What Is Light, Part I
MARCELO GLEISER: We have seen that astronomers collect light, or, more generally,
electromagnetic radiation, to study all sorts of objects in the universe, from nearby planets to
faraway galaxies. But what is light? It's all over the place, but it's a very
strange thing. You can't
hold onto it, it doesn't seem to have weight, and we completely depend on it in our lives.
The nature of light has been a mystery since very early on. Skipping to Isaac Newton, in the 17th
century, he would say that light is made
of tiny little bullets
--
atoms of light. This is what we call
the atomistic description. Newton was influenced by Greek philosophers from a long time ago
who believed that everything is made of tiny particles
--
indivisible little bits of stuff called
atom
s.
However, also in the 17th century, the Dutch physicist Christiaan Huygens had a different point
of view. He believed that light was made of waves
--
that it waved through space as it propagated
from one point to another, something like a water wave. So
you can see that, even 400 years ago,
people were already conflicted about the nature of light. Is it made of particles or is it a wave?
The discussions continued until the turn of the 19th century, when the English scientist Thomas
Young showed, quite co
nvincingly, that light is indeed a wave. He built what is now called a
double slit experiment, where a wave of light coming from a source is made to pass through two
holes. Once that happens, the waves of light will interfere with one another, sometimes
en
hancing their intensity, sometimes cancelling it out altogether so that when you project the
light onto a screen, you will see a sequence of bright and dark fringes.
We are now going to repeat Young's experiment, but using a more modern apparatus than hum
an
hair and candlelight. What I have here is a helium neon laser, which has red light, as you can see.
And I have it aimed at the wall, but it's passing through what we call a diffraction grating or a
grid that I can adjust the width of it. And, as I chang
e the width of this diffraction grid, what's
going to happen is that the pattern projected on the wall is also changing. And it is very, very
hard to explain the result of this experiment using a particle theory of light, and that's why,
during the 19th ce
ntury, the wave
-
like theory of light won.
You can use your imagination to play with light and its diffraction properties. For example, when
you look at clouds with the sun behind them and you see a silver lining, that's an example of
diffraction
--
of ligh
t bending around an obstacle. Can you think of any other examples from
everyday life?
Another effect is known as refraction of light, when light hits an object and changes its direction
of propagation. This is what happens during a rainbow, as sunlight go
es through water droplets
in the atmosphere. If you have a prism or just a crystal ornament and you can make sunlight go
through it, you will see it being separated into the colors of the rainbow
--
from red to violet.
Or you can do that with a water spray
and a hose. Or, even better, you can surprise me and come
up with something quite different to show sunlight spreading into its colors. Let me know!
With the acceptance of the wave theory of light, it became clear that different colors are basically
wave
s of different wavelengths. A wavelength is just the distance between two successive crests
of the wave. For example, red light has a longer wavelength than violet light. Instead of
wavelength, you can also use frequency, which is the number of wave crests
that pass by a point
in one second.
In the case of light or any electromagnetic radiation, if you multiply the wavelength by the
frequency, you always get the speed of light, which physicists represent with the letter c. Since
the speed of light, c, is c
onstant, a high frequency wave has smaller wavelength and a low
frequency wave has longer wavelength.
It turns out that the light that we can see with our eyes is just a tiny window in the
electromagnetic spectrum. The typical wavelength of visible light
varies from about 400 to 700
nanometers, where a nanometer is one billionth of a meter
--
very tiny. Examples of
electromagnetic waves of shorter wavelength than visible light are ultraviolet, x
-
rays, and
gamma rays. On the other side of the spectrum, waves
that have longer wavelength than visible
light are infrared, microwaves, and different kinds of radio waves.
The energy packed in a wave of electromagnetic radiation is directly proportional to its
frequency. So red light, having lower frequency than blu
e or violet light, has less energy than
these two. The most energetic kind of electromagnetic radiation are gamma rays
--
typical of
nuclear phenomena. If a source emits gamma rays, you can be sure that nuclear physics is
involved.
Going back now to the na
ture of reality, we realize that the picture of the world that our senses
are able to construct is incredibly limited. In a sense, Plato was right
--
we all live in a cave: The
cave of our limited perception of reality. After all, our eyes evolved to captur
e only a very small
fraction of the whole electromagnetic spectrum.
All around us, there are other kinds of electromagnetic radiation
--
invisible kinds of light that are
as real as visible light, even if we can't see them. That's why one of the roles of s
cience is to
amplify our perception of reality. The tools we use are a sort of window into invisible aspects of
what's out there. The more we can see with our tools, the more complete our picture of reality is.
But, as we know, there will always be somethi
ng that is beyond what we can grasp, so that the
very essence of reality will always remain elusive.
