Diffraction and Telescopes

Diffraction and Telescopes

1. Diffraction Waves Spreading Out

   • All waves diffract at the edges when they pass through a gap or past an object. the narrower the gap or the longer the wavelength the more the wave spreads out. A narrow gap is one about the same size as the wavelength of the wave. So whether a gap counts as narrow or not depends on the wave.

2. To detect faint sources you need a wide arperture

   • Some objects in the sky are so distant that only a small amount of radiation from them reaches us. To collect enough of the radiation from these objects to see them you need to use a telescope with a hude objective lens. The diameter of the objective lens is called the aperture. Making large lenses is difficult and expensive where as big mirrors are smuch easier to make.

3. Arperture Size must be much larger than wavelength

   • Because all waves can diffract when they pass through a gap radiation entering a telescope spreads out at the edges of the aperture causing the image to blue. If you looked at radiation from a point source when there was no diffraction you would see a bright dot, if you used too small an aperture to look at the star, the dot would be dimmer and sourrounded by rings which get dimmer the further away from the image they are. The only way around this is to have an aperture much bigger than the wavelength

4. A diffraction grating can be used to make aspectrum

   • A diffraction grating has very narrow slits small enough to diffract light, When white light passes through the gaos the different wavelengths of coloured light are all diffracted diffrent amounts creating a spectrum. Astronomers can use these spectra to analuyse the light coming from stars