Let us not forget that Asimov was a chemist and not a physicist. He was
fairly accurate in his writings, but this particular passage is not good
physics.
Interference does not produce heat. In space the two beams of light would
pass through each other and while they would cancel in some parts, they
would add in other places. And of course it is very difficult to get white
light to interfere because as soon as you do this, the different wave
lengths corresponding to different colors will interfere at different
places.
When you have interference, the energy is simply moved to another point in
the light. So if you do double slit interference the black parts on the
screen are in between white parts which have double the energy. An example
of this is a wave on a string. If you have a second wave of the same
frequency and amplitude going in the opposite direction you have greater
energy in the anti-nodes and no energy in the nodes so you get a standing
wave. Now if you want to get total interference of a wave on a string, you
simply clamp it in the middle. By doing this the clamp is pushing on the
string in the opposite direction to the wave, so on one side you see no
wave. This is where the original wave and the upside down wave interfere
totally. But on the other side you have generated a back wave which
interferes with the original wave to produce a standing wave. No energy is
lost (ignoring friction and air resistance).
An example of almost total interference of white light is the coated lenses
used in better eye glasses and camera lenses. It produces a second
reflection back wave which interferes with the normal reflection to cancel
out almost all of the reflection. But then more energy is transmitted
through the surface of the lens. I say almost total, because it really only
works totally in the center of the spectrum. Actually a single coating will
only work perfectly at one discrete frequency. You still have a reduced
reflection which you can generally see in the blue region.
Asimov was a bit too clever in his explanation. He had very broad
knowledge, but any of us can be easily fooled by a specific question,
especially when it is not in our field. Asimov had at least 1 book in each
of the old Dewey decimal catalog system, and he wrote over 200 books plus
many articles. If he were still alive I suspect he would be willing to
listen to a better explanation and would try to change the article, unlike
some authors who have written absolutely dead wrong explanations. Here I am
thinking of the NTN 3rd law explanation in the Worldbook written by a PhD
astrophysicist and science author. I have been informed the article has
been changed, but I have not yet seen the new version. Meanwhile the old
one is still in circulation.
John M. Clement
Houston, TX
Hello:
We were reading in class the book "Please Explain" by Isaac Asimov, and
answer 78 explains about light interference. It says that when two white
light waves interfere destructively on a screen, we can obtain darkness in
the screen, but energy should be converted to another form: heat (screen
temperature raises up) .
A student asked me about the case that white ligth interference were in
perfect vacuum, ¿ in wich way can the energy conservation principle be
preserved ? .¿ Energy will be manifested as heat ?. I found it difficult to
answer.
Please, I would appreciate you help me to answer that query.
My best regards.
Roberto