Interference and Diffraction
Light is a wave. As such, it can undergo interference effects, much like what we see with
sound waves or water waves. In order to observe interference between two waves, the
waves should have the same frequency (or wavelength) and be ‘coherent’. By ‘coherent’,
we mean that the two waves should maintain the same ‘phase’ difference for an extended
period of time. Coherence cannot be obtained between two separate light sources, even if
they are the same ‘color’. To obtain coherent sources, we need to take a single source and
split it into two or more sources. In this activity you will observe interference effects using
various light sources and means of splitting a single source of light into multiple sources.
Preliminary Questions:
1. Rainbows in the sky are caused by
(a) interference (b) dispersion (c) absorption (d) scattering
2. A rainbow pattern observed when viewing a soap bubble is caused by
(b) interference (b) dispersion (c) absorption (d) scattering
Cornell Slit Film
The Cornell slit film consists of a dark film sandwiched between two glass plates. The
film has various patterns through which light can pass to produce interference effects.
Study the film and the accompanying guide. You will see that it consists of single slits of
different widths, double slits of different widths and spacing, and multiple slits of various
widths and spacings. The slit dimensions can be determined from the guide.
Double-Slit Interference
One of the simplest ways of observing interference is to shine a single source of light onto
two closely-spaced slits. The light passing through the slits will have the same wavelength
and phase. When the light is projected onto a distant screen, you will see bright and dark
lines (or spots), depending on whether the light from the two slits arrive in phase or out of
phase, as illustrated below.
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At the central bright line, the light travels the same distance from the two slits. At the first
dark line, the light from one slit travels ½ wavelength further than the light from the other
slit.
Prediction: How does the interference pattern on the screen change as you change the slit
separation? That is, if you decrease the slit separation does the separation of bright lines
on the screen increase, decrease, or remain the same?
Check it out: Shine the laser pointer onto one of the double-slits and project the light onto
a screen a few meters away. Note the interference pattern. Try slits with different spacing
and check your prediction. (Caution: Don’t shine the laser beam directly into someone’s
eye.)
Multiple-Slit Interference
If you use several closely spaced slits instead of two slits, you should find that the bright
lines become sharper. The spacing of the lines will depend on the separation of adjacent
slits, just as for the double-slit case. Check this out using various multiple-slit patterns.
Diffraction Grating
A ‘diffraction grating’ consists of a very large number of very closely spaced slits. Your
grating is labeled as having 500 lines/mm. This means that the slit separation is
d = 1 mm/500 = 0.002 mm. This is much smaller than the line spacings for the Cornell
slits. So you should expect to see very sharp and widely spaced bright spots when you
shine the laser through the grating and project the light onto a screen. Try it out.
You can also observe diffraction by reflection from a (reflecting) grating. A compact disk
(CD) consists of a pattern of closely spaced reflecting tracks. Shine your laser pointer onto
a CD and observe the pattern of the reflected light. Note the angular direction of the
diffracted spots relative to the orientation of the tracks. Now repeat using a DVD. Is the
angular spread of the diffracted spots from the DVD larger or smaller than from the CD?
What does this say about the density of information that can be stored on a DVD relative to
a CD?
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Wavelength dependence
Now let’s think about how the interference pattern resulting from double or multiple slits
depend on the wavelength of light. So, let’s make a prediction and check it out.
Prediction: If you use blue light instead of red, does the separation of the bright lines on
the screen increase, decrease, or remain the same? It might help to study the figure on the
first page when trying to formulate your answer.
Check it out: Now let’s check your prediction by viewing light of different colors. We
have a vertical array of light emitting diodes (LEDs) of different colors. Put the diffraction
grating near your eye and view the array. How do your observations compare with your
prediction?
Light Emitting Diodes (LEDs)
An LED can provide a simple source of light with a narrow color band. They only require
a few volts for operation and they are cheap. The sheet at the end of this handout gives
some background on LEDs. You have a red LED and a green LED. The LED can be
powered by inserting it into the LED tester. (It must be inserted with the correct
orientation. If it doesn’t light up, then reverse the connections.) Use the diffraction
grating to observe the light from each of the LEDs. You can also observe the LEDs with
the slit patterns on the Cornell slit by placing the slit next to your eye. Note again how the
angular spacing of the interference bands depends on the slit dimensions.
Interference from a White Light Source
Hopefully, you have now learned that the spacing of an interference pattern using multiple
slits increases with increasing wavelength. This means, for example, that if you view a
white light source with a diffraction grating you will see a spectrum of colors, with red at a
greater angle than blue. Check it out. Try this also using the Cornell slit patterns. It
works well when viewing the filament of a clear incandescent bulb.
Diffraction
The word ‘diffraction’ refers to the interference of light coming from different parts of a
single opening. For example, if you pass a monochromatic light source through a single
slit, then the light emerging from the top part of the slit may interfere either constructively
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or destructively with the light emerging from the bottom part of the slit, depending on the
angle at which the light is projected onto a screen. The resulting interference pattern is
more complicated than double slit interference. (Actually, when you view a double-slit
pattern, you are seeing a combination of interference from the two slits and diffraction
from each of the slits.) Generally speaking, you will see a central bright spot and much
weaker spots to the left and right of the central spot. The width of the bright spot should
increase as the slit width decreases. Check it out using the Cornell slit film and the laser
pointer.
Further Study
1. Think about ways that you can observe interference and diffraction effects without
using a diffraction grating or the Cornell slit film. For example:

Make a single slit by bring the edges of two razor blades close together.

Make a double slit pattern. Deposit a smoke film onto a glass plate. Tape two
razor blades side-by-side and use them to scratch two parallel slits.

Look through a screen.
2. What is the concept behind thin-film interference? Why do different parts of a soap
bubble have different colors?
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Notes about LEDs
An LED is a Light Emitting Diode. Although it is not a pure monochromatic light source,
it emits light in a relatively narrow band of wavelengths. LEDs can be purchased that emit
in colors ranging from red to blue.
So, how do LEDs work? A LED is a p-n junction diode, consisting of a ‘p-type’
semiconductor joined to an ‘n-type’ semiconductor. A semiconductor has an energy band
gap separating the valence band and the conduction band. In a pure intrinsic
semiconductor at low temperatures, the lower energy valence band is completely filled and
the upper energy conduction band is completely empty. A p-type semiconductor is doped
with elements that create some vacant levels in the top of the valence band (‘holes’). An ntype semiconductor is doped to create occupied levels in the bottom of the conduction
band.
I
A p-n diode conducts electricity easily when a small
voltage, V0, is applied across the diode with a
certain polarity and very weakly when applied with
V
the opposite polarity. A plot of the current through
a diode as a function of the applied voltage would
V0
look something like this.
At the turn-on voltage, V0, electrons in the conduction band make transitions to the valence
band at the junction, and their energy loss appears as photons (quanta of light) with energy
given by
eV0 
hc

,
where e is the charge of the electron, h = Planck’s constant, c = speed of light, and  =
wavelength of the light. The wavelength and color of the light is thus determined by the
turn-on voltage, which depends on the particular type of material that the LED is made of.
Turn-on voltages for LEDs range from about 2 V (red) to 3 V (blue). Laser pointers
consist of LEDs that are appropriately designed (e.g., polished end surfaces) so that lasing
action occurs. They are much more monochromatic than LEDs.
The current through the diode increases
rapidly with increasing voltage above V0.
Thus, when operating a diode it is important to
limit the current to avoid burning out the
diode. This can be done by putting an
appropriate limiting resistor in series with the
diode as shown below. The current should
typically be less than ~ 100 ma.
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R

V
LED