Diffraction and light.

[Squirrel]

Warning: long post. OK, we'fve been doing light interference in physics at the moment (the double slit experiment etc.) and there'fs a few things I don'ft get. It kind of makes sense with water waves, but not with light.

1. Do the light rays (I'fll call them rays for now because I don'ft really know what I'fm talking about) actually oscillate as they move, i.e. trace a snake-like route through the air, or do they pulse into and out of existence or something like that? I ask this because our teacher stubbornly refuses to give me a straight answer about what 'peaks' and 'troughs' are.

2. What is gravity - something to do with electrons and protons attracting each other more than they repel, or what? Also is it gravity that bends the light rays when they get close to the slits? (Slit width similar to wavelength or something.)

3. Do two light rays have to actually 'touch' in order to interfere? For something so small in so large a space, it seems unlikely that almost every light ray would be interfered with in the double slits experiment (constructively or otherwise).

4. How do you know that rays 'from the same source' are in phase and coherent? I don't know much about lasers, but with an ordinary light bulb it appears that light is being emitted from all over the place: some from near the front of the bulb, some from half a wavelength behind etc. resulting in rays in random phase positions...or do all the rays organise themselves into phase with each other?

5. When a 'peak' of one ray meets a 'trough' of another, and they 'cancel out' (dark fringe on slit expt.), where does the energy they were carrying go? If the screen was moved a little further back, and the rays allowed to travel a bit further, would they appear again as 2 rays, or just one cancelled out 'dead' ray with no energy as seems to be implied?

(Run out of space, continued below...)

[Squirrel]

6. Is all laser light polarised? I'm confused because when we were shown the slits experiment, there were these light and dark lines of laser light on the screen and our teacher drew a picture that looked like "''" (sorry if this symbol doesn't come out) cancelling out one that looked like "U". I thought, if this light is unpolarised, surely it would contain equal amounts of "''"s and "U"s anyway, and the "''"s would be in phase, but one would be flipped over upside down ("U") - the equivalent of out of phase light. So when the light is diffracted through the slits, or just shining on an object in general, doesn't the interference idea say that it should all cancel out, negating itself?

Someone can probably tell me what I've understood wrongly in about 5 seconds, but this wave/particle thing confuses the hell out of me.

[sddsddean]

Part 1 OK, I'll have ago at this, but bear in mind it is over 20 years since I did A level Physics. Light exhibits properties of both waves and particles, which is one of the major stumbling blocks in tying down how it 'performs' in certain situations. You can reasonably call them 'lightwaves' as this is how their behaviour is usually explained. So for Q1, you can assume they are of the form of a sinusodal wave (a constant up and down wave like you see on osilloscope in B sci fi movies!). This means the 'peak' is at the top of the screen and the 'trough' at the bottom. This leads to Q5. If we call the peak 1 and the trough -1, you can see the inter action between them. If 2 waves are 'in phase' , their peaks and troughs will add together to get a brighter 'beam'. However, if they are out of phase, the peak(1) and the trough (-1) meet, add together and make 0, or no light. The energy doesn't 'go' anywhere, they just cancel out. Not sure what you are getting at in the start of Q2. Gravity is totally dependant on the mass of an object. However, as the electrons and protons (I think its both) have mass, although very small, they can be affected by a larger mass. During a Total Eclipse of the sun, it is possible to see a star behind the sun appear either side of it as the light is bent round the sun by the sun's gravity, making it appear as though the star is in two different places.Gravity has nothing to do with the slits, this is the slit creating an 'interference' pattern (see your text books for explanations).

[sddsddean]

Part 2 Q3. Light waves don't actually touch as such, as they are almost non existent (I'm getting a bit sticky here!). It is more to do with how the peaks and troughs interfere with each other that causes the effect.Q4. As you say, an ordinary light bulb sends light in all directions. If I remember rightly, coherent light is produced by putting it through a polarising lens, so that all the light waves are the same way up (does that sound right?) and in phase light comes out of a laser. I could be wrong on both those, sorry! Not sure what you are saying in Q6, as the symbols haven't come out. Thinking about it, I don't think laser light is necessarily polarised. The worst comment in your question is that the teacher refused to give you a straight answer. If you don't understand something, it is his/her job to help you. I hope these ramblings have helped a bit. If you are still stuck, [post a message to that effect here] and I'll ask my mate who is a professor to explain it better.

[Hamish]

This is a huge question. Have a look at http://www.qmw.ac.uk/~zgap118/ and see if hat heps a bit. there is more than one area of physics that you aer asking about.

[anthomson]

I think to some extent you are trying to understand what light actually is ( rather than just the properties it exhibits) and I think this is still one of science's unknowns. What you have to remember is that the over-simplified notion of light as a wave is really just a convenient model that helps most people ( physics teachers included ) to understand some of its basic properties. Maybe its best not to take it too literally at this stage - unless of course you actually want to delve in to the quantum theory ( and if you understand that you can explain it to me ).

[Squirrel]

Your answers have been a great help, so far. What I mean about our teacher is that he knows all the physics stuff and is a great teacher, but sort of...carefully dodges around certain questions! Whenever I'm about to ask him a question he'll say "Not the damn light thing again!" because it drives him mad.

Also, I'm still not so sure about peak/trough thing (the symbol was meant to be an upside down "U"). Would it be better for me not to so much think about what the rays are, as think of them as, say, strips of paper or something?

[sddsddean]

Although anthompson is perfectly correct in what he/she says, for the purposes of diffraction grating and interference experiments, I think you have got to consider them to be light waves (not rays, that sounds like something out of Star Trek!!). It is the only way you will (hopefully) get to understand the subject. If you think this is hard, wait until you do Simple Harmonic Motion! Never has a subject had such a misleading first word!!

[BenDToy]

Squirrel, one thing which may help is to ignore the conventional view of a wave as like the surface of the ocean, or a blanket being flapped up and down. The wave here is more of a pulse, where the particles are compressed and relaxed, a bit more like what happens with a long spring being pushed in and out. This gets away from the "snake like" motion. That just about exhausts my very old physics knowledge!

[bonsaimaster]

wow i never new you could run out of space in a AB post!!!!

[bonsaimaster]

oh and sorry about this being three years late!