# Q & A: What is light made of?

Q:
- megan (age 14)
A:
Megan -

Light is what's called an "electromagnetic wave", just like radio waves, microwaves, X-ray waves, etc. Electromagnetic waves typically start when an electric charge jiggles back and forth.

Depending on the "frequency" of the electromagnetic wave (or how scrunched together the peaks in the wave are), you get different kinds of waves. For example, radio waves have a pretty low frequency - that is, the peaks in a radio wave are pretty far apart. Next comes microwaves, then infrared light. Next is the visible spectrum (i.e. the different colors of light that people can see), followed by ultraviolet light. Then waves with  higher frequency are called X-rays and still higher are gamma rays - these have the peaks closest together.

Sometimes you'll hear that light is made of photons. What that means is that when light is absorbed or emitted, the energy in the wave comes in lumps. The size of those lumps (or "quanta") of energy depends on the frequency. The higher the frequency the more energy per photon.

-Tamara(w. mods by mw)

(published on 10/22/2007)

## Follow-Up #1: Massless matter?

Q:
If the particle is massless how can it be matter, as matter is defined as having mass and taking up space?
- Joshua McDonald (age 17)
Cottonwood, CA U.S.A
A:

Well, we sure didn't define matter in that way.  We do know of two kinds of particles which have zero invariant mass (called "rest mass" by some, but if it's zero, you can never get these particles to be at rest -- they always travel at the speed of light, c).  The two particles are the photon (already spoken about), and a similar particle, called a gluon, which is a lot like a photon in many ways but carries the strong force instead of the electromagnetic one.

Photons can have energy (E) and momentum (p).  Einstein's special relativity says that  E2=p2c2+m2c4.  If E=pc, then we're all okay with zero mass m but nonvanishing energy and momentum.  Some people insist on writing this m as m0 but I don't see the real need.

Now whether you want to call photons "matter" or not is a quibble people in the particle physics business never worry about.  Photons are their own antiparticles, so they'd be "antimatter" if they were "matter", so we generally don't use that word when talking about photons.

We used to allow for the possibility that neutrinos are massless, and in fact, even with the latest observations that show that at least some neutrinos are not massless, it still is possible that one species of neutrino is in fact massless (although I give it a plausibility rating of very low).  Massless neutrinos also don't cause troubles for the theory; E=pc for them, too, and they are more easily classifiable as "matter", being neutral cousins of the leptons -- electrons, muons and taus are the leptons.

All the elementary particles we know of, even the massive ones, appear to be pointlike.  But that may just be tautological -- if we knew an object had some spatial extent, we'd seek the pieces it's made of.  At any given time, there is a limit to the sensitivity of our experiments, and we call all the things we cannot split into smaller pieces "elementary".

Tom

I think there's a gray area where we can resolve some structure but not enough to describe smaller pieces yet. Maybe we'd call particles in that category elementary and maybe we wouldn't. Right now I guess there aren't any like that, though the proton, for example, was in that category for a while. Anyway, Tom's central point still applies: it's not the names that are important but rather the mathematical properties that they get at.  Mike W.

(published on 10/22/2007)

## Follow-Up #2: what things are made of

Q:
Also,doesn't everything have atoms or at lest mass and weight? if it doesn't then is it really there?
- Taylor (age 14)
San Bernardino, Calfornia, USA
A:
Actually, lots of things aren't made of atoms. Light and other electromagnetic waves are an obvious example. Individual protons, neutrons, and electrons don't constitute atoms until they're combined. Under conditions of extremely high pressure, the ingredients of protons and neutrons form a sort of quark soup, which doesn't even have individual protons and neutrons in it.
As for whether everything has mass (and weight, the gravitational effects of mass), the answer is yes. Even light has mass, in that sense, as we've discussed in several other answers. It doesn't have 'rest mass', but that's a more technical issue.

Mike W.

(published on 05/19/2009)

## Follow-Up #3: light and heat

Q:
Isn't light just heat created by the sun, transferred to energy by mater, then the reflected light becomes sight?
- Taylor (14)
San Bernardino, Calfornia, USA
A:
Light (electromagnetic radiation) is one of the basic ingredients of our universe. "Heat", on the other hand is a very general term which includes all sorts of energy associated with temperature. That includes the kinetic energies of particles jiggling around, the potential energy connected with the forces between them, the energy of the light present in any hot object, etc.
Of the many forms of 'heat' in the Sun, light is the main one that leaks out into space, where some of it reaches us. Any light reaching our eyes makes a visual signal, regardless of whether it's reflected off something or direct from its source.

