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Physics · 2023 · 2 marks
CBSE 2023 · Region 1 · Set 1 · Q21
(a) Using Huygens' principle, draw a ray diagram showing the propagation of a plane wave refracting at a plane surface separating two media. Also…
(a)
Using Huygens' principle, draw a ray diagram showing the propagation of a plane wave refracting at a plane surface separating two media. Also verify Snell's law of refraction.
OR
(b)
Why is a reflecting telescope preferred over a refracting telescope ? Justify your answer giving two reasons.
Marking-scheme solution
(a)
AB is incident wave front, incident at an angle i. Let \(\displaystyle \tau\) be time taken by the wave front to travel distance BC.
\(\displaystyle \mathrm{BC}=\mathrm{v}_{1} \tau\) where \(\displaystyle \mathrm{v}_{1}\) is speed of wave in medium 1.
To determine shape of refracted wave front, we draw a sphere of radius \(\displaystyle \mathrm{v}_{2} \tau\), where \(\displaystyle \mathrm{v}_{2}\) is speed of wave in medium 2.
CE represents a tangent drawn from point C on sphere, CE is the refracted wave front.
\[\begin{aligned}
\sin \mathrm{i} & =\frac{B C}{A C}=\frac{\mathrm{v}_{1} \tau}{A C} \\
\sin \mathrm{r} & =\frac{A E}{A C}=\frac{\mathrm{v}_{2} \tau}{A C}
\end{aligned}
\]
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