Question

which part of a telescope focuses the incoming light? I thought it was the mirror but people are telling me I am wrong?

Answer 1

The type of telescope you refer to has 2 optical devices (lenses). One at either end. They both have a focus & using a ray diagram you can see how it does this.

Answer 2

As we know that telescope will have two lens in its tube.

Both lens used to refract light coming towards it. First lens of the telescope at which all light incident for the first time is known as objective lens.

This is convex type of lens and this lens converge all lights at the focal plane of objective lens.

Now the light converged at this focal plane is the first image formed by the lens and this behaves like an object for second lens which is also known as eye lens. Now eye lens will form its final image which is observed by the observer

Now we can say that Objective lens or the first lens of telescope at which light incident is used to converge all light in telescope.