P5 optics

• refraction is the change of direction that occurs when light passes at an angle across a boundary between two transparent substances
• in refraction, if the incident ray is along the normal, there will be no change in direction
• at a boundary between two transparent substances, the light ray bends towards the normal if it passes into a more dense substance
• at a boundary between two transparent substances, the light ray bends away from the normal if it passes into a less dense substance
• the refractive index of a material is how much it slows down the light which passes through it
• the equation for refractive index is n = c/cs where:

  n is refractive index

  c is speed of light in a vacuum

  cs is speed of light in the material
• a material with a higher refractive index is more optically dense than a material with a lower refractive index
• light does not slow down significantly when travelling through air, so the refractive index of air is approimately equal to 1
• snell’s law is used for calculations involving refraction, the equation is: n1 sintheta1 = n2sintheta2

  n1 and n2 are refractive indices

  theta1 is the angle of incidence in material 1

  theta2 is the angle of reflection in material 2
• as light moves across the boundary between two materials, its speed changes, causing its direction to change as well
• refraction occurs because the speed of light is different in each substance, the amount of refraction that takes place depends on the speed of light in each substance
• white light is made up of light of many different wavelengths, from red to violet
• red light has the longest wavelength, violet light has the shortest wavelength
• shining white light through glass splits it up into the different components because each wavelength is refracted by different amounts

  the shorter the wavelength, the more it will be refracted
• the diffraction of white light produces a rainbow spectrum as each wavelength is refracted in a different direction
• when a light ray travels into glass from air, it refracts away from the normal because glass is more optically dense than air so has a higher refractive index
• if the angle of incidence is increased to a certain value, called the critical angle, the light ray will refract along the boundary
• if the angle of incidence is increased past the critical angle, the light ray undergoes total internal reflection at the boundary
• total internal reflection can only take place if:

  • the incident substance has a larger refractive index than the other substance
  • the angle of incidence is greater than the critical angle
• the equation for the critical angle is sin(thetac) = n2/n1

  thetac is the critical angle

  n1 is the refractive index of the incident substance

  n2 is the refractive index of the other substance
• optical fibres are flexible, thin tubes of plastic or glass which carry information in the form of light signals
• optical fibres have an optically dense core surrounded by less optically dense cladding, this allows total internal reflection to occur at the boundary between the core and the cladding
• the purpose of cladding in an optical fibre is to protect the core from damage, and to prevent signal degradation through light escaping from the core
• signal degradation can be caused by:

  • absorption = part of the signal’s energy is absorbed by the fibre, reducing the amplitude of the signal, leading to the loss of information
  • dispersion = the received signal is broader than the transmitted signal, causing overlap of received signals, leading to the loss of information
• the two things that cause signal degradation are: absorption and dispersion
• the two types of dispersion are material dispersion and modal dispersion
• modal dispersion = light rays enter the fibre at different angles, so take different paths, so arrive at different times, causing pulse broadening
• material dispersion = light rays have different wavelengths, so travel at different speeds, so arrive at different times, causing pulse broadening
• modal dispersion can be prevented by making the core very narrow as this reduces the possible number of paths
• material dispersion can be prevented by using monochromatic light as the rays should all have the same wavelength
• absorption and dispersion can be reduced by using an optical fibre repeater which regenerates the signal during its travel to its destination
• young’s double slit experiment demonstrates interference of light from two sources
• for young's double slit experiment, you can use either two coherent sources of light, or one coherent source of light and a double slit to form an interference pattern

  if you don’t have a coherent source of light, before the double slit use a single slit to make the light have a fixed path difference, and a filter to make the light be monochromatic
• the process of young's double slit experiment:

  • coherent light is shone through two slits roughly the same size as the wavelength of the light
  • the light diffracts
  • a pattern of light and dark fringes is produced
• in young's double slit experiment:

  light fringes are where light meets in phase and interferes constructively, the path difference between waves is a whole number of wavelengths

  dark fringes are where light meets out of phase and interferes destructively, the path difference is a whole number and a half of wavelengths
• the equation for the double slit is W = (lambda D)/s

  W is the fringe spacing

  lambda is the wavelength

  D is the distance between the screen and the slits

  s is the slit separation
• path difference is the difference in distance travelled by two waves
• coherent light is light which has the same frequency and wavelength, and a fixed phase difference
• a typical source of coherent light is a laser
• light from a laser is highly monochromatic, and the beam is almost perfectly parallel, so it can be easily focused to one spot, therefore you should never look into a laser as the intense concentration of light would destroy part of the retina where it shone