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To find the separation of a pair of double slits using youngs experiment.

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Youngs experiment involved sunlight being allowed to pass through a pinhole, S and then at a considerable distance away , through pinholes S1 and S2. The two sets of spherical waves emerging from the two holes interfered with each otherinsuch a way as to form a symmetrical pattern of varying intensity on the screen AC. This experiment is shown below

 The mathematical formula that was derived from this experiment is Λ = (a * y)/D, which can be re- arranged so that the seperation of the two slits is the subject of the equation,

 a = (λ * D)/y


  • Monochromatic light (red laser)
  • Measuring tape
  • Double slit
  • Piece of paper
  • Pen


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Substitute D, y, and λ = 660nm into the formula, a = (λ * D)/y to obtain a value for the seperation of the double slits, a.


  • The measurement of D will not be completely accurate, due to human inaccuracy, but this will have minimal effect on the eventual out come.
  • The wavelength of the light could possible be inaccurate, this is unavoidable
  • The tracing of the projected dots could be inaccurate due to human error, this could affect the results remarkedly.
...read more.


The values that were obtained for ‘a’ were only a smal percentage off the actual value of ‘a’, which was 5 *10-3m Below is a table that shows the inaccuracy of the two results.

Wavelength, λ

Distance, m

Actual distance

% Inaccuracy









From thes results you can deduce that the wavelength of the light is more likely to be closer to 660nm than 680nm, assuming that the results are accurate.


This experiment has been very accurate in finding the seperation of a pair of double slits, using young’s experiment. It was also very usful in providing an insight into the concept of light travelling in waves.

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