Properties of Light
Light as energy
Light is remarkable. It is something we take for granted every day, but it's not something we stop and think about very often or even try and
define. Let's take a few minutes and try and understand some things about light. Simply stated, light is nature's way of transferring energy
through space. We can complicate it by talking about interacting electric and magnetic fields, quantum mechanics and all of that, but just remember,
light is energy. Light travels very rapidly, but it does have a finite velocity. In vacuum, the speed of light is 186,282 miles per
second (or nearly 300,000 kilometers per second), which is really humming along! However, when we start talking about the incredible
distances in astronomy, the finite nature of light's velocity becomes readily apparent. It takes about two and a half seconds, for instance,
for a radio communication travelling at the speed of light to get to the moon and back. You might find it interesting to remember, the next
time you watch a beautiful sunrise or sunset, that it actually occurred eight minutes earlier--it takes that long for the light to reach the
Earth! And, of course, every newspaper article you ever read about astronomy will always include the required statement, "A light year
is the distance light travels in one year at the speed of 186,282 miles per second, about 6 trillion miles." (Well, 5.8 trillion miles
actually). We should also highlight right up front that light is more generally referred to as electromagnetic radiation.
Okay, we used a big word. It had to happen sooner or later. But too often when we say "light" it is mistaken to mean "optical
light," which is roughly the radiation visible to our eyes. Visible light is a tiny portion of a huge smorgasbord of light called the electromagnetic spectrum. For our convenience, we break this smorgasbord up into different courses (appetizer,
salad, etc.) and refer to them by name, such as gamma-rays, X-rays, ultraviolet, optical, infrared, and radio. However, it is important to remember
that they are all just light. There are no "breaks" and no hard boundaries in the electromagnetic
spectrum--just a continuous range of energy.
Particles and Waves
Physics experiments over the past hundred years or so have demonstrated that light has a dual nature. In many instances, it is convenient to
represent light as a "particle" phenomenon, thinking of light as discrete "packets" of energy that we call
photons. Now in this way of thinking, not all photons are created equal, at least in terms of how much energy they contain. Each
photon of X-ray light contains a lot of energy in comparison with, say, an optical or radio photon. It is this "energy content per photon"
that is one of the distinguishing characteristics of the different ranges of light described above.
The "wave" model of light.
The other way of representing light is as a wave phenomenon. This is somewhat more difficult for most people
to understand, but perhaps an analogy with sound waves will be useful. When you play a high note and a low note on the piano, they both produce
sound, but the main thing that is different between the two notes is the frequency of the vibrating string
producing the sound waves--the faster the vibration the higher the pitch of the note. If we now shift our focus to the sound waves themselves
instead of the vibrating string, we would find that the higher pitched notes have shorter wavelengths, or
distances between each successive wave. Likewise (and restricting ourselves to optical light for the moment), blue light and red light are both
just light, but the blue light has a higher frequency of vibration (or a shorter wavelength) than the red light.
The colors of the familiar "rainbow" of visible light correspond to differing wavelengths of the light, here shown
on a nanometer scale. The wavelengths get successively larger as one moves from left to right. Optical light runs from about 400 to 700 nanometers.
It's the same way as we move throughout the electromagnetic spectrum. Each range of light we have defined above corresponds to a range of frequencies
(or wavelengths) of light vibrations. These wavelengths are one of the primary indicators we use to describe light and spectra on a graph. Displaying
a spectrum as a graph instead of just a color bar allows us to measure the light.
For instance, the "rainbow" of color shown
in the figure above is what you see when you pass white light through a prism. What may not be obvious, however, is that the "intensity"
or brightness of the light is also changing along with the colors. If we converted the "rainbow" into a graph of light intensity versus
wavelength, it would look like this:
The familiar "rainbow" of the visible spectrum can be converted into a graph that shows how the intensity of the light changes along
Notice that the spectrum is brightest in the middle (yellow-green region) and drops off in both directions (toward red and blue). This was not
obvious from the rainbow version of the spectrum! Also notice that the "intensity" of the light in the graph does not stop at the
"ends" of the rainbow spectrum that is visible to our eyes! The light continues beyond what we can see in both directions, which we
can see in the graph but not by looking at the rainbow. Astronomers use graphical spectra most of the time because they can get more information
out of the light this way, and because they can still plot and analyze light that is not directly visible to our eyes!
Now we mentioned that the energy of each photon of light was also a basic property. It turns out that there is a simple relationship between
the energy of a photon and the corresponding wavelength of that photon:
E (photon) = (constant) / (wavelength).
This simple equation basically ties together the particle and wave nature of light by permitting us to convert back and forth from wavelengths
to photons and photons to their corresponding wavelengths. This equation is also in accord with what we said earlier...an X-ray photon has a
large energy (and a small wavelength) compared with a photon of optical light.
Interaction of Light with Matter: Absorption and Emission of Light
It should come as no surprise to you that atoms and molecules (which are simply bound collections of two or
more atoms) can absorb light (= energy!). If they didn't, you could simply flick a light on and off, and then sit back while the photons continued
to bounce around the room! Likewise, infrared light (= heat = energy!) wouldn't do any good in heating up your home in the winter if it didn't
get absorbed by matter. Higher energy light photons, like X-rays, tend to want to plow through more matter before they get absorbed. (Hence,
their use in medical imaging: they can pass through your "soft" tissue, but are more readily absorbed in your bones, which are denser.)
Well, it's time to develop another conceptual device to help us understand this process. In physics, we often find it helpful to pretend we
are looking at a single atom. Atoms are made up of protons, neutrons, and electrons, and each chemical element has a specific number of them--that's
what makes them different! Protons (and neutrons) are more massive than electrons, and so we sometimes visualize an atom as a miniature solar
system, with the heavy particles at the center (the nucleus) and the electrons whizzing around in specific "orbits" like planets.
