Q: The speed of light in vacuum is approximately:

3 \times 10^8\text{ m/s}. The accepted value is 2.998 \times 10^8\text{ m/s}, usually rounded to 3 \times 10^8\text{ m/s}.

Q: The refractive index of glass is 1.5. The speed of light in the glass is:

2 \times 10^8\text{ m/s}. v = \dfrac{c}{\mu} = \dfrac{3 \times 10^8}{1.5} = 2 \times 10^8\text{ m/s}.

Q: Light of wavelength 600\text{ nm} enters a medium of refractive index 1.2. Its wavelength in the medium is:

500\text{ nm}. \lambda_{medium} = \dfrac{600}{1.2} = 500\text{ nm}.

Q: Light of wavelength 400\text{ nm} enters a medium of refractive index 2. Its wavelength there becomes:

200\text{ nm}. \lambda_{medium} = \dfrac{400}{2} = 200\text{ nm}.

Question 1

According to Huygens' principle, every point on a wavefront acts as:

Accepted Answer

A source of secondary spherical wavelets. The new wavefront at a later instant is the forward envelope of all these secondary wavelets — the construction that explains reflection and refraction.

Question 2

A wavefront is defined as the locus of all points which are:

Accepted Answer

In the same phase of vibration. Every point on a given wavefront has been reached by the disturbance at the same instant, so they all vibrate in step.

Question 3

The wavefront produced by a point source of light at a finite distance is:

Accepted Answer

Spherical. The disturbance spreads out equally in all directions, so surfaces of constant phase are concentric spheres. Far from the source a small patch approximates a plane wavefront.

Question 4

Light waves are:

Accepted Answer

Transverse electromagnetic waves. The electric and magnetic fields oscillate perpendicular to the direction of propagation, which is why light can be polarised.

Question 5

The speed of light in vacuum is approximately:

Accepted Answer

3 	imes 10^8	ext{ m/s}. The accepted value is 2.998 	imes 10^8	ext{ m/s}, usually rounded to 3 	imes 10^8	ext{ m/s}.

Question 6

The refractive index of glass is 1.5. The speed of light in the glass is:

Accepted Answer

2 	imes 10^8	ext{ m/s}. v = \dfrac{c}{\mu} = \dfrac{3 	imes 10^8}{1.5} = 2 	imes 10^8	ext{ m/s}.

Question 7

Light of wavelength 600	ext{ nm} enters a medium of refractive index 1.2. Its wavelength in the medium is:

Accepted Answer

500	ext{ nm}. \lambda_{medium} = \dfrac{600}{1.2} = 500	ext{ nm}.

Question 8

Light of wavelength 400	ext{ nm} enters a medium of refractive index 2. Its wavelength there becomes:

Accepted Answer

200	ext{ nm}. \lambda_{medium} = \dfrac{400}{2} = 200	ext{ nm}.

Question 9

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

Accepted Answer

The same frequency with a constant phase difference. Only a steady phase relationship keeps the maxima and minima fixed long enough to be observed.

Question 10

Constructive interference of light occurs when the path difference is:

Accepted Answer

An integral multiple of the wavelength. A path difference of n\lambda brings the two waves into step, giving a bright fringe.

Question 11

A dark fringe is formed when the path difference between two interfering waves is:

Accepted Answer

An odd multiple of half the wavelength. A path difference of (2n-1)\dfrac{\lambda}{2} puts the waves exactly out of step, so they cancel.

Question 12

At a point where the path difference between two interfering light waves is \dfrac{\lambda}{2}, the fringe observed is:

Accepted Answer

Dark, since the waves are exactly out of phase. A path difference of \dfrac{\lambda}{2} corresponds to a phase difference of \pi, producing complete destructive interference.

Question 13

At a point the path difference between two interfering waves is 3\lambda. The fringe there is:

Accepted Answer

Bright, since the path difference is an integral multiple of \lambda. An integral number of wavelengths brings the waves back into step, giving constructive interference.

Question 14

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

Accepted Answer

\beta = \dfrac{\lambda D}{d}. Here D is the slit-to-screen distance and d the slit separation; widening the slits narrows the fringes.

Question 15

The central fringe in Young's double slit experiment is:

Accepted Answer

Bright, since the path difference there is zero. The centre of the screen is equidistant from both slits, so the waves arrive exactly in step.

Question 16

Increasing the separation between the two slits, with everything else unchanged, causes the fringe width to:

Accepted Answer

Decrease. \beta = \dfrac{\lambda D}{d} varies inversely with d, so wider slit separation crowds the fringes together.

Question 17

In Young's double slit experiment, doubling the distance between the slits and the screen makes the fringe width:

Accepted Answer

Twice as large. \beta \propto D, so doubling the screen distance doubles the fringe width.

Question 18

Diffraction of light is the phenomenon of:

Accepted Answer

Bending of light around the edges of an obstacle. Light spreads into the geometrical shadow when it encounters an obstacle or aperture comparable in size to its wavelength.

Question 19

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

Accepted Answer

Comparable to the wavelength of light. This is why sound diffracts noticeably around a doorway while light, with its far shorter wavelength, casts sharp shadows.

Question 20

A diffraction grating consists of:

Accepted Answer

A large number of equally spaced parallel slits. Thousands of slits per centimetre produce very sharp, widely separated maxima, which makes a grating an excellent spectroscopic tool.

Wave Optics — MH-CET Physics MCQs with Solutions

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Sample questions with solutions