View Full Version : Light. A Particle That Gives off Waves?

2016-Apr-07, 06:30 PM
Before I ask I'll explain that all my curiosities are mostly because of my limited knowledge of what I'm thinking about, and are almost always explained easily by filling in the gaps of my knowledge/understanding.

I often wonder how, when you see something, you always see every part of whatever from whatever angle (obviously given enough light).

I'll try to explain: if what you see is light bouncing off the object and hitting your retina (or camera lens etc.), then there has to be a particle coming from every part of that object going in every direction (so you can see it from any angle).
My first question is, how does light emit from an object (either directly or indirectly) so every part of it is visible from any angle from any distance? How do they not scatter and leave invisible spots? Think of this in distances. Say a star (or anything, really) many light years away. The photons coming from that have to be coming directly from that to whatever instrument measuring it precisely and directly. So, imagine a bomb exploding with tiny ball bearings or something. The further away you get, the more the ball bearings will scatter and get further apart, thus not hitting your retina.
That thought got me thinking about the wave/particle duality of light. Is light photons emitting a wave (to spread out)?

Getting ahead of myself, if light did scatter enough so it left invisible spots could that explain dark matter?

I'm sure brilliant minds have this covered, and less brilliant understand, and I'm open to ridicule over my ignorance.

2016-Apr-07, 06:52 PM
If you take a photograph in very low light with a short shutter time, you will not get photons into every pixel of the camera from the object being photographed.

2016-Apr-07, 06:59 PM
Maybe that's where shade comes from. A less intense wave of the particle.

2016-Apr-07, 08:35 PM
Maybe that's where shade comes from. A less intense wave of the particle.
Milky Joe. Your ball bearings model approximates light as quanta. Shade is not related to a less intense wave of a single particle, but rather a smaller flow of them...(flux)..per second. It's still huge. The human eye is a pretty sensitive detector of light...experiments have shown about three photons are detectable by a rod in the eye, stimulating the brain to "see".
As Antoniseb said, it takes time to photograph a very faint object, and the eye is not good at that....the receptor fires and has a recovery time to reset...the electronic pixel in the CCD can store accumulated hits, as can a photographic emulsion in a camera. pete

Geo Kaplan
2016-Apr-07, 09:20 PM
Maybe that's where shade comes from. A less intense wave of the particle.

At very low intensities, one must take into account the quantum nature of light. It is easy to arrange for no photons, lots of photons, and everything in between to strike a detector. See https://www.youtube.com/watch?v=MbLzh1Y9POQ as one experimental example. We have detectors that can sense single-photon events with ease. The human eye isn't quite as good, but is still impressive -- we can consciously sense fewer than ten photons at a time (the retina is still more sensitive).

2016-Apr-07, 09:45 PM
Bathed in sunlight: very roughly about 1017 photons/sec/cm2 in these parts. That's a lot. No wonder some wind up impinging on your retina.

2016-Apr-07, 10:30 PM
Yeah, but considering the vast distances, how do photons hit your eye? They must leave at every angle and at every concentration/direction.

2016-Apr-07, 10:31 PM
Not only that, but consistently.

2016-Apr-07, 10:56 PM
Yeah, but considering the vast distances, how do photons hit your eye? They must leave at every angle and at every concentration/direction.

You seem to be still thinking of photons as discreet small objects. They spread out as ripples in a pond, and impinge on anything in their path. You don't count ripples by the drop, do you?

2016-Apr-07, 10:59 PM
Not sure how current Feynman's concepts are but his lectures deal with what you are thinking about. If I understand it correctly the wave aspects of light can be explained by probability of particle paths.
There are a number of these lectures:

2016-Apr-07, 11:02 PM
Yeah, but considering the vast distances, how do photons hit your eye? They must leave at every angle and at every concentration/direction.

It might be hard to imagine, but keep in mind that the figure given above is 10 with 17 zeroes behind it, so an incredible number. So even at great distance some will strike each person's eyes.

