OCR A Level Physics

Revision Notes

5.11.5 Transmission Diffraction Grating

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Transmission Diffraction Grating

  • Dispersion is the separation of visible white light into a spectrum of its colours
    • This can be done using a glass prism or a diffraction grating
  • Transmission diffraction gratings are useful for separating light of different wavelengths with high resolution in order to:
    • Analyse light from stars
    • Analyse the composition of a star

Diffraction Grating Applications, downloadable AS & A Level Physics revision notes

Diffraction gratings are most commonly used in spectrometers to analyse light from stars

  • A transmission diffraction grating is a glass or plastic slide containing a large number of regularly spaced, parallel slits or lines
  • It is used to analyse spectral line wavelengths from the light emitted by stars
    • The angular dispersion (separation) of the colours is much greater using a transmission diffraction grating than an optical prism
    • Using diffraction gratings results in sharper fringes compared to using a double slit

Condition for Maxima for a Diffraction Grating

  • The angles at which the maxima of intensity (constructive interference) are produced can be deduced by the diffraction grating equation:

Grating equation, downloadable AS & A Level Physics revision notes

  • Exam questions sometime state the lines per m (or per mm, per nm etc.) on the grating which is represented by the symbol N
  • d can be calculated from N using the equation

Angular Separation

  • The angular separation of each maxima is calculated by rearranging the grating equation to make θ the subject
  • The angle θ is taken from the centre meaning the higher orders are at greater angles

Angular separation, downloadable AS & A Level Physics revision notes

Angular separation

  • The angular separation between two angles is found by subtracting the smaller angle from the larger one
  • The angular separation between the first and second maxima n1 and n2 is θ2θ1

Orders of Maxima

  • The maximum angle to see orders of maxima is when the beam is at right angles to the diffraction grating
    • This means θ = 90o and sin θ = 1

  • The highest order of maxima visible is therefore calculated by the equation:

  • Note that since n must be an integer, if the value is a decimal it must be rounded down
    • E.g If n is calculated as 2.7 then n = 2 is the highest order visible

Worked example

An experiment was set up to investigate light passing through a diffraction grating with a slit spacing of 1.7 µm. The fringe pattern was observed on a screen. The wavelength of the light is 550 nm.Worked Example: Diffraction Grating, downloadable AS & A Level Physics revision notes

Calculate the angle α between the two second-order lines.

Worked example - diffraction grating equation (2), downloadable AS & A Level Physics revision notes

Exam Tip

Take care that the angle θ is the correct angle taken from the centre and not the angle taken between two orders of maxima.

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