I
J
analyzing other spectra, such as through a sample
of transparent material or liquid.
To do this, you position the reference light to
shine through the top half of the slit, and shine a
broad-spectrum light source through your sample,
covering the bottom half. Use a bit of aluminum
foil to divide the slit into two parts, separating the
calibration light from the sample light.
As an example, I used the fluorescent tube as
a reference for testing some green clear plastic lit
from behind with a white LED.
Upload the double spectrum image to the spectrum analyzer with the first checkbox checked, and
you’ll get a plot of both against each other. This
analysis results in 2 graphs: the calibration on top,
like the graph we just looked at, and the sample
HOW DIFFRACTION
GRATINGS WORK
Our diffraction grating is a transparent sheet of plastic
with dark lines on it spaced 1/500
of a millimeter (2μm) apart.
Light waves pass through the
spaces between the lines, called
slits, and interfere with waves going
through adjacent slits to produce
bands of light or dark where the
interference is constructive or
destructive.
Light bands form where the
wavecrests from adjacent slits
are both an integer number of
wavelengths away from the
diffraction grating. Since this
depends on the wavelength, it
causes different colors to form
bright bands in different places,
separating them into the rainbow
we call the spectrum.
spectrum on the bottom (Figure J).
The plots have a nanometer scale at the bottom,
which we can verify against the 4 brightest lines in
mercury’s emission spectrum, as listed in the
Handbook of Chemistry and Physics (CRC Press,
2009): 435.8nm, 546.1nm, 577nm, and 579.1nm.
Since both sources pass through the same slit
and the same camera, calibrating this way guarantees that the frequencies in each spectrum match
up exactly, and therefore that the readings are
accurate.
Simon Quellen Field (
sfield@scitoys.com) is president
and CEO of Kinetic MicroScience (
scitoys.com), where he
designs scientific toys. He’s the author of several books on
science and computing.
61 Make:
