Lens Maker's Formula

Q. A hollow double concave lens is made of very thin transparent material. It can be filled with air or either of two liquids $L_1$ and $L_2$ having refractive indices $n_1$ and $n_2$ respectively $(n_2>n_1>1)$. The lens will diverge a parallel beam of light if it is filled with

1. Air and placed in air
2. Air and immersed in $L_1$
3. $L_1$ and immersed in $L_2$
4. $L_2$ and immersed in $L_1$

Q. A double convex lens made of a material of refractive index 1.5 and having a focal length of 10 cm is immersed in liquid of refractive index 3.0. The lens will behave as

1. Diverging lens of focal length 10 cm
2. Diverging lens of focal length $\frac{10}{3}cm$
3. Converging lens of focal length $\frac{10}{3}cm$
4. Converging lens of focal length 30 cm

Q. Diameter of a plano–convex lens is 6 cm and thickness at the centre is 3 mm. If the speed of light in the material of the lens is 2 10⁸ m/sec, the focal length of the lens is

1. 15 cm
2. 20 cm
3. 30 cm
4. 10 cm

Q. Assumimng new cartesian sign convention, choose the correct option for the Lens shown in figure:

1. $R_1>0$
   $R_2>0$
2. $R_1<0$
   $R_2<0$
3. $R_1>0$
   $R_2<0$
4. $R_1<0$
   $R_2>0$

Q. A thin equiconvex lens is made of glass of R.I 1.5 and its focal length is 0.2 If it acts a concave lens of 0.5 m focal length when dipped in a liquid, the R.I of liquid is

1. $\frac{17}{8}$
2. $\frac{15}{8}$
3. $\frac{13}{8}$
4. $\frac{9}{8}$

Q. A convex lens is made of a material having refractive index 1.2. Both the surfaces of the lens are convex. If it is dipped into water $(\mu =1.33).$ It will behave like

1. a convergent lens
2. a divergent lens
3. a rectangular slab
4. a prism.

Q. While calculating the focal length of a lens formula; what is different when the lens is immersed in or surrounded by some other medium than that of air?

1. ${\mu }_1$
2. ${\mu }_2$
3. $R_1\&R_2$
4. $Noneofthese$

Q. The power of a thin convex lens ${(}_a{\mu }_g=1.5)$ is +5.0 D. When it is placed in a liquid of refractive index ${}_a{\mu }_l$, then it behaves as a concave lens of local length 100 cm. The refractive index of the liquid ${}_a{\mu }_l$ will be

1. $\frac{5}{3}$
2. $\frac{4}{3}$
3. $\sqrt{3}$
4. $\frac{5}{4}$

Q. A thin biconvex lens having radius of curvature $10\text{cm}$ is placed as shown in figure. Calculate the focal length of the lens, if parallel rays are incident as shown.

1. $\frac{160}{7}\text{cm}$
2. $\frac{150}{7}\text{cm}$
3. $\frac{180}{7}\text{cm}$
4. $\frac{120}{7}\text{cm}$

Q.

In the previous question

1. $R_1$ is +ve, $R_2$ is -ve.

2. $R_1$ is -ve, $R_2$ is +ve.

3. $R_1$ & $R_2$ are both +ve.

4. $R_1$ & $R_2$ are both -ve.

Q. A biconvex lens in air has radii of curvature $20cm$ each. If the refractive index of the material of the lens is $1.5$, what is its focal length?

1. $20cm$
2. $-10cm$
3. $10cm$
4. $-20cm$

Q. A thin equiconvex lens is made of glass of R.I 1.5 and its focal length is 0.2 If it acts a concave lens of 0.5 m focal length when dipped in a liquid, the R.I of liquid is

1. $\frac{17}{8}$
2. $\frac{15}{8}$
3. $\frac{13}{8}$
4. $\frac{9}{8}$

Q. A hollow double concave lens is made of very thin transparent material. It can be filled with air or either of two liquids $L_1$ and $L_2$ having refractive indices $n_1$ and $n_2$ respectively $(n_2>n_1>1)$. The lens will diverge a parallel beam of light if it is filled with

1. Air and placed in air
2. Air and immersed in $L_1$
3. $L_1$ and immersed in $L_2$
4. $L_2$ and immersed in $L_1$