How to define light in laymans terms

[bykerboy011]

So I've got to give a speech for a class soon with the informative topic of what is light. Light is just what we call the visible portion of the spectrum. How can i go about defining this to a group of college kids in a way they can understand?

thanks

 

[jim hardy]

So is light particles or waves?

How long can you speak?

I'd make a mystery story out of it - see this article.

http://www.spectroscopyonline.com/spectroscopy/article/articleDetail.jsp?id=337288&pageID=1&sk=&date=

What is Light?

Is light a particle or a wave? The actual nature of light is a topic that has occupied minds for millennia. The earliest mention of the nature of light might have been by Pythagorus (of Pythagorean theorem fame; ca. 580–500 B.C.), whose particle hypothesis of light suggested that light particles were emitted by all objects and were intercepted by the eyes (called the "intromission hypothesis" of light). However, Plato (ca. 427–347 B.C.) suggested that light was created by the eye (the "emission hypothesis" of light) and illuminated other objects. In contrast, Plato's pupil Aristotle (384–322 B.C.) thought that light was some kind of wave. In about 300 B.C., the mathematician Euclid noted that light traveled in straight lines and obeyed a law of reflection, the concept that the angle of reflection equals the angle of incidence. Euclid also thought that light was a wave that was emitted by the eye, and light waves must move extremely fast because when you open your eyes, objects — even faraway objects — are immediately visible. (The determination of the speed of light is another fascinating story, but that's another column.) In the 10th century A.D., Persian scientist Alhazen Abu Ali al-Hasan Ibn Al-Haitham used experiments to support the intromission hypothesis of light, but apparently did not address the "particle versus wave" nature of light.

 

[Astronuc]

Light is just what we call the visible portion of the spectrum. How can i go about defining this to a group of college kids in a way they can understand?

My first reaction to the question is - if a college kid doesn't understand light - then what the heck are they doing in college?! Are our educational standards set so low these days?

In most folks everyday experience, light generally refers to 'visible' light, which corresponds to electromagnetic radiation (photons) of a narrow range of energy (E) or wavelength (λ), where E = hc/λ = hν, and ν is the frequency. Visible light originates in an atom in which an electron drops from an excited energy level to a lower (more stable) energy level. The emitted photon has wave and particle (discrete) properties.

Is one interested in the origin or nature of light, or both?

The Nature of Light - http://ocw.tufts.edu/data/30/365841.pdf (probably more than one would want to know)

Photon - http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html
Electromagnetic radation - http://hyperphysics.phy-astr.gsu.edu/hbase/ems1.html
Visible light - http://hyperphysics.phy-astr.gsu.edu/hbase/ems3.html#c2
Wave-particle duality of light - http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html
Measurement of light - http://hyperphysics.phy-astr.gsu.edu/hbase/vision/radphocon.html#c1

 

[bykerboy011]

My speech is suppose to be between 5 and 6 minutes. In my opinion, light is both, its a wave of particles.(before you begin bashing my opinion take regard that it is just a thought) But yes college standards are such a joke these days, your jaw would hit the floor if you saw how basic the standards are and yet people still can't grasp anything.

Technically, light(visible), is no different than the rest of the electromagnetic spectrum, its just energy on a different frequency. So in a sense its all the same. So does color exist?

And i understand light travels at the speed of light in a vacuum until it hits matter, than it acts as a wave. So where does that leave us on the spectrum? we see a small portion, but everything else is happening all around us at the same time we just don't see it.

this topic was definitely a hard choice, not even google can find me an answer to what is light lol Is it just something we have to experience for ourselves as language can't obviously find the words to use

 

[Astronuc]

Light is not a wave of particles.

The light we see is a collection of individual photons. Photons have wave and particle properties, and essentially a photon is a particle.

One needs to distinguish between the individual photon (and its behavior or interaction) and the collective behavior of a population of photons, much like one would distinguish between a molecule of water and a liter or gallon of water, or a gas atom and the atmosphere above one.

