P7: Waves, lenses and telescopes

Light and sound travel in waves. These waves can be:
1. Refracted
  1. This is caused when waves cross a boundary between one medium and another
    1. The frequency is the same but there is a change in wavelength
      1. This leads to a change in wave speed
        Which causes a change in wave direction
  2. The colours that make up white light are refracted by different amounts as they pass through a prism
    1. Red light is refracted the most
    2. Violet light is refracted the least
    3. This occurs because the colours that make up white light have different frequencies and wavelengths
2. Defracted
  1. This is caused when waves move through a narrow gap or past an obstacle
    1. Diffraction is most obvious when:
      1. The size of the gap is smaller or the same size as the wavelength of the wave
      2. The waves that pass obstacles have long wavelengths
    2. Radiation is diffracted by the aperture of a telescope
      1. to produce sharp images, the aperture must be a lot bigger than the wavelength of the radiation detected by the telescope
Convex/converging Lenses
1. These bend rays of light inwards as it passes through the lens
  1. If the rays of light are parallel, the rays of light will be brought to focus at the focal point
    1. This is due to refraction
  2. The greater the curvature of the lens, the more powerful it will be
    1. This means that of two lenses made of the same material, the one with the biggest curvature will be more powerful
    2. You can calculate the power of the lens using a formula
      1. Power (dioptres) = 1/ focal length
        The focal length is the distance between the focal point and the lens
Telescopes
1. A simple refracting telescope is made from two converging lenses of different powers
  1. The eye piece lens is more powerful than the objective lens
    1. The objective lens captures parallel light from a different object and brings it to a point on the focal point of the lens
      1. This point is also the focal point for the eyepiece lens. So the image acts as an object and the eyepiece lens magnifies it
        the distance between the lenses will be equal to the focal lengths of the objective and eyepiece lenses added together
2. One type of telescope is an astronomical (reflecting) telescope
  1. This uses a concave mirror instead of a convex mirror for the objective lens
    1. This allows the telescope to be larger so it takes in more light
      1. The larger the telescope, the more detail the image it produces has
    2. Concave mirrors reflect rays of parallel light and bring them to focus
Angular magnification
1. The image of a disant magnified object will appear closer than the actual object is
  1. So the angles made by ray lines entering the eye is greater
    1. the increase in angle is called angular magnification and makes the object appear closer
2. You can calculate the angular magnification of a telescope using a formula
  1. Magnification= Focal length of objective lens / focal length of eyepiece lens

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P7: Waves, lenses and telescopes

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