Which Model of Light Do Scientists Use to Explain How Light Can Interact With or Change Things?

Information technology'due south in your physics textbook, become look. Information technology says that yous tin can either model lite as an electromagnetic wave OR you can model light a stream of photons. You tin't utilise both models at the same time. It's one or the other. It says that, get look.

Here is a likely summary from well-nigh textbooks.

i. Light as a wave: Low-cal can be described (modeled) every bit an electromagnetic moving ridge. In this model, a irresolute electric field creates a changing magnetic field. This changing magnetic field then creates a changing electrical field and Smash - yous have calorie-free. Different many other waves (sound, water waves, waves in a football stadium), low-cal does not need a medium to "wave" in.

Oh, that is too simple of an explanation? How about this?

These are i form of Maxwell'due south equations. They describe the human relationship between the electric and magnetic field (well mostly the last two). If y'all like, you can use vector calculus on the above equations and then eliminate B to get:

This is the class of the wave equation. And then, Maxwell's equations do say that light is a wave.

ii. Calorie-free every bit a particle: The textbook might showtime off with some experimental evidence from the historic photoelectric effect to prove that the wave model of light doesn't always describe what happens.

It volition and so say that nosotros can model light every bit individual "things" (some books actually say particles and others just say photons). These light "things" have free energy that depends on the wavelength such that:

Hither h is Planck's constant and λ is the wavelength of the light and f the frequency. With the photon model, a brighter calorie-free but produces more photons per 2nd.

Is light a particle or a wave?

Nigh texts terminate with something similar this:

"Is light a particle or a moving ridge? This is a difficult question - the respond is that in some situations light behaves as a particle and in others it behaves as a wave."

What's incorrect with multiple models?

Nosotros always have multiple models for things that we see. Yet, they are different than this wave-particle model of light. Allow'south look at a few other models.

Momentum. When you start looking at momentum, information technology is well-nigh always (except in the awesome textbook Matter and Interactions) divers as:

This is great. It'south simple and it's useful. It goes neat with the momentum principle that says that the net forcefulness on an object is the time rate of change of momentum. Of course, you lot could besides say it is wrong. What if you accept a proton moving at 90 percent the speed of light? In that case, you lot can't use this definition of momentum with the momentum principle. Instead, y'all have to apply this model:

That's nice, correct? Some people call this the "relativistic momentum". However, I like to call this just plain momentum. Simply what does this have to do with 2 models for light? Well, what if I wanted to find the momentum of a proton going at just ten% the speed of light? Which model would I use? The answer depends on how quickly you desire to summate this and how authentic you want your answer to be. Yes, I know "quick" is relative.

Here is a plot of momentum of a proton as a role of speed for the ii models.

You tin meet that at lower speeds, the two models agree. The faster the proton goes, the less the two models hold.

Gravity. Everyone knows the model for the gravitational force, right? Y'all can write it like this:

No. That is wrong. That model only works when close to the surface of the Earth. The gravitational force is:

That is still incorrect, but better. However, we don't often use the improve model for the gravitational forcefulness most the surface of the Earth. Why? Because the mg model works well enough. Besides, the two models agree on the surface of the Earth merely like the two expressions for the proton momentum agree for "wearisome" speeds.

Quantum Mechanics. I am going to skip many of the very interesting details, simply permit me simply say that I can use the following model the behavior of a super tiny particle in a box. Here is an older post with most of the particle in a box details. Knock yourself out with that.

Or peradventure y'all would like to write it out similar this:

This is Schrodinger's equation and Ψ is called the wave office. It doesn't give you lot annihilation you could directly measure, but from it you could get the probability density - or a description of where a particle is probable to be found (or really, anything else you can know about the particle).

But await! There's more. What if you lot use Schrodinger's equation to await at a particle in a 1 dimensional box? Why would you lot do this? Because it is mathematically simple and because nosotros can use it to explore some of the results of a breakthrough organization. From Schrodinger's equation, you would find that the particle can simply be at certain detached energies. This is actually one of the fundamental points of quantum mechanics (information technology's the quant in quantum).

My favorite breakthrough analogy is a staircase. For a staircase yous can be on i stride or the next step just y'all actually can't be in between steps. In this case, yous could say that height is quantized. The same is true for a particle in a box or an electron in a hydrogen atom. There are only sure possible energy levels.

Does this quantum free energy model concord with classical mechanics? Aye. If you looked at a tennis ball billowy back and forth in a typical classroom, y'all could calculate the quantized free energy levels. However, these energy levels are so close to each other that yous essentially would never be able to experimentally verify that the ball tin can only have sure free energy levels.

Merely to be clear: the quantum model of stuff is but like the other models above. Information technology slowly gives a unlike result from the classical model of stuff.

Why Practice Textbooks Include the Photon Model of Calorie-free?

You take been very patient. I know y'all want to talk virtually photons, only I had to go the model stuff out of the manner. Just like I said, just well-nigh every introductory physics textbook talks most photons using the photoelectric issue as a basis for this model.

