# OCR Physics P7 Notes

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• Created on: 01-05-13 17:29

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Topic 1 ­ OBSERVATORIES AND TELESCOPES
LENS
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 curved
Lens with a short focal length are fatter ­ its
surfaces are strongly curved
Stand opposite to window. Hold lens and focus an
image on the wall. Measure distance from lens to
wall.
Power of lens
Fat lens bend rays of light more. Short focal length = greater power
Thin lens bend rays of light less. Long focal length = lower power
Power (dioptres) = 1
Focal length (metres)
REFRACTING TELESCOPES ­ produces images upside down
Converging lenses act as magnifying glasses
A telescope that uses lenses to gather and focus light is called a refracting telescope.
Eyepiece lens ­ lens next to your eye fatter
Objective lens ­ lens closest to the object ­ thinner
To achieve a degree of magnification, you need to have two different lenses of different powers.
Diverging lens
Diverging lend make parallel rays of light diverge. Lenses
are fatter at the edge than in the middle.
Magnification

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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 = focal length of objective lens
focal length of eyepiece lens
Aperture and image brightness
Telescopes can make stars look brighter.
Larger apertures gather more light and concentrate the rays before they enter your eye.…read more

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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 be balanced carefully
Temperature changes
Telescope structure will expand as its temperature rises and contract as it cools.

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ATMOSPHERE
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 caused when starlight passes through the atmosphere.
The atmosphere is not uniform. Some areas are denser than some. As a ray of light passes through
areas of different densities, it is diffracted and changes direction.…read more

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When the Moon is in the direction of the Sun, the side that is in darkness faces the Earth. NEW
Moon
Solar and Sidereal Day
The stars appear to travel across the sky once every 23h 56m. This is 4 minutes less than the time
taken by the Sun. The difference is because the Earth orbits the Sun, once every year.
A fixed star will reappear in the sky every 23h 56m.
This means that the Earth has rotated through 360° in

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From months 1 to 3, Mars seems to move forwards, then for 2 months it goes into reverse before
moving forwards again.
ECLIPSES
Solar eclipse ­ Moon blocks the Sun's light Lunar eclipse ­ Moon moves into the Earth
The Sun moves slightly faster The Moon moves behind the
than the Moon so its path may Earth so that the Earth blocks
take it behind the moon. For a the sun's rays from striking the
total solar eclipse, the Sun Moon.…read more

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Measurements
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 degree
360
The smaller the parallax angle, the further away the star:
Distance to star = 1
(parsecs) parallax angle (in arc seconds)
A parsec is the distance to a star whose parallax angle is one second of arc.

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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 ­ luminosity is greater
Hotter star, the peak frequency is greater ­ produces a greater proportion of
GALAXIES
Cepheid variables stars
These are stars that vary in their brightness. A star like this is expanding
and contracting so that its temperature and luminosity vary. Its diameter
may vary by as much as 30%.…read more

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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.…read more

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Photons of energies 2, 7 and 8 would be absorbed and 1,6 and 9.3 would not be absorbed.
ATOMS AND NUCLEI
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 of just a few atoms