Mike W.

(published on 05/19/2009)

## Follow-Up #4: matter and substance

Q:
If light is in fact anti-matter then wouldn't it be destructive?And don't all things need to have some sort of matter to exist, then light in fact would have to be considered matter thus protons must be considered matter correct?Because all things must be defined as some sort of substance because its there or it isn't?
- Kyle (age 14)
Pheonix,TX,USA
A:
Light and anti-light are the same thing. So if two beams of light collide and destroy each other (a very improbable process under current conditions, but possible) the output is a balanced mixture of particle/antiparticle pairs. Back in the day, when photons were a lot denser than now, those types of collisions were common.

I couldn't really follow the rest of your questions. The key point is that if we can describe how things behave, we've done all that we need to do. Deciding what names to call things ("substance", "matter", etc.) is not so important.

Mike W.

(published on 05/20/2009)

## Follow-Up #5: sight

Q:
So would light just be heat energizing atoms then those atoms go flying into out eyes to make sight? Or do we directly pick up the energy with our retinas?
- Keven (age 15)
Colton, CA, United States of America
A:
Light is not made out of atoms or anything like them. Electromagnetic waves are themselves a basic ingredient of our world. It's the light itself which enters our eyes. It is absorbed by special molecules which change their chemical configuration as a result of that absorbed energy. The new configuration changes the ability of electrically charged ions to pass through a membrane, and the resulting electrical signal triggers nerve impulses.

Mike W.

(published on 05/20/2009)

## Follow-Up #6: light and gravity

Q:
if light is affected by gravity then how is it affected? I would think that light travels straight forever till affected by an outside force as stated in a previous answer.
- Taylor (age 14)
San Bernardino, CA,USA
A:
The quick, but not quite right answer, is that light has weight (but not rest mass)  and is pulled by gravitational fields. So in the presence of gravity, it deviates from a straight line.

Trying to calculate the curvature of the beam using that picture gives only half the actual value. The more accurate answer is that gravity changes the geometry of our spacetime, so that it isn't 'flat'. That means that it simply doesn't contain anything with the properties of simple straight lines in a flat space. A calculation based on these principles (General Relativity) gives the correct answer.

Mike W.

(published on 05/20/2009)

## Follow-Up #7: What is rest mass?

Q:
SO then what is defined as rest mass? Like whats that mean? Im going to guess that its mass that is effected by gravity to slow down the atomic vibration.
- Taylor (age 14)
San Bernardino,CA,USA
A:
Take some object with total energy E and total momentum p. The rest mass m is defined to be SQRT((E2/c4)-(p2/c2)). That's a special combination because it is (in the Special Relativistic approximation) independent of what reference frame you look at the object in, even though both E and p do depend on the frame. Somebody at rest with respect to the object, who says that p=0, would give that as its mass. Hence it's called the rest mass. Its value in that rest frame is E/c2.

Mike W.

(published on 05/26/2009)

## Follow-Up #8: ions and light

Q:
I though ions were sunstable atoms? so if light were ions then wouldnt that make light atoms?
- Denis (age 12)
Colton , CA, USA
A:
Ions are atoms or molecules with a net electrical charge because they have an unbalanced number of electrons and protons. Whether they're stable or unstable depends on what environment they're in. The same is true for uncharged atoms and molecules. Light is not made of anything like that.

Mike W.

(published on 05/26/2009)

## Follow-Up #9: photon effects

Q:
could the photons light is made of, be effected by temperature? Such as absolute zero. Is it also effected by water or glass. If the speed of light is manipulable, could we then control time?
- clint
usa
A:
Photons are not affected by any ordinary temperature. The density of photons of different energies is a function of temperature, but the properties of the photons are not.
We believe that at extremely high temperatures, such as prevailed very shortly after the Big Bang, electromagnetism did not exist in its current form, but in a form more directly integrated with what's now the weak nuclear force. At those temperatures, there would not be photons but rather a different sort of boson.

Photons do behave differently in water or glass, as you can see. They travel more slowly. I don't really understand the part about controlling time.

Mike W.

(published on 08/21/2009)

## Follow-Up #10: massless light

Q:
So, if light is a wave of massless particles; how do we concentrate it with magnifying glasses and such? I mean; I know how a magnifying glass works; but how does it affect a massless particle? Another question would be: sight is a perception of light transformed by our optic nerve for us to understand what we see. Then what happens to the 'light"? Does it just disappear, go though us or what? Thanks!
- Doc (age 56)
Olalla, WA, USA
A:
There's still a wave equation describing how light moves. It's still affected by interactions with the charged particles in, for example, a piece of glass. So the wave direction can change when it enters glass, just as people realized before they started worrying about wave mass and such.

