• A?diffraction grating?is a plate on which there is a very large number of parallel, identical, close-spaced slits
• When?monochromatic light?is incident on a grating, a pattern of narrow bright fringes is produced on a screen

Diagram of diffraction grating used to obtain a fringe pattern

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

Diffraction grating equation for the angle of bright fringes

• 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

• 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?θ2?– θ1

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