Diffraction gratings and patterns

Posted in AQA AS Unit 2, AS Unit 2: Waves by Mr A on 21 Feb 2010




Diffraction grating equation



Important: The following derivation assumes that all rays incident on each part of the screen are parallel. This is a fair assumption, provided the distance from the slits to the screen is much larger than the slit separation.


Thus, for the central fringe, the rays travel exactly the same distance as one another.


For the first order fringe, each successive ray travels an extra path difference of d \sin{\theta} . Incidentally, this extra path difference must also be equal to \lambda , for constructive interference to occur.


If we proceed to the second order fringe, each successive ray must travel an extra n \lambda . It, therefore, follows that


\boxed{n \lambda = d \sin{\theta}}


Worked example (class demo)

If a red laser is shone through a diffraction grating with ? lines per mm at a screen ? m away, and the first order fringe makes an anlge of ?, what is the wavelength of the light?


Diffraction patterns

Interference and Diffraction

Posted in AQA AS Unit 2, AS Unit 2: Waves by Mr A on 20 Feb 2010



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Huygens’ Principle and Wavefronts

Posted in AQA AS Unit 2, AS Unit 2: Waves by Mr A on 7 Feb 2010


  • Huygens’ principle
  • Diffraction and Reflection
  • Refraction



Huygens’ Principle

Given that waves are caused by a source of disturbance, and that waves themselves cause disturbance as they propagate, Christiaan Huygens (1629-1695) treated waves along the principle that:


Every point on a wave may be considered as a point source disturbance, causing secondary waves that spread out evenly in all directions with a speed equal to the speed of propagation of the wave.




Diffraction and Reflection