New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Colour
I
120
Noon Summer Sunlight
5740K
--I
4O F
Noon Winter Sunlight
01 I I I I I
400 500 600
Wavelength/nm
Figure 19. Sunlight at different times of the year at mid latitudes.
Sources of Light
We have said very little about the nature of light sources so far, because we wished to introduce
the CIE chromaticity diagram first as a means of specifying their colour. Daylight itself is a very
variable quantity, as shown in figure 19 for sunlight. North light, or the blue sky, which is sunlight
scattered by small particles in the atmosphere, can exceed 7000 K colour temperature.
Fluorescent light comes from mercury discharge tubes, which give only single lines in the
spectrum, as shown in figure 20. Fluorescent powders inside the discharge tube modify this to
give a white light spectrum.
200 300 400 500 600 700
Wave I e ngt h /nm
Figure 20. Line spectrum of a low pressure mercury vapour lamp.
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