1. Book Reference : Pages 205-207
1. To understand diffraction gratings
2. To understand how changing wavelength
and slit size affect the transmitted pattern
3. To understand how the diffraction grating
equation is derived
4. To be able to complete diffraction grating
related calculations

2. A diffraction grating is plate with many parallel
slits in it.

3. 1. Light passing through each slit is diffracted
2. The Diffracted light wave from adjacent slits
interfere, (reinforce each other) in certain
directions only

4. 1. The central beam is
referred to as the “zero
order beam”
2. The other beams are
numbered outwards on
each side: 1st order, 2nd
order etc

5. 1. How does the diffraction pattern change
with wavelength?
2. How does the diffraction pattern change
with slit distance?
Virtual Physics Lab : Waves  Diffraction

6. 1. How does the diffraction pattern change
with wavelength?
The angle of diffraction between each beam and
the zero order beam increases with increasing
wavelength (Blue to Red)
2. How does the diffraction pattern change
with slit distance?
The angle of diffraction between each beam and
the zero order beam increases with decreasing
gap size

7. 1. Each diffracted
wavefront reinforces an
adjacent wavefront
2. Wavefront at P reinforces
wavefront at Y one cycle
earlier which in turn
reinforces wavefront at R
one cycle earlier
3. This forms a new
wavefront PYZ which
travels in a certain
direction and forms a
diffracted beam

8. Formation of nth order beam
Q Wavefront at P reinforces
Y wavefront from Q emitted n
d θ cycles earlier.
Wavefront from Q has
P θ
travelled n wavelengths.
QY is nλ
sin θ = QY/QP (substitute)
sin θ = nλ /d (rearrange)
Where d is the slit separation,
n is the order of the diffracted dsin θ = nλ
beam and λ is the wavelength

9. Notes
The number of slits per metre N is 1/d
As d decreases the angle of diffraction
increases. (As N increases, the angle of
diffraction increases)
Maximum number of orders is when θ = 90°
and hence sin θ = 1
∴ n = d/λ
(Rounded down to the nearest whole number)

10. A laser of wavelength 630nm is directed normally
at a diffraction grating with 300 lines per mm.
Calculate :
a) The angle of diffraction for the first two
orders [10.9° & 22.2°]
b) The number of diffracted orders produced [5]

11. Light incident normally on a diffraction grating
with 600 lines per mm contains wavelengths of
580nm and 586nm only.
a) How many diffracted orders are seen in the
transmitted light [2]
b) For the highest order calculate the angle
between the two diffracted beams [0.58°]

12. Light of wavelength 480nm is incident normally on
a diffraction grating the 1st order transmitted
beams are at 28° to the zero order beam.
Calculate:
a) The number of slits per mm for the grating
[1092]
b) The angle of diffraction for each of the other
diffracted orders [69.9°]