Question 1

The phenomenon of interference of light is based on the principle of:

Accepted Answer

Superposition of waves. Interference results from the superposition of two (or more) coherent light waves.

Question 2

Two sources of light are said to be coherent if they have:

Accepted Answer

The same frequency and a constant phase difference. Coherent sources emit waves of the same frequency with a constant (time-independent) phase difference.

Question 3

For constructive interference at a point, the path difference between the two waves must be:

Accepted Answer

n\lambda (n = 0, 1, 2, …). Constructive interference (bright fringe) occurs when the path difference is an integral multiple of the wavelength, nλ.

Question 4

For destructive interference, the path difference between the two waves must be:

Accepted Answer

(2n-1)\dfrac{\lambda}{2}. Destructive interference (dark fringe) occurs when the path difference is an odd multiple of λ/2.

Question 5

In Young's double-slit experiment, the fringe width β is given by:

Accepted Answer

\beta = \dfrac{\lambda D}{d}. Fringe width β = λD/d, where D is the slit-to-screen distance and d the slit separation.

Question 6

In Young's double-slit experiment, the central point of the screen (equidistant from both slits) is:

Accepted Answer

A bright fringe (zero path difference). At the centre the path difference is zero, giving constructive interference — the central bright fringe.

Question 7

A path difference of one full wavelength (λ) between two interfering waves corresponds to a phase difference of:

Accepted Answer

2π. Phase difference = (2π/λ) × path difference = (2π/λ) × λ = 2π.

Question 8

Diffraction of light is the phenomenon of:

Accepted Answer

Bending of light around the edges of an obstacle or aperture. Diffraction is the bending/spreading of light as it passes the edges of an obstacle or a narrow aperture.

Question 9

Diffraction effects become significant when the size of the aperture or obstacle is:

Accepted Answer

Comparable to the wavelength of light. Appreciable diffraction occurs only when the obstacle/aperture is of the order of the wavelength of light.

Question 10

For a single slit of width a, the directions of the minima in the diffraction pattern are given by:

Accepted Answer

a\sin	heta = n\lambda (n = 1, 2, …). Single-slit minima occur where a sinθ = nλ, n = 1, 2, 3, …

Question 11

In a single-slit diffraction pattern, the central maximum is:

Accepted Answer

The brightest and the widest. The central maximum carries most of the energy — it is the brightest and about twice as wide as the secondary maxima.

Question 12

Diffraction of light provides direct evidence for the _____ nature of light.

Accepted Answer

Wave. Diffraction is a characteristic wave phenomenon, so it demonstrates the wave nature of light.

Question 13

In a single-slit diffraction pattern, the point directly opposite the centre of the slit is:

Accepted Answer

The centre of the bright central maximum. At θ = 0 all secondary wavelets arrive in phase, giving the central bright maximum.

Question 14

A thin sheet (μ = 1.5, thickness 10 μm) covers one slit. The number of fringes the pattern shifts (λ = 500 nm) is:

Accepted Answer

10. Shift (in fringes) = (μ − 1)t/λ = 0.5 × 10 × 10⁻⁶/500 × 10⁻⁹ = 10.

Question 15

Light of wavelengths 6000 Å and 4800 Å illuminate a double slit. The 4th bright fringe of 6000 Å coincides with the nth bright fringe of 4800 Å, where n =

Accepted Answer

5. n₁λ₁ = n₂λ₂ ⇒ 4 × 6000 = n × 4800 ⇒ n = 5.

Question 16

For two equal-intensity beams (I_max = 4I₀), the intensity where the path difference is λ/3 is:

Accepted Answer

I₀. φ = 2π/3; I = 4I₀cos²(π/3) = 4I₀(1/4) = I₀.

Question 17

A mica sheet (μ = 1.6, thickness 2 μm) is placed over one slit (λ = 500 nm). The fringe shift is:

Accepted Answer

2.4 fringes. Shift = (μ − 1)t/λ = 0.6 × 2 × 10⁻⁶/500 × 10⁻⁹ = 2.4 fringes.

Question 18

The Fresnel distance for a beam of width 4 mm and wavelength 400 nm is:

Accepted Answer

40 m. z_F = a²/λ = (4 × 10⁻³)²/(400 × 10⁻⁹) = 16 × 10⁻⁶/4 × 10⁻⁷ = 40 m.

Question 19

The Fresnel distance for an aperture of 3 mm and light of 500 nm is:

Accepted Answer

18 m. z_F = a²/λ = (3 × 10⁻³)²/(500 × 10⁻⁹) = 9 × 10⁻⁶/5 × 10⁻⁷ = 18 m.

Question 20

The position of the 2nd minimum of a single-slit pattern (a = 0.2 mm, λ = 600 nm, D = 1 m) is:

Accepted Answer

6 mm. y = nλD/a = 2 × 600 × 10⁻⁹ × 1/0.2 × 10⁻³ = 6 mm.

Interference and Diffraction of Light — IISER Physics MCQs with Solutions

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Sample questions with solutions