| Assignment 1: Basic Theory of Light. Parts of pages 9 - 16. |
There are a lot of different 'things' represented here. You know something about just about all of them. Starting from the left side of the center bar, there are
Well, lets look at the words themselves. We've spent some time discussing the various theories of wave versus particle when discussing visible light, so look first at the word 'ray'. Does this suggest that cosmic rays act more like particles and less like waves?
Yes, they do.
And how about 'cosmic"?
OK, they come from stars and interstellar space, and the ones that bombard earth are trapped in the magnetic field around earth. Their interaction with the atmosphere produces the aora borealis or northern lights.
They may also be referred to as ultragamma rays. Ultra- means 'beyond', so these are 'beyond gamma rays'. I will not refer to them this way because my Physics references do not, so you can file that as trivia.
One more item about cosmic rays for now, and we will come back to them later. The little packets of energy called photons, if they are in the cosmic ray range of the electromagnetic spectrum, have more energy than any of the other elements of the spectrum.
So the left end of this bar is the 'high energy' end.
They travel through things that are not transparent to visible light.
Based on the comment under cosmic rays, I'll bet that you guessed that they act more like particles, less like waves.
They can do damage to biological tissues.
They come in three types: Alpha rays, beta rays, and gamma rays. Guess what? The gamma rays are to the left, the alpha rays are to the right. That is why cosmic rays are 'ultragamma' instead of 'ultraalpha'. Try saying THAT three times fast. The gamma rays are the most damaging and the highest energy of the X-rays; the alpha rays are the least damaging and the least energetic of the X-rays.
And we do tend to call them rays, so we are still more into particles than waves. But waves are gaining.
And yes, they have less energy than X-rays.
And in small doses they don't do much. But if you have been reading much, or have been in the optical business long, you know that the build-up of UV exposure over time leads to damage (or at least accelerated ageing) of biological tissues, i.e., your skin and eyes.
Yes, that means sunburn and suntan go here.
We still refer to waves in this range as rays; but they have attributes of waves as well.
Within the visible range we have different colors, because we have three different types of cells in the retina that get excited by three different RANGES of wavelength. On page 12 is a listing of major color names and the APPROXIMATE wavelength that these colors represent. The colors are ranges, and not just one wavelength. Do not memorize color names and wavelengths. Rather, memorize the order of the colors and the relative position in the spectrum.
At this area of the electromagnetic spectrum, sometimes the waves act like waves, and sometimes like particles. In the way they travel they are more wavelike, in the way they interact chemically and electrically they have more of a photon nature.
IR-A ranges from about 800nm to about 1500nm, and it is absorbed by the aqueous, lens, and vitreous of the eye. About 4% of the IR exposure reaches the retina.
IR is converted to heat when it is absorbed. Our bodies are great dissipaters of heat. The IR that reaches the retina and is converted to heat is dissipated by the blood vessels at and near the retina. But if there is an exposure that is so high in IR that the tissues cannot dissipate it fast enough, we get a retinal burn. What happens if you have a quick intense exposure to heat? You kill cells. In the skin, the result is pain, dead cells, regrowth of cells, and eventually new skin cells. In the retina there are no pain nerves, so when a person stares at the sun there is no pain. But there is to much heat, and retinal cells die. Problem is, the retinal cells do not regrow. Once the cell is dead, no other cell replaces it, and the person has a permanent blind spot. This is what happens when people look directly at a solar eclipse: because the moon is blocking the brightest of the sun's rays, the sensation of discomfort that comes from staring at the sun is absent, and those little IR rays can kill cells with no interference, and the person has a permanent blind spot that is a ring where the intensity of the IR waves were absorbed by the retina.
Longer wave IR, IR-B and IR-C, are partially absorbed by the cornea and may cause corneal and conjunctival irritation.
One type of cataract, called the glassblowers cataract, is caused by prolonged exposure to IR.
But do notice that we refer to all of them as waves. Ever hear of a radio ray? or an X-wave? No, the longer the wavelength the more wave-like and the less particle-like in nature. So most of the attributes of TV and radio transmission can be explained using wave theories.
This is the low energy end of the spectrum.
And that is about all that you need to know about this end of the spectrum
for this course!
First is that the shorter the wavelength, the higher the energy. The fact is, the only difference between a cosmic ray and a radio wave is the amount of energy in the photon/wave!
Second is that the higher the energy and therefore the shorter the wavelength,
the more the particle/wave acts like a photon. The lower the energy and
therefore the longer the wavelength, the more the particle/wave acts like
a wave. And visible light is right in the middle of the transition between
particle and wave. No wonder the argument over whether light is a wave
or a particle continued for centuries and occupied the time of the greatest
philosophers and scientists in history!
When you are ready to continue the first week's lesson click here.
Press the BACK button at the top left of the screen to return to the assignment page.
