# Physics Class 12 NCERT Solutions: Chapter 9 Ray Optics and Optical Instruments Part 16 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 35. (A) For the telescope described in Exercise 34 (a) (Reference: Chapter 9 Ray Optics and Optical Instruments Part 15) , what is the separation between the objective lens and the eyepiece?

(B) If this telescope is used to view a tall tower away, what is the height of the image of the tower formed by the objective lens?

(C) What is the height of the final image of the tower if it is formed at ?

Answer:

Focal length of the objective lens,

Focal length of the eyepiece,

(A) In normal adjustment, the separation between the objective lens and the eyepiece

(B) Height of the tower,

Distance of the tower (object) from the telescope,

The angle subtended by the tower at the telescope is given as:

The angle subtended by the image produced by the objective lens is given as:

Where,

Height of the image of the tower formed by the objective lens

Therefore, the objective lens forms a tall image of the tower.

(C) Image is formed at a distance,

The magnification of the eyepiece is given by the relation:

Height of the final image

Hence, the height of the final image of the tower is 28.2 cm.

Q: 36. A Cassegrain telescope uses two mirrors as shown in Figure, such a telescope is built with the mirrors apart. If the radius of curvature of the large mirror is and the small mirror is , where will the final image of an object at infinity be?

Answer:

The following figure shows a Cassegrain telescope consisting of a concave mirror and a convex mirror.

Distance between the objective mirror and the secondary mirror,

Radius of curvature of the objective mirror,

Hence, focal length of the objective mirror,

Radius of curvature of the secondary mirror,

Hence, focal length of the secondary mirror,

The image of an object placed at infinity, formed by the objective mirror, will act as a virtual object for the secondary mirror.

Hence, the virtual object distance for the secondary mirror,

Applying the mirror formula for the secondary mirror, we can calculate image distance (v) as:

Hence, the final image will be formed away from the secondary mirror.

Q: 37. Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards as shown in figure. A current in the coil produces a deflection of of the mirror. What is the displacement of the reflected spot of light on a screen placed away?

Answer:

Angle of deflection,

Distance of the screen from the mirror,

The reflected rays get deflected by an amount twice the angle of deflection i.e..

The displacement (d) of the reflected spot of light on the screen is given as:

Hence, the displacement of the reflected spot of light is .