Diffraction grating - Physics Courses
[Pages:5]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
dsin=m similar to two slit interference.
However, the intensity of the diffracted light is higher and the peaks are much narrower.
Question
A grating in a spectrometer has a length of 2 cm and has contains 104 lines. Find the first order diffraction angle for light with a wavelength of 500 nm.
first order m=1
d= L N
=
2x10-2 m 104
=
2x10-6 m
dsin=m
sin
=
m d
=
500x10-9 2x10-6
= 0.25
L=2.0 cm
N =104 slits
=14.5o
Diffraction grating
d two slit interference
dsin=m
multi-slit interference Diffraction grating
Higher intensity
m=2 m=1 m=0 m=-1 m=-2
m=2 m=1 m=0 m=-1 m=-2
Use of a diffraction grating in a spectrometer
Dispersion of light of different wavelengths
1
Optical compact disc
10-6 m The closely spaced dots act like a diffraction grating.
Single slit diffraction
a
barrier
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.
point sources
Two Limiting Cases
Barrier
Ray picture a>>
Light moves in a straight line a
Wave picture a ................
................
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