Light

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I'm getting a few mixed massagers hear, so I made this thread to patch a few things up:

1. Can Light be bent?

First I'm being told that light always travels in straight lines and can never bend, but then I get told that light can easily be bent by black holes and changes in density. They can't both be right.

2. Is Light electro magnetic?

Apparently, there are only 5 types of energy; light, sound, heat, electricity and kinetic, which would mean that waves on the electro magnetic spectrum (like radio waves and X-rays) are not types of energy, but I'm told that light is on the electro magnetic spectrum which means that light is electro magnetic (the same as radio waves, X-rays ect) but wouldn't that mean that light isn't a type of energy? It obviously is.

3. Is Light straight or Curvy?

As mentioned before, I got told that light travels in straight lines, but I've also heard that light is a transverse wave (meaning a curvy wave that travels up and down alternatively)………there’s a difference.

4. What gives Light its colour?

I'm being told at first that there are 3 colours; red, blue and yellow, (which means that yellow is made up of 1 type of light, yellow) but then I'm told that there is only 3 types of light; red, blue and green, and that every other colour is just a mixture of those 3 types. (which would mean that yellow is made up 2 different types of light, red and green) Next I'm told that all light is the same, but each colour has its own unique wavelength, red has the lowest blue has the highest, (which would mean that yellow is made up of 1 type of light, one with a wavelength between red and green's) and as if I wasn't confused enough, I watched a program the other day that said colour depends on how far the electron of a atom 'jumps' in one rotation.........eh?

I know the answers are probably obvious, but light is the only part of Science I don’t really understand. Little help?

Last edited by robo37 on 12 Mar 2009, 8:47 am, edited 1 time in total.

They're both right. Light always travels in a straight line, according to its own frame of reference, which can appear to be bent by gravity to an outsider, and still be a straight line to the light beam itself.


Light is energy. Individual photons are packets of energy, and light is composed of photons. Light is also a wave function. So there is a dual nature to light.


The partical function travels in straight lines, but the wave function has a polarised component. Again, that dual nature of light leads to confusion.


Red, blue and yellow are primary reflective colors. Mixing pigments of these colors in different proportions results in new pigments that reflect light of different colors than the original pigments. Red, blue, and green are primary transmissive colors. Mixing lights of the colors in different proportions results in new colors of transmitted light.


Understanding is not necessary. Just believe.

Quick answer to 5.

Light is a continuous spectrum from radio-waves (low energy/frequency) through to gamma-rays (high energy/frequency.) We can see only a narrow portion of this, what we actually call 'light.' The eye contains 3 types of frequency-sensitive cells which 'see' the light as either red, green or blue, and our perception of colour is a mixture of these.

Space-time is curved by mass.

Light follows the shortest path in the space-time manifold

electromagnetic radiation is photons. Photons mediate the electromagnetic force.

You eyes give light its color. There are three color recptors in the human eye which react with various frequencies of the incoming light. Color is subjective. Frequency is objective.

the frequency of the photon is proportional to its energy.


energy = frequency * Planck's Constant

ruveyn

Correct me if I'm wrong, but doesn't light superpose, and hence light could come in any periodic function at all?

Then color would be a crude 3 dimensional projection of an infinite dimensional function space.

And by the way, there's no 4

A quick note:

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

There are actually six acknowledged forms of energy (at this time); heat, light, chemical, electrical, mechanical, and nuclear. Each of these can be converted from one form of energy to another. When you touch a hot pot the heat from the water transfers to the pot, which in turn burns your hand. We observe the transfer of physical energy with water. Solid ice is heated. The ice is heated and changes or sublimes into a liquid. The liquid then sublimes into gas. Energy is neither created or destroyed. It is continually changing forms.

Visible light is a form of electromagnetic radiation. Electromagnetic radiation is also referred to as radiant energy because it actually carries energy through space. Electromagnetic radiation travels at a consistent speed known as the speed of light (3.00 x 10 to the 8th power m/s).

Electromagnetic radiation consists of wavelengths; cosmic rays, gamma rays, x-rays, ultraviolet rays, visible light, infrared, microwaves, and radio frequency. Each wavelength of electromagnetic radiation oscillates due to the strong influences of electronic and magnetic forces generated by the radiation. The result is radiant energy has wave characteristics.

(I wish I could draw it for you. It would be much easier to understand.)

The shorter the wavelength, the higher the frequency. The longer the wavelength, the lower the frequency. Wavelengths are measured by the distance between the peaks. This is referred to as amplitude. Cosmic rays have the shortest wavelengths, which means they have the highest energy. Radio waves have the longest wavelengths with the lowest energy.

