Question

A 5 W source emits monochromatic light of wavelength ${5000}^0$. when placed 0.5 m away. it liiberates photoelectrons from a photosensitive metallic surface, When the source is moved to a distance of 1.0 m, the number of photoelectrons liberated will be reduced by a factor of :

• A. 4
• B. 8
• C. 16
• D. 2

Answer 1

The correct option is A 4

The intensity of light is inversely proportional to the square of the distance

$1\alpha \frac{1}{r^2}$

$\frac{I_1}{I_2}=\frac{{\left(r_1\right)}^2}{{\left(r_2\right)}^2}$

Given, $r_1=0.5m,r_2=1.0m$

$\therefore$ $\frac{I_2}{I_1}=\frac{{\left(0.5\right)}^2}{{\left(0\right)}^2}=\frac{1}{4}$

Now, Since no. of photoelectric emitted per second is directly proportional intensity, So no. of electrons would decrease by factor $4$.