# P7.2: Telescopes and Images

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• Created by: Lili
• Created on: 21-06-13 13:15
How Does Light Travel?
It travels is waves.
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What is a Substance Called that Allows Light to Travel Through It?
A Medium.
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How Does the Medium Affect the Speed of a Wave?
If a wave travels from one medium to another, its speed changes.
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What Happens When the Speed of a Wave Changes?
The wavelength also changes. The wave may then change direction. This is Refraction!
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Why Do We Use Telescopes?
Telescopes magnify things that are large distances away. They allow us to see things that the naked eye can't see. For example with we can see the moon in the nights sky, however with the use of a telescope we are able to see the craters on the moon.
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What is a Convex Lens?
Convex lenses are thicker in the middle than at the edges. They are made of glass.
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What is a Refracting Telescope?
They use convex lenses, and refract light.
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How Does a Refracting Telescope Work?
Parallel rays enter a convex lens; The parallel rays come to a point called focus; The rays the converge.
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How Do You Draw a Ray Diagram?
Use Arrows to Show the Direction Light is Travelling in, A Ray Through the Centre of a Lens Doesn't Change Direction, A Ray Parallel to the Principle Axis Passes Through the Focus, A Ray Through the Focus Emerges Parallel to the Principle Axis.
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Example of a Ray Diagram.
E.G.
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How Does Light From Distant Stars Reach Earth?
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How Does Light From Planets and Moons in Our Own Solar System Reach Earth?
Light Rays from different parts of the object arrive at the lens at different angles. Rays from the top of the object go to the bottom of the image. The image is upside-down or inverted.
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What is the Focal Length?
The distance from the optical centre of a lens or its mirror of focus.
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What is the Equation for Working Out the Power of a Lens?
Power (Dioptres) = 1/Focal Length (meters, to the power of -1)
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What is an Optical Telescope?
An optical telescope has 2 convex eyepieces, the objective lens and the eyepiece lens.
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What is an Objective Lens?
An objective lens has a long focal length (low power). It collects light from the star. It forms an image of the star inside the telescope.
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What is an Eyepiece Lens?
An eyepiece lens has a short focal length (high power). It magnifies the image formed by the objective lens. The magnified image is what you see.
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What is the Distance Between the 2 Lenses?
The distance is equal to the sum of the 2 focal lengths.
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What is the Equation for Angular Magnification?
Magnification = Focal Length of Objective Lens/ Focal Length of Eyepiece Lens
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What is a Reflecting Telescope?
A reflecting telescope uses a concave mirror, instead of an objective lens.
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Name Some Advantages of Reflecting Telescopes. 1
It is easier to make a big mirror than a big lens, you need big mirrors to view weak radiation from faint or very distant objects. It is hard to make a big lens with no imperfections.
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Name Some Advantages of Reflecting Telescopes. 2
A big convex lens is fat in the middle, glass absorbs light on its way through the lens, so faint objects look even fainter. Mirrors reflect all colours the same.
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What is Dispersion?
In lenses and prisms, refractions splits white light into its colours. This is dispersion.
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What is the Spectrum for Dispersion in a Prism?
It goes from the lowest frequency, and longer wavelength: Red, Orange, Yellow Green, Blue, Indigo, Violet : To a higher frequency and shorter wavelength.
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Other than in Lenses and Prisms Where Else Can Dispersion Occur?
It also occurs at a diffraction grating (narrow parallel lines on a sheet of glass). When white light shines on the grating, different colours emerge at different angles, forming a spectra.
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What is Diffraction?
It is when waves go through a gap, they then bend and spread out.
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How Does the Size of a Gap Affect Diffraction?
The effect of diffraction is greatest when the size of the gap is similar to - or smaller than - the wavelength of the waves.
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What is the Aperture?
It is the light-gathering area of a telescope's objective lens or mirror.
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What Happens if Diffraction Occurs at the Aperture?
The image becomes blurred. Optical telescopes have apertures much bigger than the wavelength of light to reduce diffraction and form sharp images.
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## Other cards in this set

### Card 2

#### Front

What is a Substance Called that Allows Light to Travel Through It?

A Medium.

### Card 3

#### Front

How Does the Medium Affect the Speed of a Wave?

### Card 4

#### Front

What Happens When the Speed of a Wave Changes?

### Card 5

#### Front

Why Do We Use Telescopes?