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Converging lens

Any lens that is fatter in the middle than at the edge.
The longer the focal length, the larger the real image will be.

Estimating focal length

Lens with a long focal length are thin ­ its
surfaces are not strongly…

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For example ­ telescope used to see the moon is labelled 50X
Moon appears 50 rimes closer
Moon's angular size seem 50 times as great

Telescopes do not make distant object look any bigger. They spread out a group of stars allowing
them to be seen separately.

Magnification =…

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Telescope mountings

Telescopes with magnification greater than about 6X needs support.
They must be moveable ­ rotation around two axes at right angles to each other enables a
telescope to point in any direction.
Use a tripod to support telescopes
Bearings must have low friction
Weight of the telescopes must…

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The objective lens gathers light from both sides of a galaxy. The image is inverted.

Some telescopes orbit around the Earth. This is to avoid atmospheric pollution and refraction of
light. Also the atmosphere absorbs some electromagnetic radiations such as Xrays, gamma rays
and some infrared.

Twinkling (scintillation) is…

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When the Moon is on the opposite side of the Earth to the Sun, we can see the whole of its
illuminated side. FULL Moon
When the Moon is in the direction of the Sun, the side that is in darkness faces the Earth. NEW
Solar and Sidereal Day…

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This is because the earth is orbiting the sun as well but at a different rate. The direction we see a
planet depends on where both the planet and the Earth are in their orbits.

From months 1 to 3, Mars seems to move forwards, then for 2 months it…

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The closer the star, the greater is its parallax angle.


Parallax angles are measured in fraction of a second of arc. There are:
360° in a full circle
1° of arc = 60 minutes
1´ minute of arc = 60 seconds
1 second of arc is 1 of a…

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Starlight is analysed using a spectrometer. This is attached to a telescope so that it produces a
spectrum showing all the frequencies present.
The spectrum is turned into a graph which shows the intensity for each frequency in the spectrum.

Hotter star, the area under the graph is greater ­…

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stars. The light was shifted towards the red end of the spectrum. The further away the galaxy, the
greater the speed of recession.

Hubble constant

Speed of recession = Hubble constant x distance
The Hubble constant shows how the speed of recession is linked to the distance.
The Hubble constant…

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Photons of energies 2, 7 and 8 would be absorbed and 1,6 and 9.3 would not be absorbed.

Ernest Rutherford realized that alpha and beta particles are smaller than atoms. Hans Geiger and
Ernest Marsden carried the experiment:

Use gold foil and roll it to a thickness…


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