# I Huygen’s Principle and Light

#### FAS1998

My understanding of light has been that it travels in a perfectly straight line unless reflected or refracted by some object. Treating light like physical objects (like pool balls bouncing off the sides of a table) has been useful for situations where geometry can be used to find things like phase differences between beams of light.

Huygens principle says that this is incorrect. Light going through a slit will spread out as if each point on the wavefront is a source emitting light in all directions. If we treated light as a physical object in the case of single slit diffraction, we would just see light where the slit is, and see no light where the light is being blocked.

When can/can’t light be treated like physical objects?

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#### A.T.

Treating light like physical objects (like pool balls bouncing off the sides of a table)
Seems like by "physical objects" you mean "particles".

Huygens principle says that this is incorrect. Light going through a slit will spread out as if each point on the wavefront is a source emitting light in all directions.
That's the wave model of light.

When can/can’t light be treated like physical objects?
https://en.wikipedia.org/wiki/Wave–particle_duality

#### Drakkith

Staff Emeritus
2018 Award
When can/can’t light be treated like physical objects?
Because light travels like a wave and interacts like a particle. In other words, the EM wave (which is what light is) travels like a wave and interacts in discrete amounts, as if it were a particle. Kind of. It's complicated and non-intuitive.

#### Ray Vickson

Homework Helper
My understanding of light has been that it travels in a perfectly straight line unless reflected or refracted by some object. Treating light like physical objects (like pool balls bouncing off the sides of a table) has been useful for situations where geometry can be used to find things like phase differences between beams of light.

Huygens principle says that this is incorrect. Light going through a slit will spread out as if each point on the wavefront is a source emitting light in all directions. If we treated light as a physical object in the case of single slit diffraction, we would just see light where the slit is, and see no light where the light is being blocked.

When can/can’t light be treated like physical objects?
Light is both a wave and a particle---or perhaps neither. If you perform a single or double-slit experiment with very low intensity light, you can detect individual photons (light particles) striking the screen behind the slits. When many, many photons have been so detected, they build up the familiar interference pattern that characterizes wave propagation. Where the individual photons end up on the screen seems to be totally unpredictable (i.e., random), but the patterns of light and dark areas you get with waves do, in fact, emerge when many photons have gone through the slits. (There are special cameras that can detect the photon strikes one at a time and retain the information, thereby allowing one to build up the interference pattern one particle at a time!) Google "individual photons in interference patterns" so see details of some relevant experiments.

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#### sophiecentaur

Gold Member
Light is both a wave and a particle-
I have to question the use of the word "is", here. All you can say is that light can be treated as a particle or a wave, depending on the circumstances, and be able to make predictions. Plus, the word "particle" is terribly mis-used in most discussions about photons. Even using a concept like 'photons going through slits' is potentially confusing if you are trying to keep the little bullet model in your head. The word "particle" is oxymoronic; it is very small (a point) yet at the same time it occupies all of space. I blame that Feynman Diagram (Google has loads of info ) and its wiggly line with an arrow on it. He knew what he meant but how many other people do?

#### A.T.

I have to question the use of the word "is", here.
Why just here? You can make that nitpick about any physical model.

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#### sophiecentaur

Gold Member
Why just here? You make that nitpick about any physical model.
In the case of the photon / particle idea, it's much deeper than that. I am sure you wouldn't want to champion the little bullet model for photons, would you?

#### Ray Vickson

Homework Helper
I have to question the use of the word "is", here. All you can say is that light can be treated as a particle or a wave, depending on the circumstances, and be able to make predictions. Plus, the word "particle" is terribly mis-used in most discussions about photons. Even using a concept like 'photons going through slits' is potentially confusing if you are trying to keep the little bullet model in your head. The word "particle" is oxymoronic; it is very small (a point) yet at the same time it occupies all of space. I blame that Feynman Diagram (Google has loads of info ) and its wiggly line with an arrow on it. He knew what he meant but how many other people do?
I qualified my first sentence by saying "Light is both a wave and a particle---or perhaps neither." I am well aware of how tricky and non-intuitive the modern view actually is, but nevertheless, when photons are detected after transiting the slits, they are small enough to activate single micron-sized silver halide crystals (in old-fashioned film), and these individual crystals are much smaller than the size of the light-dark bands of the interference pattern. That means that when the photon finally decides to manifest itself in that particular experiment, it does so as a "small bullet". Basically, the wave-function collapses and we detect a point-like object.

I did not want to ge into all of that with the OP, who came across as a student who may be struggling with some basic concepts.

