Question: Q. 1. (i) Define wavefront. Use Huygens’ principle to verify the laws of refraction.
(ii) In a single slit diffraction experiment, the width of the slit is made double of the original width. How does this affect the size and intensity of the central diffraction band ? Explain.
(iii) When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of obstacle. Explain why?
R [Delhi, OD 2018]
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Solution:
Ans. (i) Try yourself, Similar to Q. 1, Very Short Answer Type Questions.
$1 / 2$
For Verification : Please see the Revision Notes, ‘Refraction of plane wave by Huygens Principle.’ 1/2
(ii) Angular width of the central fringe $=\frac{2 D \lambda}{a}$ where, $a$ is the width of slit. Hence if the width of the slitis made double, then fringe width becomes half.
The intensity of central fringe becomes four tipres. This is because area of central diffraction band would become (1/4)th.
$1 / 2+1 / 2=1$
(iii) This is because of the diffraction. The diffraction of waves from the edges of circutar obstacle interfere constructively at the centre of the shadow. It results the formation of bright spot at the centre of the obstacle.
$1 / 2+1 / 2=1$
Note for the Students
- (ii) & (iii) parts of this question are from Topic-2 Kindly refer its revision notes.
Commonly Made Errors
- Some of the errors made by the students were :
(1) Correct diagrams were not drawn.
(2) The arrows were not marked.
(3) Angle ’ $i$ ’ and ’ $r$ ’ were not marked correctly.
(4) Wavefronts were not drawn/marked.
