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  • (a) Define a wavefront. Using Huygens’ principle, verify the laws of reflection at a plane surface. (b) In a single slit diffraction experiment, the width of the slit is made double the original width. How

(a) Define a wavefront. Using Huygens’ principle, verify the laws of reflection at a plane surface.
(b) In a single slit diffraction experiment, the width of the slit is made double the original width. How does this affect the size and intensity of the central diffraction band? Explain.
(c) 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 the obstacle. Explain why.

 

 
 
 
 
 

Answers (1)

a) Wavefront: Locus of all points which are vibrating in the same phase or it is a surface of constant phase

Let 'v' is the speed of the wave in the medium
\tau = time taken by wavefront to advance from point B to point C
Then the distance,
BC = V \tau
Let CE = reflected wavefront
then AE = BC
Consider triangle AEC and triangle ABC. By from figure they are congruent So angle BAC and EAC are equal
implies angle i = angle r
This is the law of reflection
b) we know, the size of central maxima is
        \beta = \frac{\lambda D}{d}
where,
\lambda = wave length of light
D = Distance between slit and screen
d = width of slit
Given that width of the slit is made double

\beta \alpha \frac{1}{d}
\beta\: become\; \frac{\beta }{2}
Therefore the size of central maxima reduces to half
The intensity of the central diffraction band is increased. This is because when the size of central diffraction reduces to half the area of central diffraction pattern become one-fourth so the intensity of the central diffraction band will become four times.
c ) When a tiny circular obstacle is placed in the path of light from a distance source, a bright spot is shown at the centre of the obstacle. This is because of diffraction of light in case of diffraction the constructive interference occurs so central maxima are always bright.

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Safeer PP

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