Light plus light can produce dark
Introduction
Pass laser light through two slits to produce an interference pattern on a distant screen. This interference pattern of bright and dark regions shows that light behaves like a wave. Shine the light through one slit and the laser beam spreads into a line of light. Open a second slit and dark regions appear in this line of light showing that light plus light equals dark.
Materials
Laser (Any visible laser such as the new solid
state, battery operated laser pointers which sell for for under $ 20
and produce about 1 milliwatt of light or less.)
clay (a few tablespoons)
piece of white cardboard
empty 35 mm slide holders or a microscope slide
2 human hairs
some tape
method 1
2 microscope slides
black enamel spray paint
2 sharp knives
method 2
a 35 mm slide holder
aluminum foil
tape
razor blades
Assembly
Method 1
Use enamel spray paint to paint one side of the microscope slide
black. Allow it to dry.
(Optional, blacken the slide with soot from a candle.)
Make a narrow slit in the paint 2 cm from one end of the slide using
the knife blade. To make a straight slit, you can use a second
microscope slide as a straightedge. We use knife blades rather than
razor blades because they make wider slits. Wider slits allow more
light to get through and so make a brighter pattern. Wider slits make
narrow patterns of laser light, however, the patterns will be wide
enough.
Make 2 closely spaced narrow slits in the paint about 2 cm from the
other edge. Make the 2 slits by drawing the knife along the
straightedge, moving the straightedge and then cutting the second
slit. You might have to experiment to obtain the best distance
between the slits. The slits must be close enough together that the
laser beam illuminates them both at the same time, yet far enough
apart to give a visible pattern of dark bands. The two slits should
be 0.5 mm apart or less.
Method 2
Tape a piece of aluminum foil over the hole in one slide holder.
Put the slide down onto a piece of cardboard.
Use the razor to cut a slit into the aluminum.
To cut 2 slits use 2 razors held side by side.
Tape down one hair across the empty 35 mm slide holder, or on a clear
microscope slide. Tape down a second hair close to the first one. You
have then made the opposite of two slits, two anti-slits.
To do and notice
Shine the laser through the single slit and onto the white cardboard. Mount the laser and the microscope slide 2 cm or so apart using clay. Mount the white cardboard 2 meters from the laser (for example tape it to a wall.)
A laser shines a beam through a slit onto a screen. The beam spreads via diffraction.
Notice the pattern on the wall. The laser beam
spreads out when it goes through a single slit. It spreads along a
line perpendicular to the slit. You may see that it spreads into a
line of light and dark blobs. The central blob will be the widest and
the brightest. Examine the central blob.
Shine the laser through the 2 slits. Notice that the central blob of
the line of light is broken by many dark regions.
Use the knife to cover one of the slits. Then uncover the slit.
Notice that the dark regions appear when the slit
is open and they disappear when it is closed.
Thus light plus light can make dark.
Replace the microscope slide with the hairs. Notice that the hairs
produce a pattern of blobs of light similar to that produced by the
slits.
Whats going on.
The laser produces a very pure color of light,
i.e. light of one wavelength.When the laser beam illuminates the 2
slits, the light going through the slits is in phase, i.e. the crests
and troughs of the lightwaves arrive at the slits together. (We say
that the laser light is coherent.) The light from the slits
illuminates the cardboard screen. Where the light arrives at the
screen having travelled the same distance to the screen from each
slit, the waves arrive in-phase and there is a bright blob.
However, to the side of this point, the path of light from one slit
to one point on the screen is longer than the path from the other
slit. When the path difference is equal to one-half a wavelength the
crest of the lightwave from one slit arrives at the wall just as the
trough from the other slit arrives. These two waves cancel and
produce darkness.
Further to the side, the path lengths differ by one full wavelength.
Once again the crests line up and a bright blob appears.
To study the interference of light further see the snack Chain
Interference Analogy.
etc
In 1801 Thomas Young observed the diffraction of light through a
slit. This observation indicated that light was a wave. After all,
light plus light equalled dark. It was easy to conceive of two waves
adding out of phase to create darkness whereas it was hard to
visualize two particles coming together to cancel each other.
Interference is a poor name for this phenomenon, When two light beams
come together, they simply add to each other. The two beams do not
interfere with each other in the english sense of the word. Two light
beams pass through each other without affecting each other at all.
Light interacts with charges, light itself does not possess electric
charge. So light does not interact with light it simply adds
together.
etc
To cut good slits in aluminum foil fold over the aluminmum foil
several times, e.g. double it over twice, cut the slits, use the
pieces of aluminum foil from the center of the stack.
Scientific Explorations with Paul Doherty |
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21 Apr 99 |