Color is a rather arbitrary interpretation of light based on the interaction of light with our eyes. Once the photon interacts with our retina, it no longer exists, but there are other photons that precede or follow each photon that emanates from what we observe.

 

[bykerboy011]

I understand that, photons act differently when by itself vs in the light from the sun. But what I am having a hard time grasping is when in nature do we ever experience photons by themselves. I mean obviously we can detect them with different sensors adjusted to certain levels of the ES, but essentially photons stay together and fly in packs...

When you flip the light switch off on a lightbulb, where does the light go?...

 

[ChiralWaltz]

oh boy, big splashy liquid helices of photons

I would give mention to how we can direct light in a circular polarized orientation and also how nature does it.

 

[Astronuc]

I understand that, photons act differently when by itself vs in the light from the sun. But what I am having a hard time grasping is when in nature do we ever experience photons by themselves. I mean obviously we can detect them with different sensors adjusted to certain levels of the ES, but essentially photons stay together and fly in packs...

Light from a source travels in many directions. Go our a few million km, and one would find light from a point source is diverging.

When you flip the light switch off on a lightbulb, where does the light go?...

When one turns off a light switch, the production of photons ceases, because the excitation of the atoms producing the light ceases. The light (photons) that encounters surfaces is either absorbed or reflected. Reflected light will ultimately be absorbed somewhere. Light that is absorbed simply contributes to the energy (usually in the form of low level heat) within the material that absorbs it.

 

[SteamKing]

Light is the opposite of dark.

 

[Danger]

When you flip the light switch off on a lightbulb, where does the light go?...

This time of year, probably Cleveland.

 

[jim hardy]

When you flip the light switch off on a lightbulb, where does the light go?...

Back into the bulb, through the wires and into the generators which pull the water down the river.
oops, conventional light goes the other way.

 

[FlexGunship]

In my opinion, light is both, its a wave of particles.

But yes college standards are such a joke these days, your jaw would hit the floor if you saw how basic the standards are and yet people still can't grasp anything.

Surely the irony here is tragic. Emphasis added.

When you flip the light switch off on a lightbulb, where does the light go?...

Is this the topic of your discussion? You should be able to answer this question in far less than five minutes:

Light from a light bulb is produced in different manners depending upon the type of light bulb in question. When you flip the light switch off, the production of this light ceases.​

Your question is either simple or misleading. When you stop writing, where do the words go? When you stop yelling, where does the noise go?

 

[bykerboy011]

my question has gotten completely sidetracked. basicallly what I am doing is a speech on what is light? simple as that but its also my final and i need to come up with something good, something that will make my fellow students pull their iphone out of their a** and look at life a little differently.

But the best answer for the light bulb question i could come up with is the the light disperses into the room at the speed of light and energy is ceased to be produced.

 

[ZapperZ]

What is wrong with simply explaining that light is an electromagnetic wave? How complicated do want to do this? Are you expecting to get questions on the photon picture of light?

Zz.

 

[dipole]

Yea, I wouldn't bother trying to explain light in the context of QED/QFT because you're really getting into some very murky water there when trying to understand what a photon is in terms of pictures and analogies which aren't mathematical equations, and honestly it's probably impossible to do so anyways.

The classical picture is difficult enough for laypeople to get, and probably more accurate anyways when put in terms of simple pictures and analogies.

 

[Fredrik]

The two main theories of light are classical electrodynamics and quantum electrodynamics. In the former, light is described as a wave solution of Maxwell's equations for the electric and magnetic fields. Those two fields can be combined into one, the electromagnetic field. So light can also be described as a wave solution of Maxwell's equations for the electromagnetic field. If we simplify this statement a bit, we get what Zz said, it's an electromagnetic wave.

In quantum electrodynamics, light consists of photons, but what are photons? This is really hard to explain. The theory doesn't really define "photons", it defines things like "1-photon states" (of the electromagnetic field). So it's going to be very hard for you to give them a meaningful definition of "photon".

I think I would mostly talk about the classical description, and mention that there's also an even better theory in which light is described in terms of particles. I would also mention that these are not particles in the classical sense of "a pointlike object with a well-defined position", and that they would have to study quantum field theory to understand it.

