ing corresponding to the large distanced, and a long spacing that
relates to the small dimensionw.aa/1. A diffraction pattern formed by a real double slit. The width of each slit is fairly big compared to
the wavelength of the light. This is a real photo. 2. This idealized pattern is not likely to occur in real life. To get
it, you would need each slit to be so narrow that its width was comparable to the wavelength of the light, but
that’s not usually possible. This is not a real photo. 3. A real photo of a single-slit diffraction pattern caused by
a slit whose width is the same as the widths of the slits used to make the top pattern.Discussion Question
A Why is it optically impossible for bacteria to evolve eyes that use
visible light to form images?12.5.8 Coherence
Up until now, we have avoided too much detailed discussion of
two facts that sometimes make interference and diffraction effects
unobservable, and that historically made them more difficult to dis-
cover. First there is the fact that white light is a mixture of all the
visible wavelengths. This is why, for example, the thin-film inter-
ference pattern of a soap bubble looks like a rainbow. To simplify
things, we need a source of light that is monochromatic, i.e., con-
tains only a single wavelength or a small range of wavelengths. We
could do this either by filtering a white light source or by using a
source of light that is intrinsically monochromatic, such as a laser
or some gas discharge tubes.
But even with a monochromatic light source, we encounter a sep-
arate issue, which is that most light sources do not emit light waves
that are perfect, infinitely long sine waves. Sunlight and candlelight,
for example, can be thought of as being composed of separate little
spurts of light, referred to as wave packets or wave trains. Each
wave packet is emitted by a separate atom of the gas. It contains
some number of wavelengths, and it has no fixed phase relationship
to any other wave packet. The wave trains emitted by a laser are
much longer, but still not infinitely long.
Section 12.5 Wave optics 823