the diffraction grating

the diffraction grating

1. consists of a plate with many closely spaced parallel slits ruled on it

2. when parallel beam of monchromatic light is directed at a diffraction grating

   • light transmitted in certain directions only
     • light passing through each slit - diffracted
     • diffracted light from adjacent slits reinforce each other in certain directions only and cancel out in all other directions
   • central beam
     • 'zero order beam'
       • same direction as the incident beam
   • angle of diffraction increases if:
     • light of a longer wavelength is used
     • grating with closer slits id used

3. equation

4. diffration gratings in action

   • mde by cutting parallel grooves very close together on a smooth glass plate
     • grooves
       • some transmits incident light and reflects/scatters some
       • act as coherent emitters of waves
   • spectrometer
     • study spectrum of light from any light source and to meausre light wavelengths v.accurately
     • angle of diffraction - measured v.accurately
   • spectrum analyser
     • electronic spectrometer attached to a computer that gives a visual display of the variation of intensity with wavelength

5. types of spectra

   • continuous
     • from a filament lamp - continous spectrum of colour
       • most intense part of the spectrum depends on the temp of the light source
         • the hotter the source, the shorter the wavelength of the brightest part of the spectrum
         • by measuring wavelength, we can get temp of light source
   • line emission
     • glowing gas in a vapour lamp
       • emits light at specific wavelength
         • spectrum contains of narrow vertical lines of different colours
       • wave lengths of line - characteristic of a specific chemical element
   • line absorption
     • continuous spectrum with narrow dark lines at certain wavelengths
       • dark lines due to the elements in the glowing gas

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