium · Huygens principle

   Which of the following is a correct general property of wavefronts (in an isotropic medium), following directly from the geometry of Huygens' construction?

   • A.Wavefronts are always parallel to the direction of propagation
   • B.At every point, the ray (direction of energy propagation) is perpendicular to the wavefront at that point
   • C.All points on a wavefront must be equidistant from the original source, regardless of the medium's uniformity
   • D.Wavefronts only exist for plane waves, not for spherical or cylindrical waves
   Show answer and explanation

   Answer: B. At every point, the ray (direction of energy propagation) is perpendicular to the wavefront at that point

   By definition, a wavefront is a surface of constant phase, and in an isotropic medium the direction of wave propagation (the ray) at any point is always normal to the wavefront there — this underlies the entire Huygens secondary-wavelet construction.

   A wavefront is the locus of points having the same phase of oscillation at a given instant. In an isotropic medium, energy propagates outward from each point in the direction that is locally perpendicular to the wavefront's surface — this perpendicular direction is what we call a 'ray.' This is a fundamental geometric fact used throughout Huygens' construction, where each point of a wavefront is treated as a source of secondary spherical wavelets, and the new wavefront (their common tangent) advances along the ray directions, i.e. normal to the old wavefront. Option C is a common overgeneralization valid only for the special case of spherical waves from a point source in a uniform medium.

   Common mistake: Overgeneralizing the spherical-wavefront-from-point-source picture to all wavefronts, or reversing the perpendicular/parallel relationship between rays and wavefronts.

6. Question 6 · medium · Huygens principle

   A plane wavefront in air is incident obliquely on a plane mirror. Using Huygens' construction for reflection, which statement correctly describes how the reflected wavefront is built?

   • A.Since the wave enters and stays in the same medium (air), it cannot change direction at all upon reflection
   • B.The mirror surface itself becomes a single point source, emitting one large spherical wavelet that represents the reflected wave
   • C.Each point on the incident wavefront that reaches the mirror becomes a source of a secondary spherical wavelet in the same (air) medium; because points farther along the mirror are reached later, the common tangent to these staggered wavelets forms a new plane wavefront tilted such that the angle of reflection equals the angle of incidence
   • D.The reflected wavefront speed becomes different from the incident wavefront speed because the wave 'bounces'
   Show answer and explanation

   Answer: C. Each point on the incident wavefront that reaches the mirror becomes a source of a secondary spherical wavelet in the same (air) medium; because points farther along the mirror are reached later, the common tangent to these staggered wavelets forms a new plane wavefront tilted such that the angle of reflection equals the angle of incidence

   Points on the incident wavefront strike the mirror at different times (since the wavefront is oblique); the resulting staggered secondary wavelets in air combine into a new plane wavefront obeying angle of incidence = angle of reflection.

   In Huygens' construction for reflection, imagine the oblique plane wavefront advancing toward the mirror: the point closest to the mirror strikes it first and immediately generates a secondary wavelet, while a point farther away strikes later. All these secondary wavelets travel at the same speed (still air, unchanged medium) as spherical wavelets. Drawing the common tangent (envelope) to all these wavelets — generated at different times from different points along the mirror — produces a new plane wavefront. Simple geometry (equal radii for wavelets that have had equal time to expand, combined with the staggered starting times) shows that this reflected wavefront makes the same angle with the mirror as the incident wavefront, i.e. angle of incidence equals angle of reflection. The medium and hence the speed are unchanged throughout, ruling out options C and D; option B misdescribes the construction by collapsing many source points into one.

   Common mistake: Treating the entire mirror as a single wavelet source instead of every point the wavefront successively reaches, or wrongly assuming a speed change occurs on reflection.

7. Question 7 · medium · Huygens principle

   A narrow parallel beam of light (already a plane wave) passes through a small circular aperture comparable in size to its wavelength. According to the wave picture (Huygens' principle), what happens to the wavefront just beyond the aperture, and why?

   • A.The wavefront spreads out (diffracts) beyond the aperture because each point within the aperture opening acts as a source of secondary wavelets, and their combined envelope is no longer a simple plane extending only within the geometric shadow of the aperture
   • B.The wavefront remains a perfect plane wave of the same width as the aperture, with light strictly confined to the geometric shadow, since Huygens' principle predicts no spreading for narrow apertures
   • C.The wavefront disappears entirely beyond the aperture due to destructive interference at every point
   • D.The wavefront's frequency changes because it passed through a small opening
   Show answer and explanation

   Answer: A. The wavefront spreads out (diffracts) beyond the aperture because each point within the aperture opening acts as a source of secondary wavelets, and their combined envelope is no longer a simple plane extending only within the geometric shadow of the aperture

   Huygens' principle treats every point within the aperture as a secondary wavelet source; when the aperture size is comparable to the wavelength, these wavelets combine to spread the light beyond the simple geometric shadow — this is diffraction.

   According to Huygens' construction, every unobstructed point of the wavefront within the aperture becomes a source of secondary spherical wavelets. When the aperture is large compared to the wavelength, these wavelets combine to approximately reconstruct a plane wave confined to the geometric shadow (with small edge effects). But when the aperture size is comparable to the wavelength, the wavelets from across the narrow opening interfere significantly with each other in ways that produce a broader spreading pattern beyond the aperture (diffraction), rather than a simple sharp-edged beam. This directly illustrates why Huygens' wave picture (not simple ray/geometric optics) correctly predicts diffraction phenomena.

   Common mistake: Assuming light always travels in a strictly straight-edged beam through any aperture, ignoring diffraction effects predicted by Huygens' secondary-wavelet picture.

8. Question 8 · medium · Optical instruments

   A ray diagram of an astronomical telescope in normal adjustment shows: parallel rays from a very distant object entering the objective, converging to a point on its focal plane, then diverging to strike the eyepiece, and finally emerging as parallel rays after the eyepiece. Based on this ray diagram, which statement about the final image is correct?

   • D.The final image is virtual, inverted (relative to the object), and formed at infinity
   • B.The final image is real, erect, and formed at the eyepiece's focal plane
   • C.The final image is virtual, erect, and formed at infinity
   • A.The final image is real, inverted, and formed at the eyepiece's focal plane
   Show answer and explanation

   Answer: D. The final image is virtual, inverted (relative to the object), and formed at infinity

   The objective forms a real, inverted intermediate image at its focal plane (which coincides with the eyepiece's focal plane in normal adjustment); the eyepiece then acts as a simple magnifier on this image, producing a final virtual image at infinity that remains inverted relative to the original object.

   From the ray diagram: parallel rays from the distant object converge at the objective's focal plane, forming a real, inverted intermediate image. Since this intermediate image lies at the eyepiece's focal point (normal adjustment), the eyepiece — acting as a simple magnifier with the object at its focus — sends out parallel rays for each point of the intermediate image. Parallel emergent rays mean the final image is formed at infinity, and since parallel rays never actually converge, this final image is virtual. No further inversion is introduced by the eyepiece (a single converging lens acting as a simple magnifier with object at focus preserves orientation of the image it receives), so the final image remains inverted relative to the original distant object.

   Common mistake: Assuming the final image seen through the eyepiece is real because it 'looks' focused, rather than recognising that parallel emergent rays produce a virtual image at infinity.

Questions about Optics for NEET

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Questions are original NEET720 compositions reviewed for correctness, syllabus fit and option quality. Counts update as the bank grows (1,050 active practice questions in this chapter today).