Diffraction Gratings
Brook Edgar
Teacher
Explainer Video
Introduction to Diffraction Gratings
A diffraction grating is similar to a double slit, but instead of two slits, it has many closely spaced parallel slits.
Light is transmitted only in certain directions, where light diffracted through each slit is reinforced by light from adjacent slits and cancels out in all other directions. In other directions the light cancels each other out.
If a coherent source is used, an interference pattern is seen on the screen. The central maximum is known as the zeroth order. The next bright fringes on either sides are called the order maxima and so on.
Slits per metre
In exam questions, you are often given the number of slits per metre or millimetre on the grating, N.
However, if you look at the formula given to you on the equation sheet, the number of slits per meter, N, is not used.
Formula:
You need to remember the equation, , in order to change from the number of slits per metre into the distance between the slits on the grating.
For example, if the diffraction grating reads 300 lines/mm.
If light of wavelength is used, we can calculate the angle between the central maximum and the first bright fringe, order number .
Worked Example
If there are lines per . Calculate the slit separation in meters.
If light of wavelength is used, determine is the angle
between the and order.
Calculate the maximum number of bright fringes visible on the screen.
Answer:
Teacher Tip: In question 3, don't forget to include the central maximum - this is a common exam mistake. We see a total order number of 5, we see 5 bright fringes on either side of the central maximum. We round down as we cannot see the sixth bright.
Practice Questions
Light of wavelength is incident on a diffraction grating, with slit separation .
Determine the number of maxima produced.
-> Check out Brook's video explanation for more help.
Answer:
maxima are produced.
In a diffraction grating, the separation between slits is . Light with a wavelength is used.
Calculate the angle between the third order and the fourth order.
-> Check out Brook's video explanation for more help.
Answer: