Diffraction of waves
M.K.Rolley
Wave Behaviour
Diffraction. Occurs when waves pass through gaps.
Waves spread out as they
pass through gaps.
Wave Behaviour
3. Diffraction. Occurs when waves pass through gaps.
Diffraction works best if
gap  wavelength.
Wave Behaviour
3. Diffraction. Occurs when waves pass through gaps.
If gap is too large most of
energy gets through, but
most of waves pass
straight through so
diffraction is ineffective.
Easy to see with water...
But what about light?...
Problem is small size of
wavelength…and how do we know light
is a wave anyway?
 Need gaps about the same size as the
wavelength if we are to see effects but
Newton thought waves were particles
so who sorted it out for us??

He did in 1801….
Thomas Young 1773-1829
Discovered how eye focussed,
astigmatism, colour theory and that
light had wave properties and was a
transverse wave.
 did work in surface tension, elasticity
(Young’s modulus)
 Was an avid Egyptologist
 Spoke Latin ,French, Greek,
Italian,Hebrew, Arabic and Persian (At 14)

The Problem: Newton thought that light was a
stream of particles but if it is, how is a shadow
like this explained?
What about
these bits?
Young’s two slits experiment:

Young challenged Newton’s theory (not
done at the time) using apparatus
similar to that below:
What Young saw:
These are called ‘fringes’
Note areas of ‘canceling’ of light
intensity
 Note also degree of diffraction …higher
degree of spreading the shorter the
wavelength

What it meant:
The dark areas could only be explained
using the principle of superposition
(where a ‘crest’ meets a ‘crest’ there
will be reinforcement ie a bigger crest
-a bright fringe. Where a crest meets a
trough there would be cancellation ie a
dark fringe
 Hence Newtons particle theory was
shown to be incorrect or to be more
precise only partially correct because...

ITS ACTUALLY BOTH!
Later on it was shown that there are
some effects of light that are only
explained if light has a particle nature
 It can also be shown that:

Wavelength = Slit separation x fringe width
distance from slits screen