CBSE Class 12 Physics 2023 Outside Delhi Set 1 Paper

Question : 33 of 35

Marks: +1, -0

(a) (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

12 {\cm}

forms a three times magnified virtual image of an object. Find the distance of the object from the mirror.
OR
(b) (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 eyepiece of a compound microscope are

1.0 {\cm}

and

2.5 {\cm}

respectively. Find the tube length of the microscope for obtaining a magnification of 300.

Solution:

(a) (i) Huygens' principle:
Each point on primary wavefront act as a source of secondary wavelets sending out disturbances in all directions in the similar manner as the original source of light does.
Verification of laws of reflection:
Refer the answer of Q26 (OR) of CBSE 12 DELHI, Set-1.

\;\text " (ii) "\;

\;\;\text " Magnification "\;\;=-\;{v}/{u}

\;\;\text " Or, "\;\;3\;=-\;{v}/{u}

\; ∴ \;v\;=-3 u

Applying mirror formula,

\;\;{1}/{v}+\;{1}/{u}=\;{1}/{f}

\;\;\text " Or, "\; \;\; -\;{1}/{3 u}+\;{1}/{u}=-\;{1}/{12}

\;\;\text " Or, "\; \;\; \;{2}/{3} u=-\;{1}/{12}

\; ∴ \;\; u=-8 {\cm}

(b) (i) Refer the answer of Q33 (OR) (i) of CBSE 12 DELHI, Set-1.
Limitation of refracting telescope:
● Refracting telescope suffer from chromatic aberration.
● Large size Refracting telescopes are too heavy and too costly due to large size of lenses
(ii) Given,

f_0\;=1 {\cm}

F_E\;=2.5 {\cm}

m\;=300

\;\text " Since, "\;\;m\;=\;{ {\L} × {\D}}/{f_0 × f_{ {\E}}}

\;\text " or, "\;\;300\;=\;{ {\L} × 25}/{1 × 2.5}

\; ∴ \;L\;=30 {\cm}

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