P7.2: Light, telescopes and images:

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Convex Lens:

The ray refracted when an object goes through and medium with a different optical density.

Converging Lenses:

These are lenses that are “fatter in the middle”. They make light converge which means that they come to a point.

The ray of light bends because it changes speed as it moves from one transparent medium to another.

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How to draw a ray diagrams:

Ray Diagrams:

1. Principal Axis

2. Vertical Axis

3. Focal point – same distances from the centre

4. Pick one that will go through the centre – it will not be bent (it will still slow down and speed up)

5. One that will go through the focal point – it refracts so it is parallel to the principal axis.

6. Where the two rays cross is where the image appears.

7. The ONLY rays that can be accurately predicted are those that go through the centre and focal point.

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Parallel Rays:

Explain why light from any star appears as 'parallel rays'.

From any object, light travels in all directions.

Stars are so far away so we consider they are at infinity so the only rays of light which were directly travelling towards earth enter the telescope. We can presume they are parallel to each other.

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Refraction of Water Waves:


Refraction of Water Waves:

The frequency never changes during refraction – it is determined by the sources of the waves.

Refraction causes water to change direction. All waves behave like this when they enter a medium in which they travel at a different speed. Light waves travel more slowly in glass than in air so they are refracted at air-glass and glass-air boundaries. Lenses change the direction of light rays because refraction occurs as light enters and leaves the lens.

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Water Waves:

Water waves:

Water waves travel faster in deep water than in shallow water. As the wave crosses the boundary, the frequency of the wave stays the same but the wavelength gets shorter. This is because the waves are slowed down; the progress of each wave is reduced by the time the next wave arrives.

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Telescopes 1:

Refracting Telescopes:

2 converging lenses:

1. Objective lens (forms a real, inverted image at focal point of lens) – low power lens, long focal length, thinner in the middle

2. Eye piece lens (acts to magnify image formed by objective) – high power lenses, short focal length, fatter in the middle

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Telescopes 2:

Telescope Magnification:

Magnification = focal length of objective lens (fo)   /    focal length of eye piece lens (fe)

BEST MAGNIFICATION = biggest focal length of fo and smallest focal length of fe

magnification of 50 (or 50x)


a) object appears 50 times larger with naked eye

b) object appears to be a one fiftieth of the distance

c) the telescope makes the object's angular size look 50 times bigger

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