In an astronomical telescope in normal adjustment, a straight line of length $L$ is drawn on the inside part of the objective lens. The eye-piece forms a real image of this line. The length of this image is $l.$ The magnification of the telescope is:
1. $\frac{L}{l}+1$
2. $\frac{L}{l}-1$
3. $\frac{L+1}{l-1}$
4. $\frac{L}{l}$

Two identical thin plano-convex glass lenses (refractive index = $1.5$) each having radius of curvature of $20$ cm are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of a refractive index of $1.7$. The focal length of the combination is:
1. $-20$ cm
2. $-25$ cm
3. $-50$ cm
4. $50$ cm

If the focal length of the objective lens is increased then the magnifying power of:

The angle of a prism is $A$. One of its refracting surfaces is silvered. Light rays falling at an angle of incidence $2{A}$ on the first surface return back through the same path after suffering reflection at the silvered surface. The refractive index $\mu,$ of the prism, is:
1. $2\text{sin}A$
2. $2\text{cos}A$
3. $\dfrac{1}{2}\text{cos}A$
4. $\text{tan}A$

For a normal eye, the cornea of the eye provides a converging power of $40~\text{D}$ and the least converging power of the eye lens behind the cornea is $20~\text{D}$. Using this information, the distance between the retina and the cornea-eye lens can be estimated to be:
1. $2.5~\text{cm}$
2. $1.67~\text{cm}$
3. $1.5~\text{cm}$
4. $5~\text{cm}$