How Does Galileo's Telescope Work?

A Bit of History:

Galileo Galilei didn’t invent the telescope. Even so, it was when the great Italian scientist pointed a telescope toward the night (and even the day) sky that the world changed forever.

Galileo discovered the planet Venus has phases, just like the Moon. No one could have known that until the telescope was invented. Galileo saw that the Moon had craters and mountains, and that the Sun had moving black spots on its face. He also discovered four new moons circling the planet Jupiter.

All these discoveries helped show that the Earth is not the centre of the universe, that the planets orbit a changing and imperfect Sun, and that the planets are other worlds, something like the Earth but very far away. With his telescope, Galileo literally remade the world.

Galileo’s telescope was a refracting telescope. Isaac Newton, who was born the same day Galileo died, invented another kind of telescope, called a reflecting telescope. Reflecting telescopes use mirrors instead of lenses to make faraway things look bigger. Reflecting telescopes can be made much more powerful than refractors, and Newton’s invention paved the way for all the great telescopes to come.

How It Works:
Refracting telescopes depend on one amazing fact. As light passes through glass, it slows down. Slowing down a light beam makes it bend. Why? Imagine you’re pulling a wagon along a sidewalk, when the wheels on one side slip off into the grass. The wheels turn slower in the grass than they do on the sidewalk, and the wagon moves toward the grass. In the same way, when a light beam passes through a glass lens inside a telescope, it moves toward the lens. When the light beam comes out the other side, it’s bent!

The shape of the lens means light near the top of the lens is bent down and light near the bottom of the lens is bent up. Somewhere inside the tube the light beams cross, but before they can spread out again the eyepiece lens bends the light beams again and sends them to the eye.

Because the light beams cross, the image ends up upside-down. This doesn’t matter much when you’re looking at Mars or the Moon (remember there’s no real up or down in space), but refracting telescopes used to see objects here on Earth often have another set of lenses to flip the image right-side up again.

Refracting telescopes are simpler than reflecting telescopes, but they have an important limitation. Remember that the light passing through the glass lens gets bent. It turns out that different colours are bent different amounts, and that causes the light to become unfocused. Isaac Newton solved this problem by replacing the lenses with mirrors.

When light hits a mirror, it doesn’t bend. Instead, it bounces off. Just like a ball bouncing off a wall, a light beam comes off a mirror the same way it comes in. In other words, the angle in equals the angle out. And that rule is true for all the light, no matter its colour.

The primary mirror in a reflecting telescope is curved just the right amount to bounce all the light onto the secondary mirror. From there, the light passes through the eyepiece lens, which bends the light into the eye.