- State Huygen’s principle. Using this principle draw a diagram to show how a plane wave front incident at the interface of the two media gets refracted when it propagates from a rarer to a denser medium. Hence verifiy Snell’s law of refraction.
- When monochromatic light travels from a rarer to a denser medium, explain the following, giving reasons:
- Is the frequency of reflected and refracted light same as the frequency of incident light?
- Does the decrease in speed imply a reduction in the energy carried by light wave?
CBSE DELHI - SET 1 2013
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- Law of Reflection : Let XY be a reflecting surface at which a wavefront is being incident obliquely. Let v be the speed of the wavefront and at time t = 0, the wavefront touches the surface XY at A. After time t, the point B of wavefront reaches the point B' of the surface.
First of all the secondary wavelet starts from point A and traverses distance AA' (=vt) in first medium in time t. In the same time t, the point B of wavefront, after travelling a distance BB', reaches point B' (of the surface), from where the secondary wavelet now starts. Now taking A as centre we draw a spherical arc of radius AA' (= vt) and draw tangent A'B' on this arc from point B'. As the incident wavefront AB advances, the secondary wavelets starting from points between A and B', one after the other and will touch A'B' simultaneously. According to Huygen’s principle wavefront A'B' represents the new position of AB, i.e., A'B' is the reflected wavefront corresponding to incident wavefront AB.
Now in right-angled triangles ABB' and AA' B'
$\angle\text{ABB}' = \angle\text{AA'} B' \text{(both are equal to } 90^{o})$
side BB' = side AA' (both are equal to $v\text{t}$)
and side AB' is common
i.e., both triangles are congruent.
$\therefore\angle\text{BAB}' = \angle\text{AB}'\text{A}'$
i.e., incident wavefront AB and reflected wavefront A'B' make equal angles with the reflecting surface XY. As the rays are always normal to the wavefront, therefore the incident and the reflected rays make equal angles with the normal drawn on the surface XY, i.e.,
angle of incidence i = angle of reflection r
This is the second law of reflection.
Since AB, A'B' and XY are all in the plane of paper, therefore the perpendiculars dropped on them will also be in the same plane. Therefore we conclude that the incident ray, reflected ray and the normal at the point of incidence, all lie in the same plane. This is the first law of reflection. Thus Huygen’s principle explains both the laws of reflection.
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- If the radiation of certain frequency interact with the atoms/molecules of the matter, they start to vibrate with the same frequency under forced oscillations. Thus, the frequency of the scattered light (Under reflection and refraction) equals to the frequency of incident radiation.
- No, energy carried by the wave depends on the amplitude of the wave, but not on the speed of the wave.
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