MadSci Network: Physics

Re: Thomas Young's Light Experiment

Date: Mon Aug 2 19:10:45 1999
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Physics
ID: 932618229.Ph


The interference between rays of white light, as illustrated in most text 
books, is in reality very difficult to duplicate because they tend to be 
 simplifications that are designed to explain a particular topic and not to 
describe an experimental procedure. To demonstrate the wave like 
interference effects caused by two beams of white  light interacting, which  
Young demonstrated in 1800, requires that the light in the two paths have 
exactly the same distribution of wavelengths and that each wavelength 
(color) in one path be exactly in step (in phase) 
with the same wavelength in the second path to within a fraction of a 
wavelength in distance. Scientists call this SPATIAL COHERENCE between the 
beams. Until the development of the first laser in 1960, coherent sources 
of light, particularly white light, were very difficult to obtain. 

The spectrum of white light ranges from red wavelengths that are about 0.7 
micrometers long to violet wavelengths that are about 0.4 micrometers long.  
Quantum mechanics teaches us that there are over 320 thousand, million 
(trillion) different wave lengths of light (colors) in the white light 
visible spectrum. The red waves are almost two times as long as violet 
waves and wavelength dependent interference and diffraction phenomena are 
easily obscured and smeared by this large number of wavelengths in white 
light, each of ray of which must have the correct phase relative to its 
neighbor in the beam . 

Today, single wavelength, coherent, laser light makes it very easy to 
duplicate Youngs experiments. Red wavelength diode laser pointers are now 
available for less than $10 in many electronics and hardware stores. To 
duplicate Youngs experiment with a laser pointer we can we poke two very 
small pin holes, spaced about 1/2 millimeter (about 1/64 inch) apart, in a 
sheet of aluminum foil.  Then we can tape the foil and a laser pointer to a 
table surface with the laser pointer about one meter (3 feet) from
the aluminum foil and with the laser beam illuminating the two pinholes. By 
projecting the resultant beams passing through the pinholes onto a white 
sheet of paper located about two or more meters (6 feet) away from the pin 
holes in the foil, we can see dark and bright interference bands in the 
projected laser spot. Scientists call these bands fringes. 

If you conduct the laser experiment you will notice that the fringes of the 
interference pattern do not form until the paper is about a meter (3 feet) 
away from the pinholes and that the fringes become much more distinct as 
you move the paper greater distances from the pinholes. After the coherence 
problem, sufficient light intensity for experimentation was another problem 
faced by Young in his white light experiments. You must reach the 
Fraunhofer (far field) region of the pinholes before the interference 
fringes between two beams can be formed as variations in light intensity. 
The Fresnel (near field) region, which  begins at the pinholes and extends 
to the begining of the far field, is the reqion in which the fringes are 
being generated; however they are not visible as variations in light 
intensity. The Fraunhofer distance (F) from the pinholes can be calculated 
by dividing the square of the distance between the pinholes (D) by the 
wavelength of the light source (L) divided by 2. 

F = (2 x D^2) /L

For example: 
If the distance between the pinholes is one millimeter and the laser 
pointer operates at a wavelength of 0.63 micrometers, the far field begins 
about 317 cm (125 in) from the pin holes. 

If the pin holes are spaced 1/2 millimeter apart,  the far field begins 
about 80 cm (32 in) from the pin holes. 

While the tight laser beam can form bright fringes projected on the 
paper, doing this same experiment with white light passing through the pin 
holes would form extremely faint fringe patterns. However with a bright 
light bulb or sunlight we can still demonstrate Youngs experiment.

Let me describe a simple experiment that Young used to observe white light 
interference effects that overcomes the problems with coherence that I have 
discussed above.

The materials required for a white light experiment are :

1) a cardboard or plastic tube or pipe approximately  5 cm (2 in) diameter 
and about 1 meter (3 feet) long. 

2) Several 10 cm (4 in) squares of aluminum foil. 

3) Masking tape or Scotch tape.

4) A sharp needle or pin.

5) A table top or similar surface on which to mount the experiment.

6) A clear glass light bulb with a linear filament.

Form a tight aluminum foil diaphragm over one end of the
tube and tape the outside of excess foil tightly around the tube to make a 
cap so that you can slide the foil diaphragm on and off the tube while 
aligning the light rays during the experiment.

With a needle or a pin, poke the smallest hole possible in the center of 
the foil diaphragm. Do not push the pin all the way through the diaphragm 
for the pinhole will then be to large. Next look into the open end of the 
dark tube and practice aiming the pinhole toward a hot light bulb filament, 
that is located about 3 meters (9 feet ) away from the pin hole (later you 
might try a candle). You should be able to see the projection of the pin 
hole on the dark inside walls of the tube to help you align the beam. 

When you use a candle in place of the bulb you will observe that they 
are a very weak source of light, especially after passing through the small 
pinhole. This is why scientists such as Newton and Young used mirrors to 
direct sun light into their darkened laboratories for a light source in 
place of candles. However, on  cloudy days they still had to use candles or 
stop experimenting.

Using a pinhole is a technique that makes all of the white light rays 
inside of the tube have the same distribution of wavelengths and phases 
(spatial coherence), because all of the rays reaching the end of the tube 
come from the same bundle of light entering the tube from the light source. 
The pin hole rejects other off axis light rays from the light source that 
will have different wavelength and phase distributions that would spoil the 
inteference effect. The spatial coherence produced by the pinhole light 
source is not as good as that obtained with a laser, but it was good enough 
for Young to first demonstrate the wavelike nature of light. 

Now make a second aluminum foil diaphragm and cap on the open
end of the tube and secure it with tape. Poke two small pin holes, side by 
side, in the center of the diaphragm spaced about one half of a millimeter 
(about 1/64 inch) apart. 

Now to observe Youngs interference fringes you will
need to tape the tube onto a rigid surface or tripod and align the beam 
from the single pin hole so that it illuminates the two pin holes on the 
second diaphragm. This is best done by moving the light bulb back and forth 
or by tilting and rotating the tube on the head of a tripod.

The projected white light beams that emerge from the two pin holes will be 
extremely dim at the far field distance and it is amazing that Young could 
see and study them. To see the dim fringes you must mask out all of the 
extraneous light from the bulb and be sure that your eyes are dark adapted, 
and then you might be able to see the fringes in the white light beam 
projected onto a paper. However, we can more easily view the white light 
interference fringes in a slightly different manner. 

A lens can be used to transform near field light beam patterns into the far 
the field patterns in a very short distance. This method also makes a much 
brighter light spot with which to conduct experiments. Our
eyes have an excellent lens to do this transformation of the near field 
pattern into the far field patteren required to observe fringes. This 
transformation occurs in a distance of 2.5 cm (1 in), the focal distance 
distance from the lens to the retina! 

If we look very closely through the two pin holes and observe the lighted 
single pin hole at the other end of the tube, with proper alignment, we 
will observe a long band of bright and dark fringes extending on each side 
of the single pinhole. These are Youngs fringes being transformed  and 
generated inside of our eye. You might also observe that the fringes have 
slightly rainbow colored edges because the different colors in the beam are 
slightly out of step (out of phase) with each other. 

With patience and a very dark room I was also able to use this experiment 
to see fringes from a candle flame as Young observed them.

Best regards, Your Mad Scientist
Adrian Popa



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