Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12

# A Photographic Plate is Placed Directly in Front of a Small Diffused Source in the Shape of a Circular Disc. It Takes 12s to Get a Good Exposure. - Physics

MCQ
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A photographic plate is placed directly in front of a small diffused source in the shape of a circular disc. It takes 12s to get a good exposure. If the source is rotated by 60° about one of its diameter, the time needed to get the same exposure will be ___________ .

• 6s

• 12s

• 24s

• 48s

#### Solution

24s

Here,

$t_1 = 12 s$

$\theta_1 = 0^0$

$\theta_2 = {60}^0$

$t_2 = ?$

Let the distance be r.

Let the incident luminosity be $E_o.$

We have,

$E_\theta1 = \frac{E_o {\cos\theta}_1}{r^2}$

$t_1 \alpha \frac{1}{E_\theta1}$

$\Rightarrow t_1 = \frac{r^2 k}{E_o {\cos\theta}_1}$

$\Rightarrow 12 = \frac{r^2 k}{E_o \cos0}$

$\Rightarrow \frac{r^2 k}{E_o} = 12$

Similarly,

$t_2 = \frac{r^2 k}{E_o {\cos\theta}_2} = \frac{12}{\cos( {60}^0 )}$

$\Rightarrow t_2 = 12 \times 2 = 24 s$

Concept: Light Process and Photometry
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#### APPEARS IN

HC Verma Class 11, Class 12 Concepts of Physics Vol. 1
Chapter 22 Photometry
MCQ | Q 9 | Page 454
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