Geo Kaplan
2016-Apr-07, 11:27 PM
Yeah, but considering the vast distances, how do photons hit your eye? They must leave at every angle and at every concentration/direction.

Not so much "every concentration" -- why do you think that?

Let's say that your light-emitting object throws off a certain number of photons per unit time, on average. As Jens has mentioned, there are enormously many photons generated by a star (again, per unit time). Now, as these photons spread out, their density drops as the inverse square of distance. But if you start with a gigantic number of photons (our sun throws off something like 10^45 photons per second -- that's a lot!), the probability that some will strike your eye can still be high enough for you to see them. That's why you can see stars in the sky. And the ones you can't see, well, they're too dim/far away to be detected by your eye.

2016-Apr-08, 01:01 AM
I estimated that a 6th magnitude star puts a few thousand photons per second into each eye, and that they are concentrated on a few nerve endings. That is enough to make those cells put out a steady series of nerve impulses, and the brain perceives a spot of light. A lower flux may stimulate some sporadic impulses, but when they are coming from only one spot on the retina the brain will ignore them. An extended patch of light such as a nebula may get numerous cells firing that way, and the eye-brain system will associate them and perceive a faint fuzzy wuzzy.

2016-Apr-08, 01:16 AM
Take a sphere. On its surface, place 100,000,000,000,000,000 equally-spaced dots.

If that sphere were several dozen light years in diameter, then, by Hornblower's calculations above, a section of it the size of your iris would still contain several thousand dots. Each is a photon.

2016-Apr-08, 02:46 AM
One can imagine the shower of photons from a star into a camera as being like rain falling on a rain gauge with a small opening, such as my old Brookstone gauge with a collection aperture about an inch and a quarter in diameter. In a light rain it will go many seconds at a time before catching a drop, but if the rain keeps up all day the gauge will give a reasonably accurate measure of how much falls in the immediate area. This would be somewhat like the Hubble telescope imaging a 30th magnitude object, which I think it can do by exposing for several days. This would be an average of less than one photon per second striking the 2.4 meter mirror. Nevertheless, if the guiding is good the imager will build up the same sort of Airy disk as with a bright star in a snapshot. Welcome to the weird and wonderful world of optical quantum mechanics.

2016-Apr-08, 08:47 AM
Hi MilkyJoe,
No, light is not a particle that gives off a wave - rather, it is a particle that is also a wave.

Let's start with your ball-bearing idea. Let's say there's a gun that fires ball-bearings one-at-a-time randomly in two directions (left or right) eventually hitting detectors in those directions.
For any given ball bearing, from the moment it's fired, it possesses a fixed position and velocity, so when it gets to one of those detectors, that one fires (and of course the other one doesn't).

Now compare this to light. A photon is emitted, and it follows both (all) paths simultaneously, as a "wavefront" spreading outwards. When the wavefront hits one of the detectors, there's a probability that it will be detected there. If it is detected, then it is 100% detected (meaning 0% probability that the other detector will subsequently detect it).
That's the famous "collapse of the wavefunction", demonstrated by all the Double-Slit experiments.

OK, so going back to light from the star. When each photon is emitted, it spreads outward as a wavefront. When a piece of that wavefront hits our eyes, there's a probability that our eyes will detect it (ie, that we'll see it). If we do, then nowhere else will be impacted by that specific photon.
The wavefront and probabilities are (so far as we know) continuous, not discrete, so no matter how far from the star we are, there will always be a non-zero probability that we will detect any one photon (as opposed to the ball-bearing model, where no ball-bearings may be following the specific line to our eyes).

Notice that this doesn't change, say, Hornblower's account of accumulating photons to get better photos from extended exposures.

And no, dark matter is something else entirely. That is something (that has gravity) that doesn't affect light at all - so light passes straight through it (unlike dust and all the other stuff that we know about).