Wave Optics — Class 12 MCQs with Answers

Wave Optics — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Young's experiment demonstrates:

   AInterference of lightBReflection onlyCRefraction onlyDPhotoelectric effect
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   Answer: Interference of light

   Explanation: Young's double-slit setup overlaps light from two coherent slits, and the alternating bright/dark fringes it produces are a wave signature — only interference explains that pattern, not reflection or refraction.

2. Q2 of 6Easy

   Diffraction is:

   ATotal reflectionBSplitting of white lightCBending of light around obstaclesDPolarisation
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   Answer: Bending of light around obstacles

   Explanation: Diffraction is the bending of light into the geometric shadow around obstacles or through narrow slits — a direct consequence of light's wave nature, distinct from reflection or refraction.

3. Q3 of 6Medium

   Fringe width β =

   Aλ/dBd/λDCλD/dDλD
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   Answer: λD/d

   Explanation: Fringe width in YDSE is β = λD/d — proportional to wavelength and screen distance D, inversely proportional to slit separation d; swapping D and d in the formula is a common error.

4. Q4 of 6Medium

   Assertion (A): The finite resolution of optical instruments such as telescopes and microscopes is limited by diffraction. Reason (R): The colours seen when a CD is viewed in white light are due to refraction of light.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is true but R is false

   Explanation: A is true: diffraction spreads the light from each point, which limits how close two objects can be and still be seen as separate. R is false: the colours seen on a CD are due to diffraction effects, not refraction.

5. Q5 of 6Hard

   YDSE: d=1mm, D=1m, λ=600nm. A slab t=10μm, n=1.5 covers one slit. Fringe shift:

   

   A0.5 mmB10 mmC5 mmD1 mm
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   Answer: 5 mm

   Explanation: Fringe shift Δ = (n−1)tD/d = (0.5)(10×10⁻⁶)(1)/(10⁻³) = 5×10⁻³ m = 5 mm — it's (n−1), the extra path beyond what air alone would give, not n itself.

6. Q6 of 6Hard

   Assertion (A): Direct sunlight viewed through a narrow slit does not give clear diffraction fringes. Reason (R): The Sun subtends an angle of about ½° at the eye.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true: clear fringes need a source that subtends a very small angle, such as a straight filament or a tiny reflection of the Sun. R is true and is exactly the reason: the Sun is too large a source, so the patterns from its different parts overlap and wash out.

Wave Optics notes: key concepts (Class 12 Physics)

Light treated as a wave: Huygens' principle and wavefronts, coherence and interference in Young's double-slit experiment with its fringe width, diffraction at a single slit and the width of the central maximum, how interference differs from diffraction, and polarisation with Malus's and Brewster's laws.

Huygens' principle

Every point on a wavefront acts as a source of secondary wavelets, and the new wavefront is their common tangent. It explains reflection and refraction geometrically.

Wavefront

The locus of all points vibrating in the same phase. The direction of propagation is always perpendicular to the wavefront.

Types of wavefront

A point source gives spherical wavefronts, a line source cylindrical ones, and a source at effectively infinite distance gives plane wavefronts.

Coherent sources

Two sources with a constant phase difference and identical frequency. Sustained interference is impossible without them, which is why two bulbs never interfere.

Interference of light

The superposition of coherent waves, producing alternating bright and dark fringes. Energy is redistributed, never created or destroyed.

Young's double slit experiment

The classic demonstration of the wave nature of light, producing equally spaced fringes of uniform brightness on a distant screen.

Conditions for bright and dark fringes

A bright fringe forms where the path difference is a whole number of wavelengths, a dark fringe where it is an odd number of half-wavelengths.

Fringe width

The spacing between consecutive bright fringes, λD/d. Widening the slit separation narrows the fringes; using a longer wavelength widens them.

Effect of immersing in a liquid

The wavelength shortens by a factor n inside the medium, so the whole fringe pattern contracts by the same factor.

Diffraction

The bending of light around obstacles or through narrow apertures, appreciable only when the aperture is comparable to the wavelength.

Single slit diffraction

Produces a broad central maximum flanked by much weaker secondary maxima, with minima where a sin θ equals a whole number of wavelengths.

Width of the central maximum

Twice as wide as any secondary maximum and inversely proportional to slit width — narrowing the slit spreads the pattern out.

Interference versus diffraction

Interference fringes are equally spaced and equally bright and come from two sources; diffraction bands are unequal in width and intensity and come from one aperture.

Polarisation

The restriction of the electric field's vibration to a single plane, possible only because light is a TRANSVERSE wave. Sound can never be polarised.

Malus's law

The intensity transmitted by an analyser varies as the square of the cosine of the angle between the transmission axes of polariser and analyser.

Brewster's law

At a particular angle of incidence the reflected light is completely plane polarised, and the tangent of that angle equals the refractive index.