Continuous spectrum of visible light
Theoretical introduction:
White light contains wavelengths from the entire visible range, and is therefore a mixture of all
colors. Fig. 1 show the continuous spectrum of visible light, indicating the wavelength intervals that
correspond to specific colors. These intervals have various widths, and their boundaries may be
perceived somewhat differently by different individuals.
Fig. 1 The visible spectrum
In this lab task, you will observe the spectral components obtained by decomposing the white light in
a monochromator instrument. You will identify the boundaries of the spectral ranges as perceived by
your eye / visual system.
In the monochromator, a white light beam is passed through a diffraction grating. The elements of
the grating then act as sources of mutually interfering light waves. The light beam that leaves the
grating at angle 𝛼 is dominated by the wavelength 𝜆 that satifies the condition of constructive
interference:
𝐷 sin 𝛼 = 𝑘 𝜆 ,
where D is the spacing of the elements of the grating and k is an integer number (usually 𝑘 = 1).
A monochromatic beam of desired wavelength may therefore be obtained by selecting the beam
diffracted at the angle 𝛼 given by this equation, and removing (screening out) the beams diffracted
at other angles. The selected beam exits the monochromator and may be observed on a suitable
screen.
Equipment:
1) SPECOL monochromator
2) Observation screen (round glass aperture for observation of the monochromatic light ray).
Task:
Determine the interval of wavelengths for each observable spectral color.
Fig. 2 Scheme of the SPECOL instrument panel.
(1) – knob regulator with nonium; (2) – light shutter.
Fig. 3 Detail of the instrument panel.
(3) – the observation screen
Fig. 4 Detail of the knob regulator.
Execution:
1. Switch on the voltage source.
2. Use the knob regulator (1) in Fig. 2 (detail in Fig. 4) to select the smallest wavelength on the
nonium scale.
3. Set the light shutter (2) to the position I.
4. By turning the knob regulator, gradually increase the wavelength until you perceive visible
light on the observation screen (3).
5. Keep increasing the wavelength and record the changes in color. For each color that you can
distinguish, record the range of wavelengths in which you perceive the color. Terminate your
observations once you stop perceiving any light on the screen.
6. Record the the observed wavelength ranges in the protocol table.
7. Compare the widths of the wavelength ranges for the basic spectral colors: violet, blue,
green, yellow, orange and red.
Questions:
1.
2.
3.
4.
How does the diffraction angle of light depend on the wavelength?
Name the sequence of spectral colors, going from longest to shortest wavelength.
Which color has the widest wavelength range? Which color has the narrowest wavelength range?
What are the wavelengths of ultraviolet and of infrared radiation?