 

[collinsmark]

Sorry to be nit-picky, but I need to correct some terminology here.

The term "light" applies to infrared radiation all the way up through ultraviolet; visible light is small sliver somewhere near the middle.

For some reason, frequencies below infrared are not considered "light" (such as radio waves and microwaves) nor are frequencies above ultraviolet (x-rays, gamma rays and cosmic rays). This is really just a terminology thing: they're all electromagnetic waves, and can all be thought of as photons too, but only the band from infrared to ultraviolet is called "light." (This is mostly a matter of language more than it is anything technical.)

But there is some logic to this insanity. Light (from infrared through ultraviolet) is created and absorbed by electrons transitioning between excited states of atoms. When an electron is in an exited state it might fall to a lower energy state releasing a photon in the process. Similarly, a photon might bump up an electron to a more energetic state in an atom.

Radio waves and microwaves are created and absorbed by moving electrons around in a conductor, but the electrons involved are not necessarily in any "excited state" They remain in the conduction band. Also microwaves can be created (and absorbed) by thermal interaction of polar molecules -- again though, nothing that involves excited electrons. Since it doesn't really involve "excited states" it's not called "light."

X-rays involve completely ionized electrons accelerated to extremely high velocities and then smash into stuff, releasing a x-ray photons in the process. This is above and beyond the normal interaction of simply "excited" electrons, so its not called "light."

Gamma rays involve nuclear interactions, not excited states of electrons. Thus it's not called light. Cosmic rays are more mysterious, but they are definitely not the simple, familiar excited electron state transitions.

[Edit: When defining these bands of electromagnetic radiation, from radio waves up through cosmic rays, there is always some overlap when it comes to the photon interaction. There are not necessarily well defined boundaries.]

 

[bykerboy011]

Finally, someone actually understands me lol Collinsmark you are officially my hero that is exactly the response I've been looking for. So the reason we can't see xrays or microwaves is because its not in a range on the spectrum where photons are released from which our eyes than pick up?

 

[collinsmark]

Finally, someone actually understands me lol Collinsmark you are officially my hero that is exactly the response I've been looking for. So the reason we can't see xrays or microwaves is because its not in a range on the spectrum where photons are released from which our eyes than pick up?

Actually yes, that sums it up pretty well.

Our eyes have rods and cones that are sensitive to certain frequencies of light. The mechanism involves electrons in atoms in certain molecules (in some sort of cellular sub-structure that I'm not at all familiar with) getting bumped up from one energy state to a different, higher energy state. Somehow or another (again I don't know anything about the details) produces a synaptic response. Although I don't know the details, I'm guessing that the excited state of an atom in a particular type of molecule induces a electro-chemical reaction (perhaps with some other molecules) which then is ultimately registered as the synapse, but that's just my guess.

You might want to research rods and cones to figure out more about how they work (I can't be of much help there).

But yes, x-rays won't cause the particular excited state transition to happen that would otherwise end up as a synaptic response: instead, an x-ray photon would just blow the electron clean off the molecule. Radio wave photons don't have enough energy to excite the electron, so they won't cause the necessary excited state transition either. (Edit: radio wave/microwave photon would cause the molecule to vibrate, but not cause an electron excitation.)

On a subtle, yet important note, the same can be said of infrared photons and ultraviolet photons too. Although these types of photons can cause electron state changes in general, they are not the correct energy for the particular electron state changes that would cause the right state change that ultimately registers a synapse response. So we can't see ultraviolet or infrared light with our eyes. But for whatever reason, infrared and ultraviolet are still called "light," unlike x-rays or radio waves.

 

[bykerboy011]

Ok now it all makes sense, the point i was shooting for with regards to my speech, is along the line of "visible light" being the same thing as xrays or microwaves just energy level turned up or down. Kind of my way of trying to get people to think about life a little more.

What gets me is i know snakes see more on the infared and bees see more ultraviolet, so with that being said this has to mean that were being subjected to most the spectrum all day every day but just can't see it...

