# B Can a photon travel slow enough to be seen by the naked eye

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1. May 2, 2018

### me-Lights

Please see my attachment of a recent observation of light traveling through a medium.

There is no source, just my observation

The packets of light appear to be visible, distinct , moving at different speeds, and display the various colours for each wavelength.

Is this normal / possible in a medium?

Please note that it can be recreated visually (randomly)

Thank you

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2. May 2, 2018

### DennisN

Hi me-Lights,
I do not understand your pdf, particularly what equipment you used (a prism?) and how you got the various images (moving/rotating the prism?), which to me seems like quite ordinary spectra.

You asked "can a photon travel slow enough, to be seen by the naked eye?"
The problem is not the speed of light, the problem, or more correctly, one problem, is that a photon actually is a very tricky object. And I think it could be a good idea to have a look at this article:
"See an electron lately?", which describes some things about our eyes and e.g. photons.

3. May 2, 2018

### sophiecentaur

I agree. Different distances travelled by Different wavelengths of the light is not what the picture implies. The 'transit time' is of the order of a few ns and the dispersion varies by about 2% over the optical range.
What the OP is seeing is the difference in the angles of refraction being magnified by the 'throw' of the emerging beam of light. If the optics produced a parallel beam then there would be no difference. To observe the speed difference directly would require the sort of measurements that were done in the past, with a large separation of source and detector and a very fast optical shutter. Search for Fizeau measurement of c for a good read.

4. May 2, 2018

### Andy Resnick

Your diagram shows the effect of group velocity dispersion. Your title makes no sense.

5. May 2, 2018

### phinds

@me-Lights to expand on Andy's comment that you title makes no sense:

(1) you need to stop and think about what "see" means. You see when em waves hit your retina. The refractive index of air is essentially zero so light propagates at approximately c in air thus any photon that hits your eye is traveling at near-c. So thinking that a "slowed down" photon can hit you eye doesn't make any sense.

(2) Light propagation can be slowed down in various mediums BUT ... what hits your eye is still a em wave that has traveled to you through air, so again, it is traveling at near c, so again, thinking that a "slowed down" photon can hit you eye doesn't make any sense.

6. May 2, 2018

### Staff: Mentor

Photons are unrelated to what you're seeing. It's generally best to think of light classically, as an electromagnetic wave, except when you're working with phenomena that cannot be explained without using quantum electrodynamics, and that's not the case here.

Instead, what's going on here is that light of different wavelengths travels at different speeds in a medium (in a vacuum, all light moves with speed $c$). White light is a mixture of different wavelengths, which separate because they're travelling at different speeds in a medium. Our eyes, optic nerves, and brain interpret the different wavelengths as different colors so you're seeing the effect of the different wavelengths moving at different speeds in the medium.

7. May 2, 2018

### me-Lights

Thank you all for your responses, very informative

I did note that I can observe the light in a medium.

So here's how I got the observation:
2) make sure it's off
3) shine a torch onto the TV screen and you should be able to observe the light, similar to my diagram (which was simply a reconstruction)
PS: a lit candle or cigarette lighter will do the same,but torch much better

Thanks again

8. May 2, 2018

### davenn

that's the information that you should have been giving us at the start so that everyone didn't have to guess as to what you were doing
It leads to different situation to what you initially described

We ONLY see a photon when it interacts with our eyes ... it's speed is irrelevant for that to happen

9. May 3, 2018

### Staff: Mentor

Are you familiar with how a prism can be used to project a spectrum onto a wall? You're doing something similar: light waves emitted by the torch, separated by frequency as the move through the screen, eventually traveling to your eyes where you "see" them.

10. May 3, 2018

### Staff: Mentor

You can observe the photons that are scattered from their original path into your eye, but you cannot observe the photons which are not scattered into your eye and that continue along their path.

Your diagrams demonstrate not the packet nature of light, but appears to demonstrate wave interference of the light as it is reflected from the pattern of pixels on the screen. The pixels form a diffraction grating, causing interference, which generates the spectrums you are seeing along with the gaps in between each one. If you look at the first picture in the link I just gave, you can see the gaps in between each order.

11. May 3, 2018

### me-Lights

Thank-you once again

12. May 5, 2018

### Droidriven

As for the "slowing down enough".....

Is this, in any way, relavent to the aspect of possibly slowing it down enough to see "if" it were even possible to see it?

Not that it actually can, just "theoretically"?

http://www.bbc.com/news/uk-scotland-glasgow-west-30944584

Considering that photons themselves are what makes it possible to "see" anything in the first place because of photons that are being emitted by a source or photons from that source being reflected/refracted by another object....the photons that we are trying to observe would have to reflect other photons to our eyes in order to see the photon doing the reflecting.

This seems seems to be similar to asking if it is possible to hear a certain sound wave by using other sound waves to hear it instead of hearing the sound itself, hearing a sound by hearing other sounds.

This leaves me with a simple.......huh?

Last edited: May 5, 2018
13. May 5, 2018

### sophiecentaur

You have to be careful when reading a bit of pop science news. Kenneth Macdonald, the 'Science Correspondent' uses the term "Wave particle duality". That should ring a warning bell. It is not a valid reference, I think.
"Theoretically" - what theory? Do you mean Hypothetically?

14. May 5, 2018

### DennisN

"Seeing" is a very tricky word when it comes to these small things...
We see objects like humans, stones, cars etc. because they reflect light, and the light itself is made up of quanta called photons.
So if we think about seeing a photon like we see, let's say, a car, this would mean that the photon would have to be able to reflect other photons which then hits our eyes. Now, the fun thing is that this does not happen (caveat: light to light scattering can happen, but it is very improbable), so this means we can not see photons like we see cars.

15. May 5, 2018

### Droidriven

I stated this in my reply before you sent me this.

I understand the scenario, I was just kinda saying "hey, this might be close, but still isn't possible", hence the "if" it were even possible.

16. May 5, 2018

### Droidriven

Yes, hypothetically, to make the distinction clearer.

17. May 5, 2018

### DennisN

To the OP and others, another thing is that quantum mechanics make the prospect of seeing a photon even more unusual and interesting. I will make an analogy (note it is only an analogy): detecting a photon in experiments with detectors is a bit like being deaf and standing with your back against a person who is going to throw a ball. You don't know when he throws the ball, if he throws the ball or how it travels. You will only know if it hits you, and when it hits you, the ball gets destroyed. But please note that this is an analogy, the photon does not act like a ball!

Last edited: May 5, 2018
18. May 6, 2018

### sophiecentaur

Yes. You can only 'see' a photon once so all you can hope for is to observe the difference between (a lot of) photons hitting one detector and then the remaining photons hitting another detector further down the line. That detector could be your eye.

19. May 8, 2018

### Shane Kennedy

A photon, and light in general, cannot be "seen" unless it hits a detector. That detector can be dust suspended in the air, a photocell, or the retina of an eye. Single photons can be detected, by highly specialised methods and mechanisms.