Sarthaks Test
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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

1 Answer

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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

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