Light
WAVE NATURE OF LIGHT
Is light what we see? Is there light where there is no one to see it?
Dualism of light
Isaac Newton believe light to be a ray of
corpuscles (particles)
Robert Hooke believed light
to have the wave-like nature
Newton won… until over a century after his death…
LIGHT INTERFERES!
1801 – Thomas Young produces
interference of light
Light is a chameleon of science
Particles which interfere like waves?
A wave that travels in straight lines
Three views on light
Waves
Light rays
Photons
Most common
Visible light – part of EM
spectrum
Interference and
diffraction of light
Light travels in straight
lines
Optics: mirrors, lenses,
etc.
Photoelectric effect
Photons as particles
(quanta of energy)
Quantum mechanics:
Not so much
Covered in HS physics
Waves or not?
Waves diffract (like sound). Light doesn’t
seem to
Waves or not?
When the opening is comparable to the
wavelength, shadows are cast
Light does diffract when it
goes through a tiny slit
Young’s Experiment
Constructive and destructive interference
Constructive and destructive interference
The waves coming from the slits interfere constructively or
destructively, depending on the difference in distances between
the slits and the screen.
  d sin 
sin   m

m  0,1,2,3, 
d
sin   m 
1
2


d
m  0,1,2,3,
Example
Red light (664 nm) is used in Young’s experiment with slits separated
by 0.000120 m. The screen is located a distance 2.75 m from the slits.
Find the distance on the screen between the central bright fringe and
the third-order bright fringe.
q = 0.951
x3 = 0.0456 m
Formula for ‘bright spots’
sin   m

d
d sinq = ml
m  0,1,2,3, 
m = 0,1,2,3,…
xm = L tanq
Providing the screen is very-very distant,
sin q = m
tanq » q » sinq
l
d
ml
xm » L
d
Guess: what would the formula look like for dark spots?
Measuring wavelength of light
A double-slit interference pattern is observed on a screen 1.0 m behind two slits
spaced 0.30 mm apart. The distance from the center of one particular fringe to the
center on the ninth bright fringe from this one is 1.6 cm. What is the wavelength of
the light?
l = 540nm
Double-slit interference pictures
Single-slit diffraction
a
L
Huygens’ Principle
Diffraction
The wavelets going straight forward
all travel the same distance to the
screen. Thus they arrive in phase
and interfere constructively to
produce the central maximum
Note the brightness (intensity)!
Dark spots
Ray 2 is a/2 away from ray 1. If the two rays arrive counter-phase (trough /
crest), destructive interference occurs. Ray 2 travels a longer distance by
a
Dr12 = sin q
2
Destructive interference will occur if:
Dr12 =
l
2
Dark spots
Position of destructive interferences
(dark spots)
L a
sin q » q » tanq
pl
xm » L
a
p = 1,2,3, 4...
Note that p is not 0! That would be
the central bright stripe
How wide would the stripe be?
How does the picture change??
2l
w=L
a
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Grating Diffraction
ml
xm » L
d
Sometimes More IS More