# Light & Sight

1. Feb 10, 2012

### John15

Light travels as waves
Waves interfere with each other
We see reflected light.
We can see the source of light and the things illuminated but not the light inbetween Why?
Light is reflected in all directions from all things why does it not interfere between the object we are looking at and our eyes, light is also reflected from the eyes which should also interfere with light coming in.
Example throw a single pebble in a pond you get easily recognisable wave , same with 2 or 3 but throw a handful in and you just get a mess of waves.

2. Feb 10, 2012

### JaredJames

To see light, it has to reflect off something.

3. Feb 10, 2012

### sophiecentaur

Space is a 'linear medium' which means that the fields of one wa e fo not affect the fields of a wave it's crossing. So they don't modify each other.

4. Feb 10, 2012

### Staff: Mentor

That isn't correct:

To see light, it has to hit your retina.

5. Feb 10, 2012

### Staff: Mentor

You answered your own question. When you throw a handful of pebbles into a pond you get a whole mess of waves instead of all the waves cancelling each other out because waves have to be perfectly timed and aligned to cancel each other out and neither a handful of pebbles, nor reflected light is coherent enough for that.

6. Feb 10, 2012

### John15

Many thanks for the replies but still a bit confused.
why does light have to reflect off something in order to become visible?
In order to see something as a sharp image the light has to come in a straight line without any interference, so are we saying that this light is able to travel through what must be a mess of waves inbetween without interference, am not talking about waves cancelling out but being jumbled up for want of a better phrase.

7. Feb 10, 2012

### JaredJames

My mistake. I was thinking of it along the lines of you either have to look at the source of the light to see it, or it has to reflect off of something.

In both cases, you need the light to enter the eye. It can't "pass near by".

8. Feb 10, 2012

### davenn

and expanding on that ....

@ OP yes it travels as waves and thats fine to think of it as waves for some applications
but it also travels as particles ... photons... and it those photons entering your eye that get detected.
The more photons, the brighter the light.

Dave

9. Feb 10, 2012

### Dremmer

10. Feb 10, 2012

### cepheid

Staff Emeritus

11. Feb 13, 2012

### John15

So am I right in thinking that light travels as waves from source but acts as particle when reflected? which is what we see.

Still not sure why it does not interfere though.
Think of a wave tank, standing at one end waves from other end are easy to make out, now add waves coming from top, bottom and sides plus from your end and all you get is turbulence and making it very difficult to make out individual waves.

I do have another question about waves travelling from source but lets try to conclude this first.

12. Feb 13, 2012

### sophiecentaur

It also behaves as waves when reflected and as particles when it travels -if that's the way you want to explain it. Waves are certainly good enough for Optics and Radio studies You should stop trying to categorise this sort of thing. You'll find it's like trying to get hold of a bar of soap in the bath.

Interactions between EM and matter can often be thought of, conveniently, is terms of Photons but, otoh, a lens is 'matter' and treating the action of a lens by considering photons would just be madness.

Last edited: Feb 13, 2012
13. Feb 13, 2012

### jim hardy

"""We can see the source of light and the things illuminated but not the light inbetween """

14. Feb 13, 2012

### sophiecentaur

Do you know how they work, Jim?

15. Feb 13, 2012

### Drakkith

Staff Emeritus
Light can be explained as a wave in almost all aspects. Diffraction, reflection, and interference are all perfectly described by treating light as an electromagnetic wave. However, when light finally gets to something and is absorbed is can only be treated as a particle that gives all of its energy up and no longer exists. This is how the molecule in your eye's cone and rod cells works, it absorbs energy from the photon and changes shape, setting off a chain of events that ends with you "seeing" something. The key is that the light has to enter and interact with your eye. The reason we cannot see the light in between is that two waves don't "bounce" off of each other. One light wave passing by another does not reflect or refract off of it. (Other than normal interference effects which is not the same) Think of your water waves. If you make two waves they don't bounce off of each other and go in completely different directions or create new waves at the point they interact.

Light can only interfere when the wavelength and such are almost exactly alike, meaning it is "Coherent". Normally light entering your eye does not meet this requirement. This is similar to why sound waves do not normally interfere and keep you from hearing the speakers in your TV.

It may make it difficult, but it by no means makes it impossible to determine which wave is which. Your ears do this all day long.

I recommend picking up a book on optics. I have several, including Optics for Dummies which is pretty nice. But any book about basic optics will be able to explain it all.

16. Feb 14, 2012

### Naty1

Try reading this online article and note the illustrations:

http://en.wikipedia.org/wiki/Light_interference#Optical_interference

You'll find a lot the relates to your questions like:

"different points in the source" could be the gas molecules in a fluoresescent light, the filament in an incandescent light, or our local star, the sun....

This relates to various 'points' having different energies, so the amplitude and frequency [color] also has variations...it's not easy to get monochromatic (single frequency) light....hence we see versions of 'white light' meaning light appears sort of white visually but is actually a combination of colors:

http://en.wikipedia.org/wiki/White_light

17. Feb 14, 2012

The nature of light is such that it enables us to see things but we don't actually see light itself.