My thought about light is if we could actually “see” a light beam, it would be fuzzy because a light beam looses photons as it travels. The beam itself without any outside influence travels in wavelengths, so the direction may be straight, but the wave nature of the influences of radiation cause it to oscillate as it travels. It is sort of like how a snake moves across the ground. It's body waves, but it moves in one direction.

Definitely, light can be bent. Astronomers have made the observations you have stated in this thread about light bending around objects like black holes. Also, if you pass visible light through a crystal or prism the light does appear to bend. The wavelengths of visible light split when passing through a prism or crystal to form red, orange, yellow, green, blue, indigo (although some people do not acknowledge indigo as a part of the spectrum, others do), and violet.

Each wavelength of visible light reflects a specific color which corresponds to a specific measurement within the spectrum. 400 nm is on the violet end of the spectrum. 750 nm is on the red end of the spectrum. We can see visible light because of chemical reactions that it triggers in our eyes.

There are many more interesting things that could be discussed, such as spectral lines, emission lines, and absorption lines. Energy levels are completely fascinating too. My thought is, the above response answers your specific questions. There is an online book at this link that explains a lot of this with greater detail. The Fundamentals of Stellar Astrophysics by George W. Collins II. I downloaded some of the chapters onto my desktop. Here is the link:

http://ads.harvard.edu/books/1989fsa..book/

There are some other great learning books on this sight as well.

Cyndi

Great site. Thanks Cyndi

ruveyn

It goes in a straight line from it's own POV (so to speak), but from other perspectives it curves.

It's like how on a bicycle or motocycle you can go through a banked turn while keeping the handlebars straight (if the turn banked enough). So it's a straight path as far as the bike's steering is concerned, but on the road map it's a turn. So the path depends on curves in the road, or in the case of light the curves in space(-time), or the 'curves' in the density of the medium the light is travelling through (like going from relatively un-dense air into dense glass, which is how lenses and prisms bend light).




It's not the light beam itself that's oscillating or curving back and forth. The classical (as in pre-quantum physics) view of light is that it's a wave that is composed of oscillating electric and magnetic fields. The fields are oriented such that the direction that they point is out to the 'sides' of the direction that the wave moves -- and that's why it's called a transverse wave. (More strictly, the E&M fields are oriented at right angles to the direction of propagation of the wave. And they're also at right angles to each other.) A picture is worth a thousand words here:



The red and blue arrows represent the E & M field vectors at the point that their tails meet as the wave passes by. One thing the picture doesn't show is that wave is actually a wave in a field -- there's one of those diagrams for every point in space. Like a wave blown into a field of grain by the wind, each blade of grass representing the E or M field at that point.

A light beam makes a geodesic path in the space-time continuum. Which means it is infinitesimally straight but makes a curve is the space-time continuum has a non-zero curvature tensor.

ruveyn

Short answer: "yes"...ish. Light follows the shortest path, or, if you will, the quickest path. The shortest path between two points is a straight line -- if everything in between is linearly uniform. Black holes really mess up uniformity, as does change in density. Basically, light is still traveling straight, but the meaning of "straight" gets slightly tweaked. It's more complicated than that, but that's how they can both be right.



Yes, light is electromagnetic. If you make it to collegiate-level physics (second-year algebra, calculus), you'll learn that electricity and magnetism are two sides of the same coin -- where you have one, you have the other (this means light would technically fall under "electricity"; weird, I know, but it makes sense after a few months of EM physics).



Both -- it's called "wave-particle duality". It sounds weird, but that's Quantum Mechanics. There is a formula that relates an object's size-speed (momentum) to an equivalent wave form, the de Broglie equation. For human-scale objects, e.g. a football at passing speed, the wave equivalent has a wavelength smaller than Planck length, so the wave ("curvy") nature is moot, and the particle ("straight") nature dominates; the wave of an ocean wave has an equivalent particle mass smaller than a neutrino, so the particle nature is moot, and the wave nature dominates. As the size of the "thing" shrinks, the equivalent wave's wavelength grows larger. With very small particles (atoms, baryons, leptons, etc.), the wavelength becomes human-measurable (nanometer, millimeters). It is around this range of values that light (EM radiation in general, actually) starts acting as a particle ("straight") in some cases and as waves ("curvy") in other. At the same time, even.

And there is no difference. FWIW, de Broglie's equation implies there is no difference between a straight-traveling particle and a (transverse) wave. Took me three years to wrap my head around that. Doesn't make any intuitive sense, but the mathematics works out elegantly, much to the horror of many students. At human sizes, you perceive a distinction, as the differences between the particle nature and wave nature are so vast that you get to ignore one or the other, but at tiny tiny sizes, the distinction gets blurred.



