Feathers are made of closely spaced parallel fibers, and these can give
diffraction effects. If you have a feather, look through it at a single
bright light bulb and try to see the diffraction patterns.
The colors of a soap film are a diffraction effect. If you make a film
by dipping a loop of wire in soapy water and then hold it vertically, you
will see bands of color forming. These indicate that the film is wedge-shaped
-- it's thicker at the bottom, because the liquid drains downwards -- and
the color that is reflected depends on the thickness. You can tell that
this is a diffraction effect, because the apparent color of the film depends
on the angle from which you view it.
The second picture shows how the first picture was made. Here
we're shining a spotlight onto a uniform white surface (a piece of white
posterboard). The light reflects from the white board and shines onto
the soap film. Turn the film holder slowly until you can see the
strongest color pattern in the film. It's also important to have a
black screen (black fabric or poster board or paper) behind the
soap film (to the right of the white screen in our set up),
so that light reflected from the film doesn't get confused with light
coming from things behind the film.
The white card and the black background are important to making the colors
visible.
Does a diffraction grating also reflect light as well as transmit it?
Compare the color effects in reflection from a diffraction grating to the
color effects in transmission through it.
Another way to make a spectrum is to use a prism. This is not the
same mechanism at all! (However, the colors of a diamond's sparkle
and of a summer storm's rainbow are due to the prism effect).
We will talk about prisms later; but you could try to compare a prism spectrum
to a diffraction grating spectrum now.
When you see a rainbow outside caused by sunlight on water drops, what
is the color order? Which color is on the inside of the arc? Sometimes
there is a second rainbow: are the colors in the same order? Are
the colors in the fainter spectra in a diffraction grating in the same
order as the grating's more central brighter spectrum?
(You don't have to wait for a rainy-day rainbow -- you can make your
own with a lawn sprinkler. You need to stand so that the shadow of your
head is near the sprinkler mechanism).
Will a diffraction grating work if it is wet or soapy? What happens
if it gets oily? (We do not recommend that you do this to many of
your gratings - only to a few to see what results.)
