# Physics P7

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Physics P7 revision notes
Observatories and telescopes
· Stars are so distant that their light is effectively parallel
· Converging (convex) lenses and concave mirrors can be used to focus
parallel rays of light.
o The more curved the lens, the more powerful it is.
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· Power of lens in dioptres =!"#\$% !"#\$%& in metres
· A converging lens forms a real image of a distant source of light
· A simple telescope uses two converging lenses. The eyepiece is more
powerful than the objective
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· Magnification = !"#\$% !"#\$%& !" !"!#\$!%!
· Most astronomical telescopes use concave (parabolic) mirrors
o They reflect all rays of colours in the same way
o you can make very large mirrors (up to 10m).
· A large telescope is needed to collect the weak radiation from a faint or
distant source
· With electromagnetic waves, the aperture of a telescope must be much
larger than the wavelength of radiation it detects
Ray Diagrams for telescopes
Reflectors
· When parallel rays of light (e.g. from a star) reach a mirror, they will obey
the law of reflection:
angle of incidence = angle of reflection.
· In the picture below, this means that angles ao and bo are the same.
Incident ray
(light approaching the mirror)
Angle of incidence

Normal line
(90 to the angle of the mirror) bº
Angle of reflection
Reflector ray
(light reflected from the mirror)
· Telescopes used curved (concave/parabolic) mirrors. If parallel rays hit the
mirror head on (called "parallel to the axis of the reflector") they are
reflected to the focus point, like this:

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F
Parabolic reflector
· But if parallel rays comes from another direction, they are focused at a
different point, like this (the blue dot)
F
· The focal point is where you look at the image, So, for a parabolic mirror
telescope to be able to look at different stars, the reflector has to be able to
be moved about, or some other way has to be found.…read more

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Light from one
side of the galaxy
Light from the other
side of the galaxy
· A telescope with two lenses works like this:
o Parallel rays of light from a distant object enter the objective lens
o The objective lens focuses the parallel rays, producing a larger image
o The eyepiece is a magnifying glass which looks at and magnifies the
image created by the objective lens.
Naked eye astronomy
· Because of the way the Earth rotates, all objects appear to travel east-west.…read more

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A lunar eclipse is when the Earth is between the sun and the moon.
· Different stars are seen in the night sky through a year, as the earth travels
around its orbit.
Galaxies
· The Milky Way Galaxy is a spiral galaxy with a black hole in its centre
· The Sun is in the Milky Way Galaxy
· Nearby stars to the Earth show parallax.…read more

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The spectrum of a star has a continuous range of frequencies, plus lines that
indicate chemical elements present.
· A molecular model can be used to explain why the pressure and volume of a
gas vary with temperature.
· Kelvins (K) use the same gauge as centigrade, but start at absolute zero
(­273°C). So 273K is 0°C.
· Rutherford's alpha scattering experiment showed that an atom has a small,

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Stars with a mass 8 times or more than the sun become red supergiants.
These have enough mass to compress the core further, until most of it is iron.
o The outer layers of the star are no longer held up by the pressure of the
core, so collapse inwards. The core becomes very dense and the outer
material collides with it and bounces off. The result is a Supernova.…read more

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Chile, Hawaii,
Australia and the Canary Islands
· Some telescopes are put into space to reduce the effects of the atmosphere,
which refracts and absorbs electromagnetic radiation

merci beaucoup

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