A spectroscope contains a diffraction grating that separates
electromagnetic radiation into its component wavelengths. The
spectroscope can be used to measure absorption
or emission spectra.
Using the spectroscope
When using a spectroscope, eliminate as much external light
as possible so that the only light entering the instrument is
due to the light sourcebeing examined. If the source is a gas emission tube, DO NOT
TOUCH it! Gas emission tubes operate at high voltages and will
cause electric shocks if touched.
Video: DO NOT touch the emission
tube! ( 586K )
To view a spectrum, the light slit must be aligned correctly.
Video: Proper alignment of slit ( 1.97 M ) Text description
When the slit is properly aligned, examine the spectrum through the spectroscope.
Video: Examining the spectrum ( 3.83 M )
When recording several spectra, take all measurements at approximately
the same angle and distance from each light source.
The white lines on the spectroscopes indicate the positions of the light slits.
The student using the spectroscope on the left sees a red line at 615 nm, while the student using the spectroscope on the right sees the red line at 630 nm. Explain this discrepancy.
Who obtained a more correct reading? Answer
The spectrum is displayed on a wavelength scale inside the
spectroscope, to the right of the slit. The numbers on the scale
mark hundreds of nanometers, and the thick dashed lines are 50
nanometer increments. Each thin solid line on the scale represents
When measuring an emission spectrum, record the wavelengths
and colors of the individual lines.
Because of the thickness of the emission lines, readings should
be estimated to the nearest five nanometers.
An emission spectrum with a blue line at about 445 nm,
a green line at 525 nm, and a red line at 635 nm.
When measuring an absorption spectrum, record the ranges of
wavelengths for which no light is visible.
An absorption spectrum with absorptions at 460 - 470 nm,
550 - 556 nm, 557 - 559 nm, and 605 - 610 nm.
Calibrating the spectroscope
Readings obtained from a hand-held spectroscope may not correlate
precisely with known values measured with more sophisticated instruments.
Therefore, calibrating the spectroscope is necessary.
First, obtain a set of known emissions for a gas. Then, using
a gas emission tube containing that gas, record the wavelengths
observed using a spectroscope. Make a graph of known wavelengths
versus observed wavelengths. This plot can now be used to calibrate
other readings from the spectroscope. Show me an example
Which of the following could be true about the solution
A. It is absorbing orange light.
B. It is transmitting orange light.
C. It transmits all colors of light except orange.
D. It absorbs all colors of light except orange.
Given the following plot of absorbance vs. wavelength as
obtained using a Spectronic 20, sketch the expected absorption
spectrum that would be seen through a hand-held spectroscope for this species.
Given the following list of known emissions (wavelengths
in nm), identify the gas responsible for the emission spectrum
Radon: 745, 705
Argon: 707, 696
Krypton: 587, 557
Xenon: 467, 462, 450
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