Question 188: Ray Optics

Question

The thickness at the centre of a plane convex lens is 3 mm and the diameter is 6 cm. If the speed of light in the material of the lens is 2 $$	imes$$ 108 ms$$-$$1. The focal length of the lens is ____________.

Accepted Answer

$${R^2} = {3^2} + {(R - 0.3)^2}$$
$$ \Rightarrow $$ $${R^2} = 9 + {R^2} + 0.09 - 2 	imes 0.3R$$
$$ \Rightarrow $$ $$2 	imes 0.3R = 9.09$$
$$ \Rightarrow $$ $$R = 15.15$$ cm
$$\mu  = {C \over V}$$ = $${{3 	imes {{10}^8}} \over {2 	imes {{10}^8}}}$$ = 1.5
$${1 \over f} = (1.5 - 1)\left( {{1 \over R}} ight)$$
$$f \simeq 30$$ cm

Answer

0.30 cm

Answer

30 cm

Answer

15 cm

Answer

1.5 cm

Ray Optics

The thickness at the centre of a plane convex lens is 3 mm and the diameter is 6 cm. If the speed of light in the material of the lens is 2 $\times 108 ms-$1. The focal length of the lens is ____________.

Choose the correct answer: