more about interference

more about interference

1. double slits

   • coherent sources
     • emitting light waves with a constant phase difference

2. light from two bulbs won't form an interference pattern

   • two light sources emit light waves at random

3. wavelength and colour

   • fringe separation depends on the colour of light used
     • white light
       • continous spectrum of colours
     • fringe separation depends on the wavelength of light
       • the greater the wavelength, the greater the fringe separation

4. light sources

   • vapour lamps and discharge tubes
     • produce light with a dominant colour
       • e.g. sodium vapour lamp
         • yellow/orange glow
           • due to the 590nm
         • other colours also emitted
           • but colour with wavelength 590nm more intense than any colour
         • monochromatic light source
           • spectrum dominated by a light of a certain colour
   • light from a filament lamp/ Sun
     • composed of the colours of the spectrum
       • if beam of white light directed at a colour filter
         • light from filter a particular colour
           • contains a much narrower range of wavelengths than white light does
   • light from a laser
     • highly monochromatic
       • specify its wavelength to within a nanometre
         • wavelength depends on the type of laser that produces it
       • beam power concentrated in a very small area
         • dangerous if it enters the eye
           • eye focus the beam on a tiny spot on the retina and the intense concentration of light would destroy the retina
     • convienent source of coherent light
       • inside a laser
         • each emitted photons causes more photons to be emitted as it passes through a light-emitting substance
           • 'stimulated' photon in phase with the photon that caused it

5. white light fringes

   • blue light fringes closer than red light fringes
   • central fringe - white
     • every colour contributes at the centre of the pattern
   • inner fringes - blue on inner side and red on outside
     • red fringes more spaced out and two fringe patterns don't overlap
   • outer fringes merge into a indistinct background of white light
     • different colours reinforce and overlap

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