# P7 - Topic 1

• Telescopes
• Lenses
• Refracting telescopes
• Reflecting telescopes
• Bigger and better telescopes
• Ray diagrams for telescopes
• Windows in the atmosphere
• Images of the stars
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• Created by: Anna
• Created on: 20-04-10 19:31

## Telescopes

• Galileo made the first observations of the moon using a telescope

Pulsars:

• Pulsars were discovered by Jocelyn Bell
• Pulsars are strange sources of regularly pulsing radio waves
• Astronomers believe pulsars come from rapidly spinning neutron stars
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## Lenses

• You can make a miniature image of a distant object by using a single converging lens
• Rays of light enter the lens, and because of the lens shape they are refracted first on entering the lens and again on leaving
• Refraction is when a ray of light bends as it passes at an angle from one material to another
• A horizontal line passing through the centre of the lens is called the principal axis
• The distance from the centre of the lens to the focus is called the focal point. The longer the focal length of a lens, the larger the real image that the lens produces of a distant object will be

Focal length:

• A lens which surfaces aren't strongly curved and that is thin has a long focal length and low power
• A lens with strongly curved surfaces which is fat has a short focal length. It bends the rays of light more, so has high power.

power (in dioptres)= 1 / focal length (in metres)

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## Refracting telescopes

• Any converging lens can act as a magnifying glass

Refracting telescopes:

• A telescope that uses lenses to gather and focus light is called a refracting telescope
• A refractor has two converging lenses. The eyepiece lens is the one next to your eye (with the fatter lens). The objective lens is the one closest to the object you are observing (thinner lens).
• The object you see will be inverted (upside down). Galileo's telescope had a diverging lens as an eyepiece, and the image was the right way up.
• A diverging lens makes parallel rays of light diverge. Lenses that are fatter at the edges than in the middle are diverging lenses.

Calculating magnification:

• magnification = focal length of objective lens / focal length of eyepiece lens
• Telscopes make stars look brighter
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