There is a reason for this. Albert Einstein won the Nobel Prize in 1921 in office for his caption of the photoelectric effect. Of course, Einstein did some other awesome stuff. In item, the full general and special theory of relativity. Just the Nobel Prize didn't mention this - just the photoelectric effect. Yet, during Einstein's acceptance speech for the Nobel Prize, he talked about relativity and non the photoelectric effect.

But hither is the crazy part (I know, y'all probably recall this whole post is crazy): the photoelectric result can exist explained with a classical wave model of light along with a quantum model of thing. Really, it can. Skipping the details, permit me just say (and you can wait in your quantum mechanics book to verify this) that if you have a particle with energy E1 and y'all want information technology to transition to the energy level Eastward₂ you tin can exercise that by adding a time-varying potential such that:

Hey! That looks strangely similar to the equation for the free energy of a photon. Yup. If y'all like, you can utilize low-cal with a frequency of f to induce the transition from 1 energy level to another. Even better, information technology doesn't matter if this transition is from a college to lower or lower to higher free energy level. This oscillating perturbation tin can explain both assimilation AND emission of lite.

What about the photoelectric result? Well, all the results you lot see experimentally can be explained if the electrons in the metal can only exist at certain free energy levels (quantum model of matter) and the low-cal is a wave. Actually, some of the older quantum mechanics textbooks prove this as an example trouble.

But so why is the photon model in textbooks? I would say it is because of educational inertia. Who writes the textbooks? If you answer "people", and so y'all are right. But where do these "people" learn physics? If you said "textbooks", that would be a fairly nice respond. So, people larn from textbooks that have photons. Side by side they write a textbook, so conspicuously they will have photons in their books. Simple.

Low-cal is Quantized

My main point here is that the photon isn't what you lot call up information technology is. It isn't a tiny little brawl of low-cal. It isn't light as a particle. However, light is nevertheless pretty weird. There is a breakthrough nature to the electrical and magnetic fields in light (breakthrough theory of radiations). But almost of the stuff you expect at can exist explained using a classical wave model of light and a quantized model for affair.

Appeal to Authorisation: I acknowledge that sometimes, things get confusing. In instance whatever of my arguments don't brand whatever sense, I will add some opinions from experts (meaning people that know more I practise).

Perhaps the most recent is this quote from W.E. Lamb, Jr'south paper "Anti-photon" - Lamb Jr, Willis E. "Anti-photon." Applied Physics B sixty.2-3 (1995): 77-84.:

"Information technology is high time to surrender the use of the word 'photon', and of a bad concept which will soon be a century former. Radiations does not consist of particles and the classical, i.east., non-breakthrough limit of QTR is described by Maxwell's Equations for the EM fields, which practise not involve particles."

Or peradventure y'all would similar a quote from Einstein himself?

"All these fifty years of conscious brooding have brought me no nearer to the respond to the question, 'what are light quanta?' Present, every Tom, Dick, and Harry thinks he knows it, but he is mistaken."

Albert Einstein, letter of the alphabet to Michael Besso 1954.

TL;DR

Yes, this is long. Hither are the principal points so you don't have to read everything.

• Lite is crawly.
• Most models are wrong at some level. However, they slowly converge to other more correct models.
• Information technology is sort of silly to describe light as a particle.
• In fact, just about everything you lot see in undergraduate physics can be explained with a classical wave model of light forth with a breakthrough model of matter.
• I'm NOT denying that there is a breakthrough theory of radiations (QTR). For instance, photon anti-bunching tin can not exist described with a classical EM wave.

I wonder if I should put the tl;dr at the beginning. Oh well.

Preemptive Comments

I don't know why, but I expect some people to not exist so happy with this mail. In general, people have one of the following 2 responses to this kind of statement.

Now for some of the comments you lot might have.

• Are you saying Einstein was wrong? If and so, you are crazy. Actually, no. You tin describe the photoelectric effect with particles of lite. You just don't need to. Ok fine - Einstein was wrong about the photoelectric effect. He was still a genius and maybe the 2nd greatest physicist that we know of. Newton only edges him out because when he needed new math for his physics, he invented it. When Einstein needed new math, he learned it from mathematicians.
• (This is from my brother Neil, he has a annotate and a question) Yous just hate photons like Steve Jobs hates buttons. Tin nosotros still talk near photon torpedoes or are y'all going to ban those too? I don't hate photons. Hate is a strong give-and-take. But yes, yous can still employ photon torpedoes - but what about "light torpedoes"? Would that piece of work?
• What almost photon momentum? Nearly introductory textbooks give a nice caption of how an electromagnetic moving ridge can push button on electrically charged matter. I specially like the explanation in Affair and Interactions II (Wiley: Chabay and Sherwood). In fact, hither is my previous caption of how light tin push a comet's tail.
• What about some other particular affair dealing with photons? I will refer you to this very dainty paper by David Norwood. There. (The Use and Abuse of "photon" in Nanomechanics - pdf)

Chapeau Tip to David Norwood. Really, it's his fault that I was thinking about this whole effect. However, he did offer some nice suggestions for this post.

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Source: https://www.wired.com/2013/07/is-light-a-wave-or-a-particle/

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