How Does a Telescope Work?
by Stephen Whitt, September 1, 2008
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 ItWorks
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 color.
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.
Telescope Facts
Probably the world’s most famous telescope today is the Hubble Space Telescope. It is a reflecting telescope that orbits 600 kilometres above the Earth. Because it is above the Earth’s atmosphere, Hubble is able to see faraway objects more clearly than any telescope in history.
Telescopes aren’t limited to just the light we can see. Invisible kinds of light, like radio waves and x-rays, are important to astronomers, too. Every time astronomers use a new kind of light to look at the universe, they make new and unexpected discoveries. When scientists first used x-rays to look at the sky, they discovered black holes. When they used radio waves, they found the light left over from the birth of the universe, the event called the Big Bang.
X-ray telescopes are some of the strangest devices ever invented. Because x-rays are so powerful, they’d pass right through any mirror they hit straight on. To capture x-rays, scientists use the same effect you use when you skip a rock across a pond. The x-rays come in at an angle, hitting the mirror with a glancing blow that focuses them onto the detector. To increase the number of x-rays collected, the telescopes are built like nested barrels, with the insides of the barrels all covered in mirrors.
Food For Thought
Today, scientists all over the world are working on new kinds of telescopes that don’t use light at all. Instead, these telescopes are searching for gravitational waves. Once these waves are detected, chances are we’ll have a whole new set of strange and amazing discoveries to ponder.
But you don’t need a multi-million dollar orbiting telescope or a gravitational wave observatory to make some amazing discoveries of your own. With even a low-power telescope you can make some of the same observations Galileo made so long ago, spotting the phases of Venus, the craters on the Moon, the amazing rings of Saturn, even the four moons of Jupiter, still called the Galilean satellites.
Safety Note: Don’t EVER look at the Sun through a telescope or any other optical device. You can permanently damage your eyes!