Question 332: Ray Optics

Question

A point object in air is in front of the curved
surface of a plano-convex lens. The radius of
curvature of the curved surface is 30 cm and
the refractive index of the lens material is 1.5,
then the focal length of the lens (in cm)
is ______.

Accepted Answer

Lens-maker formula
$${1 \over f} = \left( {\mu  - 1} ight)\left( {{1 \over {{R_1}}} - {1 \over {{R_2}}}} ight)$$
for plano-convex lens
$${{R_1}}$$ = $$\infty $$ and R
2
= -R
$$ 	herefore $$ f = $${R \over {\mu  - 1}}$$ = $${{30} \over {1.5 - 1}}$$ = 60 cm

Ray Optics

A point object in air is in front of the curved surface of a plano-convex lens. The radius of curvature of the curved surface is 30 cm and the refractive index of the lens material is 1.5, then the focal length of the lens (in cm) is ______.

Choose the correct answer: