Diffraction grating - Physics Courses

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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