# 1. Write down lens maker’s formula for a convex lens.

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$\frac{1}{f}=\Big(\frac{n_2}{n_i}-t\Big)\Big(\frac{1}{R_1}-\frac{1}{R_2}\Big)$ is lens maker’s formula.

1. Write down lens maker’s formula for a convex lens.

2. “If a convex lens is immersed in water its converging power decrease. Do you agree with it? Justify your answer.

3. A convex lens of refractive index n2 is placed in different media. Explain optic behavior in each. If n1 is refractive index of surrounding media.

• in medium with n2 >n1
• in a medium with n2 < n1
• in a medium n1 = n2

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1. For convex lens R1 = +ve and R2 = -ve

$\frac{1}{f}=\Big(\frac{n_2}{n_1}-1\Big)\Big(\frac{1}{R_1}+\frac{1}{R_2}\Big)$

2.  Yes.

P = $\frac{1}{f}$ = $\Big(\frac{n_2}{n_1}-1\Big)\Big(\frac{1}{R_1}+\frac{1}{R_2}\Big)$

From the above equation it is clear that, Pα $\frac{n_2}{n_1}$

lense in air, $\frac{n_{glass}}{n_{air}}$ = $\frac{1.5}{1}$ = 1.5

lense in water, $\frac{n_{glass}}{n_{water}}$ = $\frac{1.5}{1.33}$ = 1.12

In water,$\frac{n_2}{n_1}$  is less. Hence power decreases.

3. P = +ve, converging

P = -ve, diverging

P = 0, Plane glass