# Nature & Propagation of Light – Short Questions Answer | Physics Class 12

## Solved Question | Physics Short Questions with Answer – Nature and Propagation of Light Class: 12

1. What is a wave front?
Ans. During the propagation of the wave, all the particles of the medium which are located at the same distance from the source receive the disturbance simultaneously and vibrate in the same phase. Thus, a wave front of light at any instance is the locus of all particles of the medium vibrating in the same phase at that time. The shape of wave front depends on the nature of source and the disturbance of the wave front from the source.

2. What are the advantages of Foucault’s method over Fizeau’s method of measuring speed of light?
Ans. The advantages of Foucault’s method over Fizeau’s method of measuring speed of light are as follows;

• Foucault’s method can be done in laboratory but Fizeau’s method cannot be used in laboratory since it requires very large distance in the range of 10-12 km.
• Foucault’s method can be used to measure velocity of light in various media including gases but Fizeau’s method can be used to measure velocity of light in air of vacuum only

3. Differentiate between a plane wave front and a spherical wave front.
Ans. Wave front is defined as the locus of all the particle of a medium vibrating in the same phase at a given instant. The shape of the wave front depends upon the shape of the source of disturbance.

• Spherical wave front: If the source of disturbance is a point source O, then the wave front is spherical. A point source of light emits waves which spread outward in all directions. After time t, the wave or disturbance will travel a distance vt from the point source in all directions, where v is the speed of light in the medium.
• Plane wave front: At a very large distance from the point source, a small part of the spherical wave front appears place and this is called plane wave front. That is, a spherical wave front appears as plane wave front after travelling a large distance from the point source.

4. Does the energy of light change when it travels from one medium to another? Give reason.
Ans. No. When light passes from one medium to another, its speed changes and its wavelength changes but its frequency doesn’t change and its amplitude doesn’t change. The energy carried by a wave depends on its amplitude. That’s why, the energy of light doesn’t change when it travels from a medium to another.

5. Which parameter of light does not change on refraction?
When light undergoes refraction, the velocity and wavelength of light get changed but the frequency of light doesn’t get changed. This can be explained as:

Refraction is the result of interaction of incident light with the atoms (which may be viewed as oscillators). The frequency of light emitted by a charged oscillator is equal to its own frequency of oscillation. So, the frequency of refracted light is same as that of incident light.

6. State Huygen’s principle. Does it apply to sound waves in air?
Ans. Huygen’s principle states that:

• Each point on the primary (given) wave front acts as a source of secondary wavelets, sending out disturbance in all directions in a similar manner as the original source of light does.
• The new position of the wave front at any instant is given by the forward envelope of the secondary wavelets at that instant.
• Yes, Huygen’s principle can be applied to sound waves in air. If the present position and nature of wave front in air is known, the position and the nature of wave front after some time can be predicted by using Huygen’s principle.

7. What is Huygen’s principle?
Ans. Each point on the primary (given) wave front acts as a source of secondary wavelets, sending out disturbance in all directions in a similar manner as the original source of light does. The new position of the wave front at any instant is given by the forward envelope of the secondary wavelets at that instant. Corpuscular theory of light explains only the reflection, refraction of light but it can’t explain interference, diffraction and polarization of light. These phenomena can be explained by wave theory of light which is based on a geometrical construction called huygen’s principle.

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