CBSE 2023 · Region 2 · Set 1 · Q33 · 5 marks
(i)State Huygen's principle. With the help of a diagram, show how a plane wave is reflected from a surface. Hence verify the law of reflection.(ii)A concave mirror of focal length $\displaystyle 12$ cm forms a three times magnified virtual image of an object. Find the distance of the object from the mirror.(i)Draw a labelled ray diagram showing the image formation by a refracting telescope. Define its magnifying power. Write two limitations of a refracting telescope over a reflecting telescope.(ii)The focal lengths of the objective and the eye-piece of a compound microscope are $\displaystyle 1.0$ cm and $\displaystyle 2.5$ cm respectively. Find the tube length of the microscope for obtaining a magnification of $\displaystyle 300$ .
(i)
State Huygen's principle. With the help of a diagram, show how a plane wave is reflected from a surface. Hence verify the law of reflection.
(ii)
A concave mirror of focal length $\displaystyle 12$ cm forms a three times magnified virtual image of an object. Find the distance of the object from the mirror.
(i)
Draw a labelled ray diagram showing the image formation by a refracting telescope. Define its magnifying power. Write two limitations of a refracting telescope over a reflecting telescope.
(ii)
The focal lengths of the objective and the eye-piece of a compound microscope are $\displaystyle 1.0$ cm and $\displaystyle 2.5$ cm respectively. Find the tube length of the microscope for obtaining a magnification of $\displaystyle 300$ .
Marking-scheme solution
(a) 
(i) Huygen's 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 with the speed of the wave. These wavelets emanating from the wavefront are usually referred to as secondary wavelets, a common tangent to all these spheres gives the new position of the wavefront at a later time.
Verification of law of reflection
In \(\displaystyle \triangle \mathrm{AEC}\) & \(\displaystyle \triangle \mathrm{CBA}\)
\[\mathrm{EC}=\mathrm{AB} \quad(\mathrm{c} \times \mathrm{t} \text { each })
\]
\[\angle \mathrm{AEC}=\angle \mathrm{CBA} \quad\left(90^{0} \text { each }\right)
\]
By RHS congruency \(\displaystyle \Delta \mathrm{AEC} \cong \Delta \mathrm{CBA}\)
\[\Rightarrow \angle i=\angle r
\]
Verification of law of reflection
In \(\displaystyle \triangle \mathrm{AEC}\) & \(\displaystyle \triangle \mathrm{CBA}\)
\[\mathrm{EC}=\mathrm{AB} \quad(\mathrm{c} \times \mathrm{t} \text { each })
\]
\[\angle \mathrm{AEC}=\angle \mathrm{CBA} \quad\left(90^{0} \text { each }\right)
\]
By RHS congruency \(\displaystyle \Delta \mathrm{AEC} \cong \Delta \mathrm{CBA}\)
\[\Rightarrow \angle i=\angle r
\](ii) \(\displaystyle m=+3, \mathrm{f}=-12 \mathrm{~cm}, u=\) ?
\[m=-\frac{v}{u}=3 \Rightarrow v=-3 u
\]
using mirror formula
\[\frac{1}{v}+\frac{1}{u}=\frac{1}{\mathrm{f}}
\]
\[\frac{1}{-3 u}+\frac{1}{u}=\frac{1}{-12}
\]
\[u=-8 \mathrm{~cm}
\]
OR
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CBSE Class 12 Physics past-paper question from the 2023board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.