Two convex lenses of focal length \(X\) and \(Y\) are placed parallel to each other. An object at infinity from the first lens forms its image at infinity from the second lens. The separation between the two lenses should be:

1.	\(X+Y\)	2.	\(\frac{X   +   Y}{2}\)
3.	\(X-Y\)	4.	\(\frac{X   -   Y}{2}\)

A plane mirror is placed at the bottom of a fish tank filled with water of refractive index \(\frac{4}{3}.\) The fish is at a height \(10\) cm above the plane mirror. An observer \(O\) is vertically above the fish outside water. The apparent distance between the fish and its image is:

If there is no emergent light through a prism of refracting angle \(60^{\circ},\) whatever may be the angle of incidence, then the minimum value of the refractive index of the material of the prism is:

The minimum magnifying power of an astronomical telescope is \(40\). If the length of the telescope is \(205\) cm, then the focal length of its field lens is:
1. \(5\) cm
2. \(200\) cm
3. \(40\) cm
4. \(140\) cm

A boy with defective eye-sight cannot see things beyond 50 cm. The corrective lens required has the power:

1. +1 D

2. +2 D

3. -1 D

4. -2 D

If \(C_1,~C_2 ~\mathrm{and}~C_3\) are the critical angle of glass-air interface for red, violet and yellow color, then:

An astronomical telescope has angular magnification of \(40\) in its normal adjustment. If focal length of eyepiece is \(5\) cm, the length of the telescope is:
1. \(190\) cm
2. \(200\) cm
3. \(205\) cm
4. \(210\) cm