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The aim of my coursework is to calculate the wavelength of red laser light using the diffraction grating formula and the Youngs double slit formula.

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Wavelength of Red Light


The aim of my coursework is to calculate the wavelength of red laser light using the “diffraction grating” formula and the “Young’s double slit” formula. Due to the unavailability of different diffraction gratings, I had to use different slits which then I could compare the results I got from the different slits to the result I got from the diffraction grating. To improve the accuracy of my result I got with the diffraction grating, I did the experiment with and in absence of two lenses. By doing the experiment with the two lenses I hope to get closer to the real wavelength of red light as the dot on the screen will be smaller, sharper and clearer to measure.


  • Red laser light
  • Two meter rulers
  • Wall or screen
  • Slit holder
  • 300mm lines Diffraction grating
  • Variety of slits
  • Diverging lens
  • Converging lens
  • Clamp stand
  • Lens holder
  • Cello tape, blue tack
  • Pencil


  1. Attach the laser to the clamp and cello tape the power button down so it stays on so marking the red dot on the screen can be done.
  2. Adjust the diverging and converging lens to obtain a focused and small dot and place the diverging lens in front of the converging lens.
  3. Insert the 300mm diffraction grating into the slit holder.
...read more.




Table from diffraction grating

Lines per mm

Wavelength without lenses


Wavelength with lenses






From the graph it can be interpreted that with the use of lens, the closer the value is to the actual wavelength value. The assumption I made about the use of lens increasing accuracy turns out to be correct because the dots are smaller on the wall and therefore we can reduce the uncertainty in the measurement of the distance between the fringes.

Calculation for 2nd method with using slits

Young’s double slit formula =

λ = w x simage17.png


λ = wavelength to be calculated

w = width of fringe which is measured by a ruler once maxima is marked on the wall

s = slit separation which is given on the slits used

d = distance between slit and wall

Slit no. 1:

Width of fringe:                                                         0.5cm

Slit separation:                                                         0.25mm

Distance between slit and wall:                                                 2.0mimage01.png

λ = (0.5 x 10ˉ²) x (0.25 x 10ˉ³)image02.png


Slit no. 2:

Width of fringe:                                                                         0.4cm

Slit separation:                                                                         0.31mm

...read more.


I am going to use the results of the experiment I did of the diffraction grating with the use of lens as its closer to the actual wavelength value.

As you can see on the visible spectrum of the electromagnetic spectrum, we can see that my result i got from the grating experiment, with the use of lens, 653nm is in the red section.


I got even closer to the actual wavelength by using my second experiment with the use of variety of slits, all with different slit separations. The closest wavelength I got to the actual wavelength was 630nm at a slit separation of 0.18mm.

Quality of Measurement Physics coursework

...read more.

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