Question: Q. 2. (i) Define the term wavefront. State Huygen’s principle.
(ii) Consider a plane wavefront incident on a thin convex lens. Draw a proper diagram to show how the incident wavefront traverses through the lens and after refraction focuses on the focal point of the lens, giving the shape of the emergent wave front.
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Solution:
Ans. (i) Try yourself similar to Q. 1, Very Short Answer Type Questions.
1
Huygens’ Principle : Each point of the wavefront is the source of a secondary disturbance and the wavelets emanating from these points spread out in all directions. These travel with the same velocity as that of the original wavefront. $1 / 2$
(ii) The shape and position of the wavefront, after time ’ $t$ ‘, is given by the tangential envelope to the secondary wayelets.
$$ 1 / 2+1 / 2 $$
[CBSE Marking Scheme 2017]
Commonly Made Errors
- (i) Many students couldn’t define wavefront.
(ii) Many students couldn’t write all points of Huygens’ principle.
[AI Q. 3. Define wavefront. Use Huygen’s principle to verify the laws of refraction.
U[ [OD I 2018]
Ans. Try yourself, Similar to Q. 1, Very Short Answer Type Questions
For Verification, Please see the Revision Notes, ‘Refraction of plane wave by Huygens’ Principle.’ 2
[AI Q. 4. (i) Compare Maxwell’s electromagnetic theory with Huygens wave theory of light.
(ii) Define incident angle of a light wave.
Ans. (i) Both theories treat light as a wave in nature. However by electromagnetic theory light does not need any medium to propagate while by Huygen’s, wave theory a medium is must. That is why he assumed a hypothetical medium ’ether’ through which light wave travels in vacuum.
2
(ii) Angle between incident wavefront with the interface is called the incident angle of light wave. 1
Long Answer Type Questions
AI Q. 1. (i) Define a wavefront. How is it different from a ray?
(ii) Depict the shape of a wavefront in each of the following cases :
(a) Light diverging from point source.
(b) Light emerging out of a convex lens when a point source is placed at its focus.
(c) Using Huygens construction of secondary wavelets, draw a diagram showing the passage of a plane wavefront from a rarer into a denser medium.
[OD I, II, III 2015]
Ans. (i) Try yourself similar to Q. 1, Very Short Answer Type Question.
Difference from a ray :
- The ray, at each point of a wavefront, is normal to the wavefront at that point.
- The ray indicates the direction of propagation of wave while the wavefront is the surface of constant phase.
(any one) 1
The shape of the wavefront, in the three cases, are as shown :
(ii) (a)
(b)
[CBSE Marking Scheme 2015]
