Core Physics

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The Solar System

Geocentric - Earth Centred

  • Proposed by Ptolemy
  • Earth was at the centre of the solar system
  • Everything moved around the Earth in circular orbits
  • Only featured - Mercury, Venus, Mars, Saturn, Jupiter

Heliocentric - Sun Centred

  • Proposed by Nikolaus Copernicus
  • Sun was at the centre of the solar sytem 
  • Planets moved around the Sun in circular orbits
  • Planets further away from the sun move slower than the ones closer
  • Stars were in a fixed dome beyond Saturn
  • Featured 6 planets - all except Neptune and Uranus
  • Galileo Galilei provided evidence to support this theory
  • Both had no telescopes - only naked eye

Galileo Galilei

  • Used a telescope to see - mountains on the moon and four moons orbiting Jupiter

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Reflection and Refraction


  • When light bounces off a boundary between different materials
  • All waves can be reflected (echoes)
  • Redio waves are reflected off sufaces within the atmosphere
  • angle i = angle r - incidence angle = reflected angle


  • The bending of light at a surface
  • All waves are refracted at the boundary between two different materials
  • Speed of the wave changes as it passes through different materials
  • Less dense - More dense = light bends towards the normal
  • More dense - Less dense = light bends away form the normal
  • Light speed depends on material density
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Reflecting telescope

  • Converging eyepiece lens, Concave mirror, Flat mirror
  • Concave mirror forms an image of a distant object and flat mirror reflects the image to the eyepiece lens
  • Lighter and easy to manouvere 
  • Produce better quality images than reflecting telescopes

Refracting telescope

  • Converging objective and eyepiece lens
  • Objective lens converges an image of a distant object to a focal point
  • Eyepiece lens magnifies the image
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Converging Lens

  • Fatter in the middle
  • Used in - telescopes, cameras, binoculars, projectors, eyes
  • Rays of light that are paralell to the the principal axis are refracted inwards
  • Rays of light are refracted to the focal point - principal focus
  • Distance from the centre of the lens to the focal point = focal length
  • Fatter lens = Shorter focal length

Diverging lens is thinner in the middle

Focal Length

  • When the distance between the object and lens is more than 2 focal lengths, the image will be - real , diminished and inverted
  • When the distance between the object and lens is less than 2 focal lengths, the image will be - real, diminished and magnified
  • When the distance between the object and lens is less than the focal length, the image will be- virtual, upright and magnified - magnifying glass

Diminished - smaller than the object
Inverted - upside down
Real - can be projected onto a screen

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  • White light is split up into the visible spectrum using refraction through a glass prism
  • A spectometer is a device used to look at the visible spectrum

Sources of light

  • Hot stars emit blue light and cooler stars emit red light
  • Some electromagnetic waves are absorbed by gases in the atmosphere more than others
  • Visible light is not well absorbed - but large telescopes are located on top of high mountains where the air is thinner - to get the best quality image
  • The chemical composition of stars is idnetified using a spectometer to analyse the light they emit
  • Modern telescopes have built in spectometers


  • EM waves are absorbed bu=y the atmosphere, some more than others
  • X rays and Gamma rays are almost all absorbed by the atmosphere
  • Radio waves are not absorbed - used for communication
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The Doppler Effect

  • As a police car passes the sound changes from high pitch when it is approaching and low pitch when it is moving away
  • This is because the sound waves are being squashed together when approaching and stretched when moving away
  • The Doppler Effect is observed for all waves
  • It is observed in light emitted from stars
  • The wavelength decreases and frequency increases as a star moves towards an observer - blue shift
  • The wavelength increase and frequency decreases as a star moves away from an observer - red shift 

The faster the object is moving - the greater the Doppler Effect will be

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