1. Shine white light through a quartz prism or grating to disperse it onto a screen.
NOTE
2. Place a thermometer just outside the red end of the spectrum and you should notice a heating effect, showing the presence of invisible IR.
UV can be blocked by a glass prism.
3. Now place a piece of paper with Vaseline on it just outside the violet side of the spectrum and you should see it begin to glow, thereby showing the presence of invisible UV light.
You may now complete Exercise 18B (page 214). The spectrometer
A spectrometer is a precise optical instrument that is used to examine spectra of light, which enables us to calculate wavelength (Fig. 18.16).
NOTE
Although you do not have to use a spectrometer in the mandatory
experiment measuring the wavelength of light, you need to know its parts and principles of operation.
collimator slit grating telescope
levelling screws
base vernier scale
turntable FIG. 18.16 Spectrometers are used to examine spectra of light
Main parts of a spectrometer Heavy base
The heavy base keeps the instrument steady while it is being used for measurement. The circular vernier scale is attached to the base.
NOTE
Chapter 1 explains how to read the vernier scale.
Vernier scale The vernier scale is graduated in degrees and is used to measure angles of deviation. When the light passes through the diffraction grating on the spectrometer, it causes different orders of diffracted images (n = 0, n = 1, n = 2 and so on). The vernier scale is read to find the angle θ for substituting into the formula nλ = d sin θ.
Turntable and levelling screws
This is the platform that the diffraction grating is placed on. The levelling screws allow for adjustment and the turntable allows for the grating to be positioned correctly.
