Question 1

A wavefront is defined as:

Accepted Answer

The locus of all points of a medium vibrating in the same phase. A wavefront is the continuous locus of all points that are in the same phase of vibration at a given instant.

Question 2

In his original wave theory, Huygens assumed light to be a:

Accepted Answer

Longitudinal wave propagating through an all-pervading medium (ether). Huygens treated light as a longitudinal mechanical wave travelling through a hypothetical medium called the luminiferous ether.

Question 3

Huygens' principle states that every point on a wavefront acts as a:

Accepted Answer

Source of secondary wavelets, and the new wavefront is their forward envelope. Each point on a wavefront is a source of secondary spherical wavelets; the tangential surface (forward envelope) of these wavelets gives the new wavefront.

Question 4

The secondary wavelets in Huygens' construction travel with:

Accepted Answer

The speed of light in that medium. Secondary wavelets spread out in all forward directions with the speed of light in the medium.

Question 5

The wavefront produced by a point source of light in an isotropic medium is:

Accepted Answer

Spherical. A point source sends out wavelets equally in all directions, giving a spherical wavefront.

Question 6

A linear (slit) source of light produces a wavefront that is:

Accepted Answer

Cylindrical. A line source produces cylindrical wavefronts.

Question 7

Light from a very distant source (e.g. a star) reaching the Earth has a wavefront that is essentially:

Accepted Answer

Plane. At a very large distance a small portion of a spherical wavefront is effectively plane, so starlight arrives as plane wavefronts.

Question 8

The direction of propagation of light (a ray) is always:

Accepted Answer

Perpendicular to the wavefront. A ray points in the direction of energy flow, which is always perpendicular (normal) to the wavefront.

Question 9

Two points lying on the same wavefront have a phase difference of:

Accepted Answer

Zero. All points on a wavefront vibrate in the same phase, so the phase difference between any two of them is zero.

Question 10

In Huygens' construction, the backward-moving secondary wavelet is neglected because:

Accepted Answer

The intensity of the secondary wavelets is zero in the backward direction (obliquity factor). The amplitude of a secondary wavelet varies with angle (obliquity factor) and is zero in the backward direction, so only the forward envelope is taken.

Question 11

Unlike the corpuscular theory, Huygens' wave theory correctly predicted that the speed of light in a denser medium is:

Accepted Answer

Less than that in a rarer medium. Huygens' construction of refraction gives v_denser < v_rarer, which is experimentally correct (opposite to Newton's prediction).

Question 12

Applying Huygens' principle to refraction gives Snell's law in the form:

Accepted Answer

\dfrac{\sin i}{\sin r} = \dfrac{v_1}{v_2} = \dfrac{n_2}{n_1}. Huygens' construction gives sin i / sin r = v₁/v₂ = n₂/n₁, i.e. Snell's law.

Question 13

When light passes from a rarer to a denser medium, which quantity remains unchanged?

Accepted Answer

Frequency. Frequency is fixed by the source and does not change; speed and wavelength both decrease in the denser medium.

Question 14

When light enters a denser medium, its wavelength:

Accepted Answer

Decreases. Since v = νλ and frequency is constant, a decrease in speed means the wavelength decreases (λ_medium = λ_vacuum/n).

Question 15

A plane wavefront is incident on a converging (convex) lens. The emergent wavefront is:

Accepted Answer

Spherical, converging towards the focus. A convex lens converts an incident plane wavefront into a spherical wavefront that converges to the focal point.

Question 16

A plane wavefront falls normally on a concave mirror. The reflected wavefront is:

Accepted Answer

Spherical and converging. A concave mirror converges a plane wavefront into a converging spherical wavefront (towards the focus).

Question 17

A point object is placed at the focus of a convex lens. The wavefront emerging from the lens is:

Accepted Answer

Plane. A diverging spherical wavefront from the focus is rendered plane (parallel) after passing through the convex lens.

Question 18

When a plane wavefront passes through a thin prism, the emergent wavefront is:

Accepted Answer

A plane wavefront, but tilted (its normal deviated). Different parts of the wavefront travel different thicknesses of glass and are delayed unequally, so the emergent wavefront is still plane but tilted (the beam is deviated).

Question 19

The wavelength of light in vacuum is 600 nm. In a medium of refractive index 1.5, its wavelength is:

Accepted Answer

400 nm. λ_medium = λ_vacuum / n = 600 / 1.5 = 400 nm.

Question 20

The speed of light in a medium of refractive index 1.5 is (c = 3 × 10⁸ m/s):

Accepted Answer

2 	imes 10^{8} m/s. v = c/n = (3 × 10⁸)/1.5 = 2 × 10⁸ m/s.

Huygens' Wave Theory — IISER Physics MCQs with Solutions

Questions with solutions