# Telescopes for AQA A-level Physics 1. Types of telescopes
2. Ray diagrams
3. Angular magnification
4. Focal lengths
5. Angular resolution A ray diagram shows the path that a light ray entering a lens follows to form the image. To draw a ray diagram, two steps must be followed. Firstly, a ray must be drawn from the object to the lens parallely to the principal axis, which when goes through the lens, must pass through the principal focus. Secondly, a second ray must be drawn from the object passing through the centre of the lens. A third additional ray can also be drawn into the ray diagram. The normal at the point which the ray enters the lens is the line perpendicular to the surface at that point. The angle of incidence is the angle between the incident ray and the normal, whereas the angle of refraction is the angle between the refracted ray and the normal. The angular magnification of a telescope is its magnifying power. It is given by the following formula: M = α/β, where α is the angle subtended by the image at the eye and β is the angle subtended by the object at the unaided eye. This is equivalent to dividing the focal length of the objective lens by the focal length of the eyepiece lens. The focal length of a lens is the distance between its centre and the principal focus. A radian is the angle subtended at the centre of a circle by an arc who has length of the radius of the circle. 2π radians gives a full cycle of 360º. The resolving power of a lens is its ability to produce individual images for objects which are close together. For this to occur, Rayleigh's Criterion must hold. This states that the angle between the lines from Earth to each object must be of minimum angular resolution. # 1

What are the advantages of large diameter telescopes?

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Draw a diagram for a light ray entering a concave lens, labelling the normal, the angle of incidence and the angle of refraction.

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Define what a single dish radio telescope is.

Single dish radio telescopes create images of astronomical objects by using a parabolic dish which focuses radio waves onto a receiver. # 4

Explain why I-R, UV and X-ray telescopes have to be placed in space.

The atmosphere absorbs I-R, UV and -ray radiation, therefore the telescopes cannot be placed on ground and must be in space. # 5

Explain why single dish radio telescopes must be placed in isolated locations.

They must be located far away from any other radio sources to avoid interference. End of page