CBSE Class 12 · 2006 · Geometrical OpticsShort Answer

A convex lens made up of glass of refractive index

1.5

is dipped, in turn, in
(i) medium

A

of refractive index

1.65

,
(ii) medium

B

of refractive index

1.33

. Explain, giving reasons, whether it will behave as a converging lens or a diverging lens in each of these two media.

Answer: In medium $A$ ( $μ_{A} = 1.65 > 1.5$ ): diverging. In medium $B$ ( $μ_{B} = 1.33 < 1.5$ ): converging.

Solution

\frac{1}{f} = (\frac{μ _{g}}{μ _{m}} - 1) (\frac{1}{R _{1}} - \frac{1}{R _{2}})

(i) In

A

μ_{g} < μ_{m}

, so

(\frac{μ _{g}}{μ _{m}} - 1) < 0

: the sign of

1/ f

reverses and the convex lens acts as a \emph{diverging} lens.
(ii) In

B

μ_{g} > μ_{m}

, so

(\frac{μ _{g}}{μ _{m}} - 1) > 0

: the lens remains \emph{converging} (with a larger focal length than in air).

CBSE Class 12 Board · 2006 · Subjective