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

# A Photographic Plate Placed a Distance of 5 Cm from a Weak Point Source is Exposed for 3 S. If the Plate is Kept at a Distance of 10 Cm from the Source, the Time Needed for the Same Exposure is - Physics

MCQ
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A photographic plate placed a distance of 5 cm from a weak point source is exposed for 3 s. If the plate is kept at a distance of 10 cm from the source, the time needed for the same exposure is _____________ .

• 3s

• 12s

• 24s

• 48s

#### Solution

12s

Here,

$d_1 = 5 cm = 0 . 05 m$

$d_2 = 10 cm = 0 . 1 m$

$t_1 = 3 s$

$t_2 = ?$

Let the actual incident illuminance be $E_o$

Let the iluminance at 3cm distance be $E_{d1}$

Let the iluminance at 10cm distance be $E_{d2}$

$\cos\theta = 1$

$E_{d1} = \frac{E_o}{{d_1}^2}$

Now,

$t_1 \alpha\frac{1}{E_{d1}}$

$\Rightarrow t_1 = \frac{k}{E_{d1}}$

$\Rightarrow t_1 = \frac{k 5^2}{E_o}$

$\Rightarrow \frac{k}{E_o} = \frac{3}{25}$

Similarly,

$\Rightarrow t_2 = \frac{k {10}^2}{E_o}$

$\Rightarrow t_2 = \frac{3}{25} \times {10}^2 = 12 s$

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 8 | Page 454
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