A telescope is a gadget which helps faraway objects appear much closer. Though the telescope is an old invention, it is still very interesting for children and adults. To understand the working of the telescope, let's consider why we can't see faraway objects. The most probable reason is that we don't have big eyes. Suppose we had big eyes, we could collect more light from the object and create a brighter image and then we could magnify part of that image so it stretches out over more pixels on our retinas. The telescope does the same.
There are two parts in a telescope which makes this possible. These parts are:
Objective Lens: The objective lens (in refractors) or primary mirror (in reflectors) collects lots of light from a distant object and brings the light or image to a point or focus.
Eyepiece Lens: An eyepiece lens takes the bright light from the focus of the objective lens or primary mirror and spreads it out (magnifies it) to take up a large portion of the retina. This is the same principle that a magnifying glass lens uses. It takes a small image and spreads it out over the retina of our eyes so that it looks larger.
The combination of both of these lenses forms a telescope.