Diffraction and Telescopes

Question 1

What is a simple definition of diffraction?

Answer 1

Waves spreading out at the edges as they pass through a gap or past an object.

Question 2

What does the amount of diffraction depend on?

Answer 2

The size of the gap relative to the wavelength of the wave.

The narrower the gap, or the longer the wavelength, the more the wave spreads out.

Question 3

Why are mirrors better to use in telescopes than lenses?

Answer 3

Large lenses are difficult to make accurately.

Large lenses are very expensive to make.

Question 4

What is needed to detect faint sources?

Answer 4

A wide aperture

Question 5

Why is a wide aperture needed to detect faint sources?

Answer 5

To collect enough radiation from objects in space so they can be seen. The bigger the aperture the more radiation can get into the telescope and the better the image formed.

Question 6

Why must aperture size be much bigger than wavelength?

Answer 6

All waves diffract when they go through a gap. Radiation entering a telescope spreads out at the edges of the aperture causing the image to blur.
Having an aperture wider than the wavelength of radiation means the radiation passes through the aperture with little diffraction. So a sharper image is created.

Question 7

What is a diffraction grating?

Answer 7

It has very narrow slits to diffract light. When the light diffracts, different wavelengths of coloured light are diffracted. This creates a spectrum of coloured light.

Question 8

What are the spectra created by diffraction gratings used for?

Answer 8

Astronomers can use these spectra to analyse the light coming from stars.