Our eyes detect light by absorbing it with special molecules. Once a photon is absorbed by the molecule, the photon no longer exists. The extra energy from the light changes the molecules chemically, triggering changes in how easily charged particles can move through some membranes. An electrical signal results. The energy in the molecule ultimately trickles away as heat.

Mike W.

(published on 05/16/2013)

## Follow-Up #11: light and heat

Q:
Ok this is a counter example to follow up number 3. So heat is energy correct? And when energy comes into contact with atoms, the electrons move up a sub level, but when they lose energy and make the electrons go back down, the electrons emit energy in the form of light, so heat must be directly related to light because of what I previously stated.
- Nick (age 15)
new york
A:
When physicists are being picky, they say heat is a form of energy transfer. For ordinary speech, it's reasonable to say that heat is a (broad) category of energy. When things are hot there are all sorts of specific forms the energy takes. Light is definitely one of them.

Mike W.

(published on 12/22/2009)

## Follow-Up #12: light getting absorbed

Q:
If light is a molicule how can it be destroyed as you said it no longer exists
- Josh (age 13)
Ooltehwah tn usa
A:
Light is definitely not a molecule. It has no rest mass, no protons, no neutrons, no electrons. When some light is absorbed by something else (a molecule, for example) the light's energy, momentum, and angular momentum are transferred to that object.

Mike W.

(published on 01/10/2010)

## Follow-Up #13: What is light energy we sense as heat known as?

Q:
what is light energy we sense as heat known as?
- Ruby
el paso texas usa
A:
All light carries energy and if the light is intense enough you will sense or feel it.  If you put your hand over a medium warm burner on a stove you can't see it but you can feel the radiated energy. This 'light' is actually infra-red or long wavelength radiation.  If you turn up the burner it will begin to glow red, then orange.  You can really feel this one.  The emitted light actually contains a spectrum of all wavelengths.  It turns out that there is a relationship between the temperature of a warm body and the spectral distribution of the emitted radiation:  the hotter the body, the shorter the average wavelength of the radiation.  This is known as Planck's Law. See for more details.

LeeH

(published on 11/09/2010)

## Follow-Up #14: light, mass, energy, E=mc^2

Q:
How does light fit into E=mc^2? Is light a mass or energy? If it is an energy than what is the porportional mc^2? If it is a mass than how could it equal energy times the squared speed of itself?
- Cassia (age 16)
Goodhope, MO, USA
A:
Although we've addressed this before, it can't hurt to try a slightly different angle.

The most important thing to do in approaching a problem like this is to figure out what you're talking about. By that I mean not some set of words but what actual physical events we're describing.

Here energy (E) is pretty familiar. You can let light hit a painted surface and see how much the surface heats up. That's one way among many to measure the energy in a light ray. So we know how to measure E in the same units we'd use for any other E.

What about mass (m)? One way to measure mass is to weigh something, but the weight of a reasonable amount of light energy is very small, if m=E/c2. We could at least check to make sure that light has some weight, that it responds to gravity, and that was done in 1919 by watching light bend as it went past the sun. A more generally practical way to measure m for light is to keep track of momentum, the product of something's m and its velocity, v.  When things collide, like two marbles, the total momentum doesn't change although it redistributes between the things. There are many experiments in which light collides with other particles, changing their momenta. They all fit with the idea that light has momentum mc, in the direction it's going, where m=E/c2.

So for light we have E=mc2 if by E we mean any standard definition of energy and by m we mean the thing you multiply the velocity by to get the momentum.

As for whether light "is" energy or mass or whatever, I don't know what that's asking.

Mike W.

(published on 06/16/2011)

## Follow-Up #15: E=m?

Q:
E=mc2 energy = mass times the speed of light squared. But the speed of light is an arbitrary measurement, meaning we could say it is 1 something(instead of 186,000 mi/sec). In the case the C equals 1 then Einsteins equation would read E = m. So energy would equal, or essentially be mass? Is this true?
- Andy (age 31)
West Allis, WI, USA
A:
Yes. In fact, in physics we frequently use just such units.  However, you have to be a bit careful, since often the word "mass" is used to mean "invariant mass", which is not E but sqrt(E2-p2) in these nice c=1 units.The mass you're referring to is sometimes called the "effective mass". It's the coefficient m in the momentum-velocity relation p=mv. It also appears as the scalar part of the source for gravity.

