Transcript Diffraction of Light

Diffraction
 The bending/spreading of waves as they go
through gaps or around edges
 The effect is greatest when gap width is equal to
or smaller than the wavelength
Diffraction of Light
 If light passes through a very thin slit it forms a
diffraction pattern
 It is seen as a bright central fringe with dark and
bright fringes on either side
 As the gap width increases the pattern width
decreases
Two Source interference
 Remember this
pattern with water
waves?
Two Source Interference
 Remember this
pattern with water
waves?
 Anti-nodal lines are
lines of constructive
interference.
 Nodal lines are lines
of destructive
interference.
Double Slit Interference
Screen
 When
monochromatic
(single colour)
light passes
through 2
closely spaced,
thin slits, the
waves overlap
and form an
interference
pattern.
Dark
Bright
Dark
Bright
Dark
Bright
Dark
Double Slit Interference
 The pattern is seen on a screen as evenly
spaced bright and dark fringes
Double Slit Interference
n=1
n=0
n=1
 The fringes are
numbered by their
order
 The central fringe is
n=0
 The first fringe either
side of centre is n=1
 The second n=2 etc.
Double Slit Interference
 For constructive
interference (bright) the
waves must arrive at the
screen in phase so the
path difference between
the two interfering
waves must be a whole
multiple of the
wavelength
 Path diff. = nl
Path
difference
Double Slit Interference
 For destructive
interference (dark) the
waves must arrive at the
screen out of phase so the
path difference between
the two interfering waves
must be a half multiple of
the wavelength
 Path diff. = (n+1/2)l
Path
difference
Double Slit Interference
S1
q
 d=distance between slits
 n=order number
 l=wavelength
q
d
pd
S2
To
screen
 The angle of the
fringe can be
calculated from the
formula below
d sin q  nl ( p.d .)
Double Slit Interference
 The angle of the fringe can also be worked out
using the following formula
 x=distance between centre and fringe
 L=distance from slits to screen
x
q
L
Bright
x
Slits
Central
bright
q
L
Double Slit Interference
 So the spacing of the fringes depends on:
The distance between the slits
The wavelength of the light used
How far away the screen is from the slits
(NB sinq≈q for small angles)
x
d sin q  nl  d
L
Diffraction Gratings
 A diffraction grating is
a series of many (eg.
6000 per cm) very
fine parallel slits,
closely spaced, on a
piece of glass or
plastic
Diffraction Gratings
The interference pattern produced is
similar to the double slit pattern
The differences are:
There are lots of slits so fringes are brighter
Slits are closer together so fringes are widely
spread
Slits are narrow so the light is diffracted through
a wider angle (almost 180°)
Diffraction Gratings
n=2
Diffraction
Grating
 If we shine white light
onto a grating, we
red
produce a series of
n=1
spectra each side of
the central fringe
violet
 This is because white
White n=0
light is made of many
violet
frequencies (colours)
which all diffract at
n=1
slightly different
red
angles
n=2
Diffraction Gratings
 The spectrum produced by a grating is more
widely spread that that produced by a prism
 It is also the other way around (ie red is
diffracted the most)
Diffraction Gratings
 The formula for
working out angles
with a diffraction
grating is the same
for two slit patterns
 However, often N, the
number of slits per m
(or slits per cm) is
given.
 Slit spacing d is
related to N by:
1
d
N
Diffraction Gratings
 CD surfaces can act
like diffraction
gratings because it
has many finely
spaced lines on it’s
surface.
 Light is reflected off
the disc, but produces
spectra in the same
way
Diffraction Gratings
Diffraction gratings are a useful tool for
determining the chemical composition of
substances.
This is down by analysing the light
produced when the atoms are excited by
heat or electricity.
This is how astronomers can tell what
stars are made of.