Why astronomy uses waves
Stars are too far to visit. So astronomers read the waves that come from them. Light is one kind of wave. There are others: radio waves, infrared, ultraviolet, X-rays and gamma rays. Together they are the electromagnetic spectrum. Each kind of wave tells a different story. Radio waves show cold gas, X-rays show very hot gas.
Optical telescopes
An optical telescope collects light with a lens (refractor) or a curved mirror (reflector). Big telescopes use mirrors, because a big mirror is lighter and easier to hold than a big lens. Two jobs: collect light (a wider mirror sees fainter stars) and magnify (the eyepiece makes the image larger). Light collected is proportional to the area, so it goes as the square of the mirror width: a mirror twice as wide gathers four times the light. Optical telescopes work at night, in clear sky, and are best on high dry mountains.
Radio telescopes
A radio telescope is a big metal dish that gathers radio waves from space and focuses them on a receiver. Radio waves are long, so the dish must be very big. They pass through clouds, dust and the daytime sky, so radio telescopes can work in daylight and in bad weather. Many dishes can be joined into an array, which works like one giant dish and shows finer detail. Radio astronomy found pulsars and the faint glow left over from the early universe.
Space-based astronomy
Earth's air lets visible light and radio waves through, but it blocks most X-rays, ultraviolet, gamma rays and much infrared. It also makes stars twinkle, which blurs images. To see these waves, telescopes are put on satellites above the air. A space telescope sees clear, sharp pictures, day and night, with no clouds. The price: it is costly to build and launch, and hard to repair. Today astronomers combine data from ground telescopes and space telescopes to see an object in many waves at once.
Try it
In the 3D: first guess, then check. Turn clouds on. Which telescope loses its signal? Now switch on X-rays. Which one gets them? At home: look at the night sky on a clear night, then on a cloudy night. Note how many stars you can count each time. Then use a small paper tube and see how few stars fit in the view, but how bright they look.
Key formulas and definitions
- Light collected ∝ (mirror diameter)²
- Mirror twice as wide → 4 times the light
- Radio: long waves, big dish; works in clouds and daylight
- Air blocks X-rays, ultraviolet and gamma rays → use space telescopes
Worked examples
1. Telescope A has a mirror 2 m wide. Telescope B has a mirror 6 m wide. How many times more light does B collect?
Light ∝ diameter². (6/2)² = 3² = 9. B collects 9 times more light.
2. A night is cloudy. Which can still observe a distant galaxy: an optical telescope or a radio telescope?
The radio telescope. Clouds block visible light but radio waves go through them.
3. Why is an X-ray telescope launched into orbit?
Earth's air absorbs X-rays, so none reach the ground. Above the air they arrive freely.
4. Why are big optical telescopes built on high mountains?
There is less air above, so less blurring and less cloud, and the air is dry. The pictures are sharper.
Common mistakes
- Thinking a telescope only makes things look bigger. Its main job is to collect more light.
- Saying radio telescopes make sound. They only collect radio waves, which are turned into pictures or numbers.
- Believing space telescopes are better in every way. They are costly and cannot be easily repaired.
- Thinking mirror area grows with width. It grows with width squared.