Exploring diffraction gratings Here are some things to think about and do as we start the unit on
diffraction gratings and the spectrum. These questions can be answered by simple experiments
that will only take a few minutes. Do most of them, and for the rest discuss
with your group how you might do the experiment. You also may be
able to discuss what you predict will be observed (if it is a situation
with which you are already familiar).
Find a diffraction grating in the
kit. They look like this:
Hold the diffraction grating right up to your eye, and look through it.
Pretty confusing!
Turn out as many lights in the room as possible, or find a big closet without
lights, or just go outside at night. Then look through the diffraction
grating at just one light -- a flashlight,
or a clear bulb in the lamp base in the kit. Now you should see a pattern
of "rainbows" (which we will call spectra). Please play with the diffraction
grating for a minute, to find out how it behaves when you look through
different parts, turn it over, or rotate it.
Each light in sight will give several spectra (rotate the grating so that
the spectra are spread out to right and left, not up/down - this is the
conventional way to hold a grating). You'll see bright spectra to
both sides, and you'll also see dimmer spectra farther to the right and
left. If you have more than one light source in the room, there will
be several such patterns present. You'll even notice spectra associated
with light that we usually ignore - such as the reflection of a light bulb
on a shiny table top. By turning the diffraction grating and by blocking
your view of various lights, determine which spectrum is caused by which
light. Describe the spectral pattern due to just one light, noting such
things as the order of the colors, and the relative width of the spectra.
Observe how the spectrum of a clear light bulb changes as you move closer
to it. Can you use this to determine how far away a light bulb is?
Look at an ordinary frosted light bulb through the grating. Compare the
color effects you see when you are far from the light bulb and when you
are close to it.
As part of your discussion, write something in your notebook about your
answers to some of these questions -- what you decided, which questions
you were sure of, which ones don't seem to make sense.
All done? On
to the first activity!
Plan ahead: the next activity is to be done at night.
As part of your discussion, write something in your notebook about your
answers to some of these questions -- what you decided, which questions
you were sure of, which ones don't seem to make sense.