CBSE Class 12 Physics 2015 Delhi Set 1 Paper

Question : 9 of 26

Marks: +1, -0

Use the mirror equation to show that an object placed between

f

and

2 f

of a concave mirror produces a real image beyond

2 f

.
OR
Find an expression for intensity of transmitted light when a polaroid sheet is rotated between two crossed polaroids. In which position of the polaroid sheet will the transmitted intensity be maximum?

Solution:

Derivation of the required condition

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

For concave mirror

f< 0

and

u< 0

As object lies between

f

and

2 f

(i) At

u=-f

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

⇒ \;\; v=∝

\;\; u=-2 f

⇒ \;\; \;{1}/{v}=-\;{1}/{f}+\;{1}/{2 f}=-\;{1}/{2 f}

⇒ \;\; v=-2 f

\Rightarrow

Hence, image distance

v ≥-2 f

Since,

v

is negative therefore the image is real.

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