The physics behind refractive telescopes?

Question

The physics behind refractive telescopes?

Answer 1

Very briefly:

Say you're pointed at the moon.  The shape of the objective lens (the lens pointing toward the moon) causes light rays from the moon to bend inward as they go down the telescope's tube, until they focus at a certain spot, forming a tiny image of the moon near the eyepiece (typically about 1/4 inch in diameter).  (You can simulate this yourself if you take magnifying lens and let moonlight fall on it.  Hold a white screen at the right distance behind the lens, and you'll see a tiny image of the moon on the screen).

The second important piece is the eyepiece.  This literally acts as a magnifying glass which enlarges the 1/4-inch image of the moon formed by the objective lens.  You're seeing exactly the same thing as if you held the eyepiece up close to a (well-lit) tiny picture of the moon.

You might ask, "Why can't I just point the eyepiece at the real moon, and dispense with the other lens?"  The answer is that you couldn't focus it.  In order for a magnifying glass to focus, the thing you're looking at has to be very close.

So, in essence, the objective lens "brings" the moon very close to your eyepiece (where it's focusable); and the eyepiece then magnifies the moon's tiny image.