1
6.1 Diffraction
Diffraction grating
Single slit diffraction
Circular diffraction
Diffraction and Interference
•
Diffraction and interference are similar
phenomena.
•
Interference is the effect of superposition
of 2 coherent waves.
•
Diffraction is the superposition of many
coherent waves.
Diffraction grating
•
Consists of a flat barrier which contains many parallel
slits separated by a short distance d.
•
A parallel monochromatic light beam passing through the
grating is diffracted by an angle
θ
similar to two slit interference.
However,
the intensity of the diffracted light is higher and
the peaks are much narrower.
dsin
θ
=m
λ
Diffraction grating
two slit interference
d
multislit interference
Diffraction grating
dsin
θ
=m
λ
m=0
m=1
m=2
m=1
m=2
m=0
m=1
m=2
m=1
m=2
λ
Higher intensity
Question
A grating in a spectrometer has a length of 2 cm and has
contains 10
4
lines.
Find the first order diffraction angle
for light with a wavelength of 500 nm.
first order m=1
dsin
θ
=m
λ
9
6
m
500x10
sin
0.25
d
2x10
−
−
λ
θ =
=
=
θ
=14.5
o
θ
2
6
4
2x10
m
2x10
m
10
−
−
=
=
L=2.0 cm
N =10
4
slits
L
d
N
=
Use of a diffraction grating
in a spectrometer
Dispersion of light of different wavelengths
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2
Optical compact disc
10
6
m
The closely spaced dots act like a diffraction grating.
Single slit diffraction
barrier
a
λ
For single slit diffraction
the complex behavior of the
wave as a function of the
aperture a is explained by
interference between multiple
coherent point sources in the slit.
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 Spring '09
 Smith
 Diffraction, Wavelength, single slit diffraction, θmin

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