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31-1. Diffraction of Light (P703)
1. Diffraction :
screen
When light encounters an obstacle,
Diffraction, like interference,
it spreads out and bend into the geometric shadow. The diffraction pattern is formed on the screen.
characterizes the wave nature of light.
Classification for diffraction:Classification for diffraction:
S
A
B
EP
B
A
SB
A E
P1L 2L
When both light source and screen are all sufficiently distant from the obstacle,
Whereas it’s Fresnel diffraction.
it is Fraunhofer diffraction.
Lenses are used to realize Fraunhofer - diffraction
( far-field diffraction)
Any point that the wave front reaches acts as a point source of wavelet.
2. Huygens-Fresnel Principle (P680):
Significance:
Diffraction is referred to as the interference of infinite number of wavelets.
The secondary wavelets are coherent.
(1) Fresnel half-wave-zone method ( 半波带法 )
3. Diffraction by a Single Slit:
L E
O
B
A
a
Central bright fringe( 中央明纹 )
(i)
00 ，
Denote the diffraction angle as
Suppose ，，， 212
sin NNaBC ，，， 212
sin NNaBC
AB is divided into N half-wave-zones.
，θ 0ii ，θ 0ii
The rays from two successive zones cancel in pairs.
BC,AC draw BC,AC draw sinmax aBC sinmax aBC
（ 2 ） Location of the Diffraction Bands (P704):
0sin a (central bright fringe)
λ Δx I
0
x1
x2
Δxo
f
1
0
λ Δx I
0
x1
x2
Δxo
f
1
0
Angular width( 角宽度 ):
a
22 10
Linear width( 线宽度 ):
affx 2ant2 10
tansin①For small ,
Central bright band:
Angular width( 角宽度 ): a
Linear width( 线宽度 ): a
fx
② Other bright bands:
—— half width of the central band
aa
0
Discussion:
am
am
)1(
,, x ,, x
；，，： remarkableisndiffractioxaa
fx
；，，： remarkableisndiffractioxa
a
fx
，a
③
The central bright fringe becomes the image of geometrical optics-----
④
For the incident white light, colorful fringes are displayed on the two sides of white central band.
Light travels along straight line.
，0
Solution:
A slit of 1.00 mm is illuminated by light of = 589 nm.
22sin
ma
11 tanfx 22 tanfx
mm77.1)2
(12 aa
fxxx
The separation of the two minima is:
Using the condition of minima of diffraction:
and
EXAMPLE:
A diffraction pattern is seen on a screen 3.00m away. What is the distance between the first two diffraction minima on the same side of central diffraction maximum?
• Questions Questions ((思考题）思考题）
• PP726 726 5; 5; PP72672677
• Problems Problems ((练习题）练习题）
• PP7277275; 5; PP7277276; 6; PP72772777
