Physics for Geologists, Second edition

Optics 53
Figure 4.5 Diffraction of monochromatic light through a grating. There is
a symmetrical set of beams diffracted upwards.
Figure 4.6 Diffraction of polychromatic light through a grating (diagrammatic).
radiation of 133 nm, fourth-order radiation of 100 nm, and so on. At the
other end of the visible spectrum, radiation of 750nm will be diffracted
through 8.63", as will 375 nm, 250 nm, etc.
This overlap of wavelengths does not lead to practical difficulty. If the
observation is visual, all the radiation of second-order and above is outside
the visible spectrum. If the observation is photographic, emulsions can be
chosen to detect specific ranges of wavelength (within limits).
Rainbows
Before leaving the subject of reflection and refraction, consider as a digres-
sion of general interest, the common rainbow. It is caused by refraction
and internal reflection of sunlight falling on a raindrop (or spray from
a waterfall). Why is the sun low in the sky when there is a rainbow?
Why do you never see a completed circle of rainbow unless you are in an
Copyright 2002 by Richard E. Chapman