18. Feb 14, 2012

### sophiecentaur

You rattled my cage a bit here. It would be much more accurate to say that white light (or, in fact many of the colours we see) consists of a combination of different WAVELENGTHS (or frequency, as you wish). i.e a SPECTRUM. There is so much confusion between colour and wavelength and this can lead to further confusion when it's taken further. 'Colour' is something that we allocate to single wavelengths or combinations of light with different wavelengths. 'Wavelength' means just one thing so it is worthwhile using that word when that's what is meant.
Inhabitants of Planet Zog would agree 100% with our spectral analysis of a light source but, even allowing for any translation of actual words, they would not agree about colours (and neither would a Dog or Cow!) Not being picky, btw. It's far more important than that.

19. Feb 14, 2012

### cepheid

Staff Emeritus
I can't say that I agree with this. When you see an object, it is because photons that are being emitted or reflected from the surface of that object are hitting your retina. The quanta of light (or, if you prefer, just the EM waves) are the things that are actually detected by your sensory equipment.

20. Feb 15, 2012

### John15

Quote Drakkith
This is similar to why sound waves do not normally interfere and keep you from hearing the speakers in your TV.

Of course enougth speakers playing different sounds from different directions and all you get would be noise.

So could it be said then that light travels as a wave but collapses to particle like on observation/ detection.
Wavelength determines colour and energy is linked to wavelength so how does amplitude work? Would I be right in thinking that amplitude enables the wave to carry more energy per wave i.e 2x amplitude gives twice energy or similar to 2 waves travelling in partnership.

Finally, I think, waves in 3d must propagate outwards spherically, as sphere grows energy should be spread out the same way a balloon gets thinner as its blown up, so taking the sun as source by the time light reaches earth we have a sphere with a radius of over 90 million miles, how does light maintain its integrity when spread out over such an area?

21. Feb 15, 2012

### Drakkith

Staff Emeritus
You might get sensory overload and be unable to distinguish the sounds, but the waves would not necessarily be interfering with each other enough to be noticeable.
Sure.

It depends on how you are viewing the EM Radiation. For example, radio waves can be thought of as actual waves and the amplitude as a direct increase in the signal strength. However when talking of individual photons this no longer works. The energy of the photon would be E=hv, where h is Planks constant and v is the frequency.

Again, it depends on how you are looking at the light. As a wave model the light simply spreads out like any wave does and the intensity of the light drops off as distance from the emitting objects increases. In terms of photons, the Sun emits uncountable numbers of photons which then move straight out away from the Sun. The intensity drops off as distance increases because the photons spread out into space, similar to birdshot from a shotgun spreading out in the air. The energy of each photon is set and does not change and the photon does not spread out as it travels.

22. Feb 15, 2012

True our retinal cells detect the waves /photons incident upon them and this is a part of the process that gives us the sense of vision,but it is the object that we see and not the photons.We see by means of photons but how can we actually see a photon?
We use expressions such as "we can see the light rays" or "the light we see is the light contained within the visible region of the EM spectrum".Such expressions are usually good and understandable within the context they are used but there may be times when a slightly different phraseology would add some useful extra detail.

23. Feb 15, 2012

### sophiecentaur

"Seeing" can be regarded as the whole perception process which involves receiving the light right through to placing an object in our brain's model of our surroundings. Or as our eye reacting to light that enters it. Take your pick.

24. Feb 16, 2012

### John15

Drakkith a fine answer I think I may be getting there.
One thing you say that a photons energy is fixed and the photon does not spread out as it travels so could it be regarded as each photon ( I am assuming that 1 photon = 1 wavelength ) to be moving with a virtual box around it, if so is this where planks quanta came from.
Would it be possible to expand a bit on amplitude? is it natural or something we add to an EM signal for instance.
It seems to me that EM waves behave strangely. I have found many formula for the relationship between wavelength, frequency and energy, all include either h, c or both, what they seem to say is that waves travel with a constant momentum which seems to be hc as in wavelength x energy = hc . e.g. comparing it to matter short wavelength acts like a bullet while in comparison long wavelength acts like a sponge ball, with both having the same mass and move at the same speed.
One reason for these questions is that depending on colours I see some 2d surfaces as 3d where red seems further away and blue seems closer especially where black is either in the forgroud or background, e.g. I have a book with a black cover the yellow writing seems flat on the black but the blue seems to stand out above the black yet others say it all looks flat. I am trying to relate this to the colour wavelengths.

25. Feb 16, 2012

### sophiecentaur

Owch!!! Where did that come from? I think you made too big a jump somewhere along the line. In fact, if you associate a photon with the wavelength of a wave you get things completely the wrong way round. A long wavelength radiation like Radio has millions (literally) more photons than a short wavelength radiation like X Rays, for the same flux of energy. Radio Frequency Photons have too little energy to detect individually (they come in vast numbers and interact with particles at a very low energy but high energy (like gamma) photons can be detected by the individual clicks of a Geiger Muller detector. The spatial extent of a photon is very debatable, in fact, and depends very much on what experiment you are considering.

This also relates to the momentum of photons, because the momentum of high frequency photons is high.

Your post also strays into the psychology of perception of colours and of scenes. That is a very complex topic and is too far away from these fundamental matters to be treated similarly. It's just asking to get bogged down almost before you've started.