First of all, realize that colour is a human perception, while wavelength is an attribute of the photon. This makes a difference. The typical human eye has receptors for red, green, and blue. Colour is perceived based on how much each receptor is stimulated. For example, violet is perceived when the red and blue receptors are stimulated by about the same amount. There are actually two ways of accomplishing this:

1. combine a red light (650 nm) and a blue light (475 nm) in about equal proportions, stimulating each receptor type separately at the same time.
2. use a pure violet (400nm source) light, which then happens to stimulate the red and blue receptors by about the same amount together at the same time.

Red/Green/Blue happens to provide (almost) all the possible ways the human eye colour receptors can be stimulated, thereby providing the illusion of all possible colours. Any other combination of "base" colours works (Red/Yellow/Blue), as long as they can manage to generate all possible combinations of receptor stimulation. Red/Green/Blue just happens to match what the human colour receptors are. If you use a prism, you can see whether the colour light you're seeing is a composition one, or a pure one. In the example above, the first way of obtaining violet will result in a red and blue split through a prism, but the second way would result in a single band of violet through a prism. But before the splitting, they both look the same kind of violet (human perception).

BTW, the prism also relates to your first question. The prism does "bend" light, with different wavelengths of light finding different "shortest paths" through the prism.

RE: jumping electrons. Electrons basically circle around a nucleus at distinct orbital energy levels. It can hop between these energy levels under certain conditions, which right now are not important. What is significant is that (a) the electron is in one or another discrete energy level (no in-betweens), (b) conservation of energy applies. The two means that if the electron "jumps down", energy leaves the atom in some form (electromagnetic radiation). Yet another equation relates the energy of a photon to its wavelength. How much energy, and thus how long of a wavelength (and thus the "colour"), gets spit out depends on the "gaps" between the energy levels and which energy levels the electron is "jumping" between, which is distinct for each type of atom and "energy source".

Another way of summarizing "jumping electrons colour" is that the "colour" of light you get depends on how "hyper" the electron is, which determines how much energy gets spit out per "jump", which determines the wavelength of that light particle/wave, which determines the colour.

How to make the electron "jump up" is a matter of supplying energy to the atom: another kind of light/EM radiation, heat, kinetic, chemical reaction, split a nucleus, etc.

In more abstract mathematics (definitely university-level math), there exists a notion of one-to-many and many-to-one mappings. How that applies here is like this:

• If you want to have one particular colour, then you have a choice of many different mixtures of wavelengths to play with.
• If you want to have one particular wavelength, then you only get one colour choice.

The important thing to take away, though, is that colour is a human perception, which can be tricked. Wavelength is an attribute of the photon (light particle/wave). Thus colour is not identical to wavelength, but there is a relationship.



Light gets even more mysterious and confusing as you learn more physics: refraction (lenses), reflection (mirrors), diffraction (splitting), virtual images, Doppler shift, gravitational lensing, electric field crossing a magnetic field. And at some point in learning, "light" just becomes a synonym for "electromagnetic radiation", anywhere from the ultra-low frequency radio all the way up gamma rays. The kind of light perceived by the human eye gets fully qualified as "visible light".

Not to scare you away from physics; it isn't really as scary as it sounds. Just be prepared to make large adjustments your perspective of reality several times.

Thank you, ruveyn. I was kind of wondering if it might be okay to start a thread asking for other free learning sites. I do not have a lot of time during the school year because I have to really focus to learn math, but in the summer time I do. It would be really great to hear what other places we go to learn, especially physics which is my passion.

Also, I wanted to note that while cosmic rays do have a higher frequency than gamma rays, most books stop at gamma rays and do not include the cosmic rays. If this is for a school project, you might want to stop at gamma rays unless your professor or the book you are learning from includes them too.

A thoroughly good idea

ruveyn

My physics teacher said that if you send any type of violet light through a red coloured filter it always come out as red light.........how would that be possible without any red light to start with? And about that 'light always takes the shortest path ' thing, another physics teacher drew a diagram of a hot day, where light is coming straight from the sky towards the ground but then bends towards some guys eyes just before it touches it..........wouldn't it be quicker for the light to just go in a straight line?

Oh yeah thanks for the help BTW.

Last edited by robo37 on 12 Mar 2009, 2:11 pm, edited 1 time in total.

Light in a vacuum takes the shortest path which is not necessarily a straight line in a curved manifold. Think of great circle routes on a sphere. They are the shortest routes one can take and still be on the surface of a sphere.

ruveyn

On the line of thought of a picture is worth a thousand words:



In reference to the previous animation of light waves, I seem to remember that photons follow a similar motion in the complex plane.

Take with a grain of salt. I can not immediately find references and may be wholly mistaken. (though I can find some technical references with lights behavior in the complex plane, none are immediately and directly supportive of my statement)