In a reflecting telescope, a secondary mirror is used to:

Solution

Physics sir · 4 steps

The secondary mirror is just a traffic diverter: it sends the light out of the tube to your eye.

1. The problem. The large concave primary mirror brings light to a focus inside the tube, on the axis. An eyepiece placed there would block the incoming light, and so would the observer's head.
2. The fix. A small secondary mirror is placed before that focus. It reflects the converging beam sideways (Newtonian) or back through a hole in the primary (Cassegrain), so the image forms outside the tube where the eyepiece can sit.
3. Other options. Mirrors have no chromatic aberration in the first place, so the secondary is not needed for that. Spherical aberration is removed by making the primary parabolic, not by adding a secondary. And an extra mirror adds to the support problem; it does not reduce it.
4. So. The secondary mirror is there to move the eyepiece outside the telescope tube, option (d).

The idea to keep. The primary mirror forms the image; the secondary only redirects it to a convenient spot. Mirrors never disperse light, and a parabolic primary takes care of spherical aberration.

Answer from the NTA final answer key. Solution by PrepWiser.