#### sophiecentaur

Gold Member
struggling with some basic concepts.
And that's the problem. Those basic concepts are invented and selected in the baseless hope of making it possible to make a start on understanding Modern Physics too early. It's enough of a Catch22 situation as it is and the Duality Principle is still used in teaching with no thought that it just doesn't need to be.
If students are to young to be able to grasp that "can be treated as" is a better way of thinking than "is" (and that's because we can't do QM at this stage - it's too hard) then they really shouldn't have the thing inflicted on them. If they can only deal with Concrete Thought then why give them Formal Thought?
That's no more a form of censorship than delaying Integral Calculus until they have a good base of Algebra.

#### sophiecentaur

Gold Member
nevertheless, when photons are detected after transiting the slits, they are small enough to activate single micron-sized silver halide crystals (in old-fashioned film), and these individual crystals are much smaller than the size of the light-dark bands of the interference pattern.
I am not sure how much you are being Devil's Advocate here but I am answering you as if you literally mean it:
You are making a massive assumption about what happens - based on a totally mechanical thought model. The small bullet model is taken (by its proponents) as describing its whole existence over the interval between being emitted and detected. A 'bullet' doesn't only have existence when it is launched from the gun and when it hits the target so the analogy really does fail over the rest of its existence. The analogy is very very attractive (the corpuscular theory was a big hit at the time) but it just doesn't cut the mustard and we shouldn't expect anything so basically friendly for the mind to do so. The Word Police have been at work throughout the history of Science, amputating unsuitable words and ideas but they seem to be ignoring that little P word. Perhaps it should always be used in inverted commas? Or perhaps the P word could always be assumed to be Virtual Particle because it only makes its presence known at the instance of an interaction.

#### Nugatory

Mentor
When can/can’t light be treated like physical objects?
A pretty good rule of thumb is that you can ignore the wave nature of light when all the sizes and distances in your problem are large compared with the wavelength of the light you're working with. There is no conflict between this simplifying assumption and the Huygens principle; it turns out that at this scale interference between the various Huygens wavelets cancels them everywhere except along the straight line path with interference and diffraction only visible at the edges.

It's worth googling for "phased array radar". These devices use the Huygens principle to generate a straight beam of electromagnetic radiation that can be steered in any direction just by altering the phase relationships of the individual emitters, and seeing how they work is very helpful.

#### tech99

Gold Member
A pretty good rule of thumb is that you can ignore the wave nature of light when all the sizes and distances in your problem are large compared with the wavelength of the light you're working with. There is no conflict between this simplifying assumption and the Huygens principle; it turns out that at this scale interference between the various Huygens wavelets cancels them everywhere except along the straight line path with interference and diffraction only visible at the edges.

It's worth googling for "phased array radar". These devices use the Huygens principle to generate a straight beam of electromagnetic radiation that can be steered in any direction just by altering the phase relationships of the individual emitters, and seeing how they work is very helpful.
Although Huygen's Principle gives correct answers, it is conceivable that the effects arise from currents flowing in the obstruction. As far as I can see, an EM wave will not change its direction or speed unless it encounters a charge.

#### robphy

Homework Helper
Gold Member
Go to 4m53s (also a good quote at 7m25s)

#### Mister T

Gold Member
If we treated light as a physical object in the case of single slit diffraction, we would just see light where the slit is, and see no light where the light is being blocked.
Light is treated as a physical object. It behaves in a way that seems to disagree with the way you think it ought to behave?

#### Nugatory

Mentor
Although Huygen's Principle gives correct answers, it is conceivable that the effects arise from currents flowing in the obstruction. As far as I can see, an EM wave will not change its direction or speed unless it encounters a charge.
Giving correct answers is all that Huygens' principle has ever claimed. The linearity of Maxwell's equations means that any electromagnetic wave can be described as a superposition of spherical waves from point sources, and that's what Huygens's principle is.

The point of the phased array radar example is that it illustrates how Huygens' principle cleanly describes beams travelling in a straight line and other phenomena that at first glance look incompatible with it.

#### sophiecentaur

Gold Member
A pretty good rule of thumb is that you can ignore the wave nature of light when all the sizes and distances in your problem are large compared with the wavelength of the light you're working with.
Just to be on the safe side, I would point out that using the photon model at large scale, as an alternative, is equally dodgy. Photons essentially describe what goes on at either end of the light path. It would be seriously taxing of the brain to replace waves with entities that occupy all possible routes from A to B etc. etc. in any attempted description of the phenomenon.
Perhaps the explicit concept of RAYS should be given a more elevated status in this sort of discussion. They are an established part of modern Optics and are not just for lower school kids.

"Huygen’s Principle and Light"

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