[Mentor's note: unscientific speculation removed]

check this out though...http://www.physics.uc.edu/~sitko/ReflectanceSpectroscopy.pdf

sorry if i keep jumping around topics, this speech has really had me thinking...

 

[jim hardy]

[Mentor's note: quote of bykerboy011's speculative comment removed]

you're asking "what is sentience ? " ?
one of my back-of-the-brain musings too.
If it's not an electromagnetic phenomenon we won't find it with voltmeters and yardsticks.

Meta- is a prefix meaning beyond or adjacent. Sentience is beyond traditional physics hence metaphysical.. and there's a lot of "metaphysical crackpottery" about.
Try a search on these terms:
"Wolfgang Pauli Carl Jung Synchronicity"
you might find an anecdote you could use in your talk.

Have fun - public speaking is really good for a fellow.
Dale Carnegie course says you should start a speech with something from your own experience that's related to the subject..

old jim

 

[sophiecentaur]

So I've got to give a speech for a class soon with the informative topic of what is light. Light is just what we call the visible portion of the spectrum. How can i go about defining this to a group of college kids in a way they can understand?

thanks

I think the important message for you to get across is light is not a particle or a wave. It is LIGHT but it does have particle-like and wave-like properties. The wave-like properties are really quite like the mechanical waves you can see around you, in many ways and that is fairly easy to talk about with some certainty. You can measure the wavelength, it will reflect and refract according to the same sort of laws that sound and water waves do. Also, you get diffraction according to the same sums that describe the diffraction of sound etc.. These are things you can easily talk to a class about and you can even give pictures (loads available on the web) which fit in with some sort of 'reality'.
BUT, when you try to talk about the particular nature of light, you run into all sorts of difficulties and the popular model of photons as particles that comes into most peoples' heads is really no good at all. Photons are not not not the slightest bit like little bullets. They have not been measured to have any size or extent, they have no mass, if you want to consider them traveling between A and B then they could be 'anywhere' in between and you can only say where they are / were by actually destroying them in the process. You can say that they more or less have to be in two places at once, to explain how you get a simple interference pattern. Despite the fact that he was a pretty clever bloke (no doubt about that), Richard Feynman really didn't help when he insisted that photons "are particles" because the sort of particle he almost certainly meant is not what yer average bloke has in mind. I think he sort of failed to realize just how much smarter he was than the people he was lecturing to. If you, as a layman, want to talk about photons to very uninformed people then I can only suggest that you get the message across that photons are, in fact, very weird and mysterious. It is possible to describe many phenomena by talking in terms of the probabilities of photons being in some places more than in other places (like in the main beam of a torch) but you can't approach the simplest examples of diffraction without getting seriously heavy and being misunderstood.
If you want to talk about light then I could suggest an historical approach. You can start with the Tactile Theory (loads of links about it) and the later Corpuscular Theory. Then move on to the wave theory - for which there are loads of historical resources. Then you can introduce Planck's work which showed that light (all em radiation) must be quantised (when the word 'particle' sort of got re-introduced into the argument). Move on through Einstein's work on the Photoelectric effect, which confirms that em energy comes in specific quanta and that the amount of energy in a quantum is determined by the wavelength.
Of course they will expect you to mention photons and you would do them an immense favour by warning them that the popular models are not very helpful because of all the false predictions they lead to. There's a fine line to be drawn here because if you tell them "it's all too hard for you to understand so don't bother" then it will turn them off. You could, however, emphasise the mysterious nature of the photon and of the many different things that it is NOT.
God only knows what you do when it's time for questions . . . . . . . Good luck.
btw, how old are the members of the class?

 

[Fredrik]

I think the important message for you to get across is light is not a particle or a wave. It is LIGHT but it does have particle-like and wave-like properties.
...

I got the impression that in spite of the question in post #1, all he wants to know is what wavelength an electromagnetic wave must have to be considered "light".

 

[sophiecentaur]

You're probably right. Just as well.

 

[bahamagreen]

You are not really equipped to present theory about light, and your audience is not likely to be prepared to follow it. The best you can hope for is that a few people will discover something new to them and perhaps develop an interest in these things.

