Question

A ray of light is incident on a glass prism at an angle of 24°. The refracting angles at the two surfaces are respectively 20° and 25°. If the angle of emergence is 30°, the total deviation produced by the prism is

• A. 40°
• B. 9°
• C. 20°
• D. 50°

Answer 1

The correct option is B 9°

As shown in the figure, i = angle of incidence, e = angle of emergence, δ = angle of deviation, A = angle of the prism, r1 = angle of refraction from the first surface AB, $r_2=$ angle of incidence on the second surface AC.
$i={24}^o$
$r_1={20}^o$
$r_2={25}^o$
$e={30}^o$
Now, $A=r_1+r_2$
$={20}^o+{25}^o={45}^o$
$\therefore \delta =i+e-A$
$={24}^o+{30}^o-{45}^o$
$=9^o$
Hence, the correct answer is option (b).