Eli5 Why can’t microscopes and telescopes be used interchangeably?

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Eli5 Why can’t microscopes and telescopes be used interchangeably?

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It’s about scale. Microscopes focus the light reflected from the very small. Pointing them at the very large would result in seeing things with less than what can be seen without them.

If you imagine the surface of an object as a light source emitting a sphere of light in all directions. If you are very far away then the light rays you see will all be effectively parallel to each other. The radius of the sphere of light being emitted is so large that locally it seems flat.

Conversely when you are very close to the object these light rays are diverging quickly, there is a large angle between them.

Telescopes are designed to look at distant objects, the light of which is effectively parallel. They take parallel beams of light and focus them down to a single point.

Microscopes are designed to look at close objects, they take strongly diverging rays and focus them down to a single point.

The difference is how the light is focused. Both telescopes and microscopes make things look bigger, but a microscope focuses very close in, while a telescope focuses very far away. A microscope actually takes in a very small amount of light and stretches it out to make a bigger, dimmer picture. A telescope takes in lots of light and concentrates it very small, to make a brighter image.