P7: Waves, lenses and telescopes

P7: Waves, lenses and telescopes

1. Light and sound travel in waves. These waves can be:

   • Refracted
     • This is caused when waves cross a boundary between one medium and another
       • The frequency is the same but there is a change in wavelength
         • This leads to a change in wave speed
           • Which causes a change in wave direction
     • The colours that make up white light are refracted by different amounts as they pass through a prism
       • Red light is refracted the most
       • Violet light is refracted the least
       • This occurs because the colours that make up white light have different frequencies and wavelengths
   • Defracted
     • This is caused when waves move through a narrow gap or past an obstacle
       • Diffraction is most obvious when:
         • The size of the gap is smaller or the same size as the wavelength of the wave
         • The waves that pass obstacles have long wavelengths
       • Radiation is diffracted by the aperture of a telescope
         • to produce sharp images, the aperture must be a lot bigger than the wavelength of the radiation detected by the telescope

2. Convex/converging Lenses

   • These bend rays of light inwards as it passes through the lens
     • If the rays of light are parallel, the rays of light will be brought to focus at the focal point
       • This is due to refraction
     • The greater the curvature of the lens, the more powerful it will be
       • This means that of two lenses made of the same material, the one with the biggest curvature will be more powerful
       • You can calculate the power of the lens using a formula
         • Power (dioptres) = 1/ focal length
           • The focal length is the distance between the focal point and the lens

3. Telescopes

   • A simple refracting telescope is made from two converging lenses of different powers
     • The eye piece lens is more powerful than the objective lens
       • The objective lens captures parallel light from a different object and brings it to a point on the focal point of the lens
         • 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
   • One type of telescope is an astronomical (reflecting) telescope
     • This uses a concave mirror instead of a convex mirror for the objective lens
       • This allows the telescope to be larger so it takes in more light
         • The larger the telescope, the more detail the image it produces has
       • Concave mirrors reflect rays of parallel light and bring them to focus

4. Angular magnification

   • The image of a disant magnified object will appear closer than the actual object is
     • So the angles made by ray lines entering the eye is greater
       • the increase in angle is called angular magnification and makes the object appear closer
   • You can calculate the angular magnification of a telescope using a formula
     • Magnification= Focal length of objective lens / focal length of eyepiece lens