Mike W.

(published on 12/18/2011)

## Follow-Up #16: Photons, momentum and solar sails

Q:
How do solar sails work if light (or photons) has no mass?
- Caspian (age 16)
A:
This is a frequently asked question.  The answer is that the the full relativistic equation relating mass, energy and momentum is  E2 =  (Moc2)2+ p2c2.   E is the energy of a particle, Mo is the rest mass, and p is the momentum, and c is the usual speed of light. For a particle with rest mass zero, like a photon, it can still have momentum  p = E/c.  So if a photon bounces off of a sail at an angle it will give some of its momentum to the sail, and give it a little nudge.
The fact that light can carry momentum is also true even in classical electrodynamic waves that obey Maxwell's equations.   This phenomenon is easily demonstrated in the lab.

LeeH

(published on 10/29/2012)

## Follow-Up #17: What is light?

Q:
Is light a particle or a wave and how so? Can light be reflected or absorbed into itself? When light is absorbed where does it go? If it is a particle, can it be compressed? Since light travels at a very fast speed from the Sun, wouldn't it make the Earth somewhat heavier? If light does make weight force, then does that mean shadows are lighter than light? And why do we say "lighter" when it doesn't even refer to light and that's not even a scientific question but it would be great if you could answer that.
- Jess (age 13)
Sydney, NSW, Australia
A:

We've covered some of those questions in the other parts of this thread.

Light is a wave, a quantum wave. Like all other quantum waves, it has an interesting particle-like side to how it behaves. Certain types of detectors will pick up 0, 1, 2,.... blips of light, but not say 1.3. That's just like counting particles. So we sometimes say that light is made of particles called photons, but we don't mean that there are little dot-like things in it.

Light can be compressed, like other waves. I'm not sure what you mean by "reflected or absorbed into itself" but there are processes by which two photons are lost and their energy shows up in another form- say an electron-positron pair.

Light, like any form of energy, has some gravitational weight, but it's extremely small compared to other weights that are around.

I don't know how in English the different meanings came to be attached to the same word, 'light' . According to my dictionary, the one about stuff you see goes back to Indo-European "leuk-" and the one for not being heavy goes back to Indo-European "legwh-".

Mike W.

(published on 07/29/2013)

## Follow-Up #18: Light: particle or ray?

Q:
is light a particle or a ray
- kenneth (age 29)
ghana
A:

I'm not sure what "particles" and "rays" are. If you tell us something about what those words mean to you, perhaps we could give an answer. or perhaps the answer could be found earlier in the thread.

Mike W.

(published on 08/27/2013)

## Follow-Up #19: light mass and wavelength

Q:
can the momentum of light be defined by c^3*E=momentum because E(eV)=1.24/λ(μm) and because e=mc^2 so m=ec^2 and momentum=mass*velocity(c) so momentum=(1.24/λ)c^2*c or just m=(1.24/λ)c^3 thanks
- Angus (age 16)
Australia
A:

Whoops, there's an algebra goof there. It's m=E/c2, not m=Ec2. So you'd end up with, in these awkward units,
m=1.24/λ eV-μm. The particle physicists do often describe masses in units of eV.

Mike W.

(published on 10/28/2013)

## Follow-Up #20: what state of matter is light?

Q:
i've got a bunch of questions, so i'll order them 1.What state is light in??(like solid,liquid,gas,plasma) or is it in a whole different state? 2.Could we convert it to a different state?? 3.and how could something(photon) be both a particle and a wave??
- kim seung (age 16)
Seul
A:

1.) If light bounces around a while in a box, getting absorbed and re-emitted by the walls, it will form a sort of gas. That is you can think of it as a bunch of photons zipping around independently in random directions. It's very different from a more familiar gas. In an ideal molecular gas, for fixed particle density the pressure, p, is proportional to the absolute temperature, T. In a photon gas, the pressure goes as T4. The number of photons isn't fixed but depends on T.

2) It's very difficult to see how you could get a photon gas to convert to something else under ordinary conditions. At very high temperatures, colliding photons will create electron/positron pairs. So I guess you could say that a very hot photon gas is really a type of plasma.

3) We've addressed this basic question many times before. It really isn't just about photons but about all  the little ingredients of the world. Here's a link to get you started.

Mike W.

(published on 05/29/2014)