Lesson LVII
Nature of Light, Huygens’
Principle, Interference: Young’s
Double-Slit Experiment
Nature of Light
• Issac Newton – light was particle in nature
(corpuscles)
o Lasted until 1830 when scientists found that light traveled in waves, which
explained interference and is not possible with a particle explanation.
o Light has properties of wave-like and particle-like motion.
• Dual Nature of Light
o When we study light we must look at it from a particle or wave like motion,
but never both at the same time (Niels Bohr)
Huygens Principle
• Christian Huygens proposed a wave theory of light
that showed the propagation of a light wave could
be predicted geometrically by considering each
point on a wave front to be a source of a spherical
secondary wavelet.
Huygens Principle
• Similar to waves in water being created from a
pebble dropped in it, light waves move in
concentric spheres from a source.
• All wavelets move from the original wave front at
the same speed.
• Huygens’ Principle – Every point on a wave can be
considered as a source of tiny wavelets that spread
out in the forward direction at the speed of the
wave itself. The new wave front is the envelope of
all the wavelets that are tangent to all of them.
Diffraction
• Light is bent when it goes through a small opening
Diffraction
• Thomas Young was able to show light diffraction in
an experiment. He was even able to determine the
wavelength of visible light from his experiment.
• This showed how there were flaws in the
“corpuscular” theory of light.
Interference: Young’s
Double Slit Experiment
• In Young’s famous experiment, he used sunlight
which fell on a screen containing two closely
spaced slits. The slits were spaced a very small
distance apart.
Young’s Double Slit
Experiment
• If light was a particle, he expected two bright lines
to appear. Instead Young observed series of dark
and bright lines called fringes.
Young’s Double Slit
Experiment
• We imagine light approaching the slits as uniform in
wavelength (monochromatic). The waves spread
out after passing through the slit.
Young’s Double Slit
Experiment
• θ is the angle that a ray is deflected from the
middle.
Young’s Double Slit
Experiment
• The diffracted waves interact and create areas of
constructive and destructive interference.
o There is a phase shift in the wave that has to travel a longer
distance.
• One ray travels further than the other, except in the
very center of the screen where central maximum
occurs.
• Constructive interference occurs when the paths of
the two rays differ by whole number multiples of
their wavelengths.
• Destructive interference occurs when one ray has
to travel an extra distance of ½ a wavelength.
Young’s Double Slit
Experiment
• Constructive and Destructive Interference
o dsinθ = mλ where m is order of the interference
fringe.
• m = 1 is the first fringe on either side of the central
maximum.
• Constructive occurs at m=1, 2, 3, 4, etc…
• Destructive occurs at m = ½ , 3/2 , 5/2, etc…
• The intensity of the light decreases as you move
away from the central maximum.
Young’s Double Slit
Experiment
• Through a little geometry, we get
o Xm = mλL/d
Example I
• Monochromatic light falls on two slits,
0.035mm apart. A fifth-order fringe
makes an angle of 9.5° to the central
maximum. What is the wavelength of
the light?
Answer
• 1.6x10-6m
Example II
• A screen is placed 1.20m from two
narrow slits spaced 0.100mm apart.
Light fron a distant source is incident
on the slits. The light has a wavelength
of 500nm. A) Find the distance from
the central maximum to the first bright
fringe. B) Find the distance from the
central maximum to the second
minimum.
Answer
• 6.00mm, 9.00mm