Diffraction is a topic at HL and also for the SL Sight and Waves option. Nothingnerdy Flippingphysics slideshow.

1. presents
a production
Diffraction
1 Background Image: PHET

2. Simple diffraction
Wave energy spreading out when it passes an
edge or an aperture (aperture is really two edges)
"no-one has ever been able to define the
difference between interference and
diffraction satisfactorily. It is just a
question of usage, and there is no
specific, important physical difference
between them." Richard Feynman

3. Changing wavelength
Diffraction is greater if the wavelength is longer.
At the edge, some wave
energy is diffracted
obstacle
(spreads into the shadow
of the obstacle). More
energy spreads out if the
wavelength is longer.
obstacle
Images: Kiselev

4. Changing aperture
Aperture = hole = gap = slit = two edges
There is greater
diffraction when the
wave passes through a
narrow aperture.
Images: PHET

5. More diffraction
The amount of diffraction depends on both
the wavelength and the width of the aperture.
It depends on the ratio of
wavelength/ aperture width
Diffraction is greatest if the wavelength
is large compared with the aperture.

6. Diffraction examples
Radio
Electron
Water
X-ray

7. Diffraction pattern
When the Central maximum First minimum
aperture is
larger, it Second minimum
behaves not Wave intensity graph
like a single
source, but
like a number Pattern on screen
of sources
spread
across the
aperture. Incident
wavefronts Image: Kiselev

8. Light through a slit
Changing slit width,
constant wavelength
A narrower slit
(aperture) causes
the wave energy to
spread out more.
A wider slit causes
the diffraction
maxima to be closer
Images: Kiselev
together.

9. Light through a slit
Changing wavelength,
constant slit width
A shorter wavelength
causes the energy to
spread out less.
A longer wavelength
causes the diffraction
maxima to be wider
apart
Images: Kiselev

10. Path differences
Image: PHET Central max
path diff = 0
The path difference
is how much
further the wave
from the top half
of the slit has
travelled than the
wave from the
bottom.
First max First min
path diff = one wavelength path diff = half wavelength

11. Diffraction formula
The path difference for the The waves from the top
first minimum is half a and bottom halves of the
wavelength. AB is the slit of slit interfere destructively
width b. at a distant screen. We can
A assume the rays are
parallel since the screen is
far away.
90˚
b b/2
B

12. Diffraction geometry
The angular position of the diffraction Units
minimum is proportional to the wavelength
Angles
and inversely proportional to the slit width.
in rads;
Lengths
in any
x
unit so
D long as
they are
the
D = screen distance same in
x = position of first min. (or half-width central max.) each
formula.

13. Diffraction practice
We can calculate the angular position of the minimum using the
distance of the screen from the slit, the distance of the minimum
from the centre of the pattern and the small angle approximation.
A laser is diffracted at a slit of width 0.20 mm. It
forms a pattern 2.2 m away. The first minimum is
6.5 mm from the centre of the pattern. What is
the wavelength of the light? [590 nm]
If a slit diffracts 650 nm light so that the diffraction
maximum is 4.0 cm wide on a screen, what will be
the width of the diffraction maximum for light of
wavelength 420 nm? [2.6 cm]

14. Diffraction examples
The regular surface of a CD diffracts
different wavelengths of light at different
angles and produces a spectrum.
Diffraction pattern
of a square aperture
is 2-dimensional.
Photo of X-ray diffraction pattern of
DNA taken by Rosalind Franklin and
interpreted by Crick and Watson.
Image: sciencephoto.com

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