Diffraction at a single slit
a=λ
a=2λ
Semi circular
wave fronts
First minima & maxima
become visible
a=4λ
Diffraction is the spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.
Diffraction at a single slit
a=λ
a=2λ
Semi circular
wave fronts
First minima & maxima
become visible
a=4λ
Diffraction is the spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.
Diffraction at a single slit
a=λ
a=2λ
Semi circular
wave fronts
First minima & maxima
become visible
a=4λ
Diffraction is the spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.
Diffraction at a single slit
a=λ
a=2λ
Semi circular
wave fronts
First minima & maxima
become visible
a=4λ
Diffraction is the spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.
Diffraction at a single slit
a=λ
a=2λ
Semi circular
wave fronts
First minima & maxima
become visible
a=4λ
Diffraction is the spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.
Diffraction at a single slit
a=λ
a=2λ
Semi circular
wave fronts
First minima & maxima
become visible
a=4λ
Diffraction is the spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.
It can be shown that the first minima occurs when sin Ə = λ/a .
First minima
T
a
Ə
Central maxima
It can be shown that the first minima occurs when sin Ə = λ/a .
First minima
T
a
Ə
Central maxima
Q1 Find the angle at which the first minima occurs using microwaves
of wavelength 3 cm when directed towards a gap of:
1) 6cm
2) 4cm
It can be shown that the first minima occurs when sin Ə = λ/a .
First minima
T
a
Ə
Central maxima
Q1 Find the angle at which the first minima occurs using microwaves
of wavelength 3 cm when directed towards a gap of:
1) 6cm
2) 4cm
Q2 Find the angle at which the first minima occurs using lightwaves
of wavelength 500 nm when directed towards a pupil of diameter:
1) 6mm
2) 4mm
D
D
D
Points to note:
* central fringe is twice as wide as the other fringes
* intensity decreases from the centre
* Central Fringe width W = λ/a x 2D
D
Visit :
http://www.phys.hawaii.edu/~teb/optics/java/slitdiffr/
Points to note:
* central fringe is twice as wide as the other fringes
* intensity decreases from the centre
* Central Fringe width W = λ/a x 2D
Blue light has narrower fringes
So cameras and microscopes
can see more detail
using blue filters
WAVES
a = width of
the gap
For first
minimum
sin  = l/a

Or for small
angles in
radians
 = l/a
How do you get the minima
Q
A
λ
λ/2
a
C
B
D
wave trains arrive
in phase at the
central maxima.
Q First minima
AQ is λ/2 longer than CQ so is out of phase by π giving destructive interference
Corresponding points along AB which are λ/2 apart also cause
destructive interference.
How do you get the minima
Q
A
a
C
B
λ
wave trains arrive
in phase at the
central maxima.
Q First minima
How do you get the minima
Q
λ
A
a
C
B
λ/2
D
wave trains arrive
in phase at the
central maxima.
Q First minima
AQ is λ/2 longer than CQ so is out of phase by π giving destructive interference
Corresponding points along AB which are λ/2 apart also cause
destructive interference.
How do you get the minima ?
CD = λ = a sinƏ
2
2
λ = a sinƏ
Q
λ = sinƏ
a
λ
A
λ/2
a
C
D
Ə
B
Q First minima
AQ is λ/2 longer than CQ so is out of phase by π giving destructive interference
Corresponding points along AB which are λ/2 apart also cause
destructive interference.
Diffraction by a Double Slit
The double slit pattern is superimposed on the much broader single slit diffraction pattern.
The bright central maximum is crossed by the double slit interference pattern, but the intensity still
falls to zero where minima are predicted from single slit diffraction. The brightness of each bright
fringe due to the double slit pattern will be “modulated” by the intensity envelope of the single slit
pattern.
The double slit
fringes are still in
the same place
Single slit
pattern
Double slit pattern
Experimental observations from the double slit
i) For a pair of slits 0.5 mm apart:
λ red » λ blue
ii) Using white light, fringes appear from all the various wavelengths present
and do not overlap exactly, hence coloured fringes
* Inner fringes are tinged with blue on the inside
and red on the
outside
Diffraction
is the
spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.
Experimental observations from the double slit cont’d
iii) Fringes obtained using slits 0.5 mm apart drawn with different widths
(a)
(b)
(a) thin slits
Diffraction is the spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.
Experimental observations from the double slit cont’d
iii) Fringes obtained using slits 0.5 mm apart drawn with different widths
(a)
(b)
The double slit interference
pattern is modulated
by the single slit pattern
(a) thin slits
Diffraction is the spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.
Experimental observations from the double slit cont’d
iii) Fringes obtained using slits 0.5 mm apart drawn with different widths
(a)
The double slit interference
pattern is modulated
by the single slit pattern
(b)
(a) thin slits
(b) thick slits
Missing fringes
Diffraction is the spreading of wavefronts around corners and obstacles.
If the slit gets narrower diffraction increases.
If the wavelength increases diffraction increases.