(In reality, this picture is not very accurate. Electrons are not thought to be little balls "in orbit"
around a nuclear "sun." Without delving into atomic physics and quantum mechanics too far, let us just take the following statement
for granted for now: the electrons bound to any particular atom can only be found in certain, specific energy levels
with respect to the atom's nucleus. The hydrogen atom only contains one proton and one electron, and is the simplest (and most common)
element in the Universe.
If left undisturbed, our hydrogen atom likes to bind its electron as tightly as it can, and so we would find the electron in the lowest energy
level, which is called the "ground state." However, if our atom is immersed in a beam of light from, say, a nearby star, sooner or
later the atom will encounter a photon with an energy that is just the right amount to jump the electron up to the next higher energy level.
Voila! The photon gets absorbed, and is "gone" from the beam of light coming from the star! Now
our hydrogen atom is in what is called an "excited" state, sort of like a kid right before Halloween. However, as all parents know,
this is not the natural state of a child, and it's not the natural state of an atom either. If no other photons are absorbed by the atom, the
electron will eventually drop back down to the lower energy ground state. However, the atom has to lose energy to do this, and so it releases
a photon of the same energy as the one it absorbed (albeit most likely into some other direction from which it was absorbed). This process is
called emission because a photon of light is emitted by the atom, again at a very specific wavelength.
Of course, the atom could have absorbed another photon with just the right energy to jump up another energy level, or even two or three or more.
Likewise, after each of these possible excitations of the atom, the electron could jump back down one or more steps, emitting photons as it
went. If a photon with a sufficiently large energy gets absorbed, it can even cause an electron to become unbound from its nucleus, a process
that is called ionization.
We have been discussing one specific transition or "energy jump" in one atom, but of course in any physical system there are many
atoms. In a hydrogen gas, all of the separate atoms could be absorbing and emitting photons corresponding to the whole group of "allowed"
transitions between the various energy levels, each of which would absorb or emit at the specific wavelengths corresponding to the energy differences
between the energy levels. This pattern of absorptions (or emissions) is unique to hydrogen--no other element can have the same pattern--and
causes a recognizable pattern of absorption (or emission) lines in a spectrum.
This graphic demonstrates the optical spectrum one would see from glowing neon gas, both in colorbar and graphical formats. As with hydrogen,
discussed in the text, neon shows a specific set of spectral lines. Note how each bright colored line in the color bar corresponds to an upward
"spike" in the graphical format. Since most of the lines are in the yellow and red regions of the optical spectrum, a neon lamp appears
"orange" to your eye. The presence of this pattern of lines in the spectrum of a glowing cloud in space would tell astronomers that
the cloud contains neon in the gas.
This diagram shows how the spectrum of neon would appear in the spectrum of a star. Here, the background "rainbow" comes from the
atmosphere of the star, and the neon atoms in the star's atmosphere (or outer layers) absorb the stars light, leaving dark lines. Note how the
graph shows dips at each line position, producing the characteristic pattern of lines expected from neon.
Extending this a bit, it should become clear that since every chemical element has its own unique set of allowed energy levels, each element
also has its own distinctive pattern of spectral absorption (and emission) lines. (See diagrams above for neon, for example.) It is this spectral
"fingerprint" that astronomers use to identify the presence of the various chemical elements in Astronomical objects.
Light or visible light is electromagnetic radiation that is visible to the human
eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometers to about 740 nm,
with a frequency range of about 405 THz to 790 THz. In physics, the term light sometimes refers to electromagnetic
radiation of any wavelength, whether visible or not.
Primary properties of light are intensity, propagation direction, frequency or wavelength spectrum, and polarization, while its speed in a vacuum,
299,792,458 meters per second (about 300,000 kilometers per second), is one of the fundamental constants of nature.
Light, which is emitted and absorbed in tiny "packets" called photons, exhibits properties of both waves and particles. This property
is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics. Generally,
EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave,
visible region we perceive as light, ultraviolet, X-rays and rays. The behavior of EM radiation depends on its wavelength. Higher frequencies
have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior
depends on the amount of energy per quantum it carries.
An example of refraction of light. The straw appears bent, because of refraction of light as it enters liquid from air.
Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described by Snell's
where θ1 is the angle between the ray and the surface normal in the first medium, θ2 is the angle between the ray and the surface normal in
the second medium, and n1 and n2 are the indices of refraction, n = 1 in a n > 1 in a transparent
When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes,
but the frequency remains constant. If the beam of light is not orthogonal (or rather normal) to the boundary, the change in wavelength results
in a change in the direction of the beam. This change of direction is known as refraction.
The refractive quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses,
spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.
There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of
black-body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 Kelvin peaks in the
visible region of the electromagnetic spectrum when plotted in wavelength units  and roughly 40% of sunlight is visible), incandescent
light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames.
The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak
shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part
of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". Blue
thermal emission is not often seen. The commonly seen blue color in a gas flame or a welder's torch is in fact due to molecular emission, notably
by CH radicals (emitting a wavelength band around 425 nm).
Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can
be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames
(light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as
in a laser or a microwave maser.
Deceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and
bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce
visible Cherenkov radiation.
Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies
produce light by this means, and boats moving through water can disturb plankton which produces a glowing wake.
Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. Some substances emit
light slowly after excitation by more energetic radiation. This is known as phosphorescence.
Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example. This mechanism
is used in cathode ray tube television sets and computer monitors.
Certain other mechanisms can produce light:
- Cherenkov radiation
When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:
- Radioactive decay
- Particle–antiparticle annihilation
EM Spectrum Properties