With just 5-6 minutes to present, you will need to keep it simple. Fortunately, you can do quite a lot if you get them involved...

Light a candle and turn out the overhead lights to make the room dark.
Have everyone take note of the amount of light provided by the candle, ask them to predict how much brighter it will be when you light a second candle, then light a second one, then a third.

Now that you have some candles burning, hand out some mirrors and see how many mirrors they can get involved in reflecting the candle flame, and what angles allow that to happen.

Blow out the candles and while it is still dark, hand out some flashlights with different colored transparent plastic bags over the lenses so they shine red, green, yellow, blue colored light. Have them shine these on different colored things. Have them shine two different colors on the same object and look at what happens to the shadows.

Check first to see if you will need a fire marshal (because of the candles) or the school nurse (because of the glass mirrors) or a paramedic with a resuscitator (because of the plastic bags) to be present during your presentation to ensure student safety.



You might want to try these things yourself in advance.

 

[Light Bearer]

Surely the irony here is tragic. Emphasis added.

Hahahaha.

 

[sWozzAres]

Technically, light(visible), is no different than the rest of the electromagnetic spectrum, its just energy on a different frequency. So in a sense its all the same. So does color exist?

Colours (qualia) exist in the mind. You can say light at 700nm causes you to experience the colour red but colour is more than that. If I shine red and green light at you, you don't perceive red and green, instead you perceive yellow, even though there is no yellow light out there! We seem to be limited in the number of colours that we can represent. Presumably, if we could sense into the ultraviolet then we wouldn't see a new colour, we would see the same seven colours but they would they would be stretched over a wider frequency range? In the same way we hear seven tones, do-re-mi-fa-so-la-ti, short-term memory can remember seven "chunks" of information (give or minus 2). Interestingly gamma and theta waves in the brain seem to be coupled and you can fit seven gamma waves into one theta wave.

I don't know how related these phenomena are, but without minds, colour wouldn't exist.

 

[jim hardy]

Maaybe the talk would be more interesting for this particular audience if it related to everyday things people experience...
Here's a link on human receptors:
http://www.yorku.ca/eye/toc.htm
with this neat graphic:

and this from hyperphysics:

The light response of the rods peaks sharply in the blue; they respond very little to red light. This leads to some interesting phenomena:

Red rose at twilight: In bright light, the color-sensitive cones are predominant and we see a brilliant red rose with somewhat more subdued green leaves. But at twilight, the less-sensitive cones begin to shut down for the night, and most of the vision comes from the rods. The rods pick up the green from the leaves much more strongly than the red from the petals, so the green leaves become brighter than the red petals!

http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/vision/rodcone.html

also from hyperphysics:

While the visual acuity or visual resolution is much better with the cones, the rods are better motion sensors. Since the rods predominate in the peripheral vision, that peripheral vision is more light sensitive, enabling you to see dimmer objects in your peripheral vision. If you see a dim star in your peripheral vision, it may disappear when you look at it directly since you are then moving the image onto the cone-rich fovea region which is less light sensitive. You can detect motion better with your peripheral vision, since it is primarily rod vision.

That's why, at night you can see the strobed LED taillights in modern cars "flicker" as you shift your view across the lanes. (I find it really annoying)



Wikipedia says birds have another receptor that sees ultraviolet:
http://en.wikipedia.org/wiki/Bird_vision

and here's more than anybody wants to know about evolution of eyes:
http://www.pigeon.psy.tufts.edu/avc/husband/avc4eye.htm

Good luck with your talk.

 

[bahamagreen]

The compound eyes of many insects (bees, for example) allow them to detect the plane of polarization of light (the sky does not "look the same" in different directions)...

Many soaring predatory birds have up to 50 times the retinal cell density of humans - their resolution is much greater.

Some spiders have elongated primary front facing dual eyes in which the eye "ball" is extended to the back of their head providing some magnification.

The retina is comprised of ten functional layers of cells. The cortex of the brain is also comprised of ten functional layers... as if the retina might be a highly modified extension of the surface of the brain itself.