Properties of Light Library Document
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Light + Air North America Properties of Light Library Document 2 Properties Library Document of Light Properties of Light Light as energy The other way of representing light is as a wave phenomenon. This is somewhat more difficult for most people to understand, but Light is remarkable. It is something we take for granted every day, perhaps an analogy with sound waves will be useful. When you play but it is not something we stop and think about very often or even a high note and a low note on the piano, they both produce sound, try to define. Let us take a few minutes and try to understand but the main thing that is different between the two notes is the some things about light. Simply stated, light is nature’s way frequency of the vibrating string producing the sound waves--the of transferring energy through space. We can complicate it by faster the vibration the higher the pitch of the note. If we now shift talking about interacting electric and magnetic fields, quantum our focus to the sound waves themselves instead of the vibrating mechanics and all of that, but just remember, light is energy. Light string, we would find that the higher pitched notes have shorter travels very rapidly, but it does have a finite velocity. In vacuum, wavelengths, or distances between each successive wave. Likewise the speed of light is 186,282 miles per second (or nearly 300,000 (and restricting ourselves to optical light for the moment), blue kilometers per second), which is really humming along! However, light and red light are both just light, but the blue light has a higher when we start talking about the incredible distances in astronomy, frequency of vibration (or a shorter wavelength) than the red light. the finite nature of light’s velocity becomes readily apparent. It takes about two and a half seconds, for instance, for a radio communication travelling at the speed of light to get to the moon and back. You might find it interesting to remember, the next time you watch a beautiful sunrise or sunset, that it actually occurred eight minutes earlier--it takes that long for the light to reach the Earth! And, of course, every newspaper article you ever read about astronomy will always include the required statement, “light year is the distance light travels in one year at the speed of 186,282 miles per second, about 6 trillion miles.” (Well, 5.8 trillion miles 400 450 500 550 600 650 700 actually). We should also highlight right up front that light is more generally referred to as electromagnetic radiation. Okay, we used The Light Spectrum in Nanometers. a big word. It had to happen eventually. Too often, when we say “light” it is mistaken to mean “optical light,” which is roughly the The colors of the familiar “rainbow” of visible light correspond to radiation visible to our eyes. Visible light is a tiny portion of a huge differing wavelengths of the light, here shown on a nanometer smorgasbord of light called the electromagnetic spectrum. For our scale. The wavelengths get successively larger as one moves from convenience, we break this smorgasbord up into different courses left to right. Optical light runs from about 400 to 700 nanometers. (appetizer, salad, etc.) and refer to them by name, such as gamma rays, X-rays, ultraviolet, optical, infrared, and radio. However, it is It’s the same way as we move throughout the electromagnetic important to remember that they are all just light. There are no spectrum. Each range of light we have defined above corresponds “breaks” and no hard boundaries in the electromagnetic spectrum- to a range of frequencies (or wavelengths) of light vibrations. These -just a continuous range of energy. wavelengths are one of the primary indicators we use to describe light and spectra on a graph. Displaying a spectrum as a graph instead of just a color bar allows us to measure the light Particles and Waves Physics experiments over the past hundred years or so have For instance, the “rainbow” of color shown in the figure above is demonstrated that light has a dual nature. In many instances, it is what you see when you pass white light through a prism. What convenient to represent light as a “particle” phenomenon, thinking may not be obvious, however, is that the “intensity” or brightness of light as discrete “packets” of energy that we call photons. Now of the light is also changing along with the colors. If we converted in this way of thinking, not all photons are created equal, at least the “rainbow” into a graph of light intensity versus wavelength, it in terms of how much energy they contain. Each photon of X-ray would look like this: light contains a lot of energy in comparison with, say, an optical or radio photon. It is this “energy content per photon” that is one of the distinguishing characteristics of the different ranges of light described above. Intensity Wavelength 400 450 500 550 600 650 700 The “Wave” Model of Light. Wavelength Library Document Properties 3 of Light The familiar “rainbow” of the visible spectrum can be converted either. If no other photons are absorbed by the atom, the electron into a graph that shows how the intensity of the light changes will eventually drop back down to the lower energy ground state. along the spectrum. However, the atom has to lose energy to do this, and so it releases a photon of the same energy as the one it absorbed (albeit most Notice that the spectrum is brightest in the middle (yellow-green likely into some other direction from which it was absorbed). This region) and drops off in both directions (toward red and blue). This process is called emission because the atom, again at a very was not obvious from the rainbow version of the spectrum! Also specific wavelength, emits a photon of light. notice that the “intensity” of the light in the graph does not stop at the “ends” of the rainbow spectrum that is visible to our eyes! Of course, the atom could have absorbed another photon with just The light continues beyond what we can see in both directions, the right energy to jump up another energy level, or even two or which we can see in the graph but not by looking at the rainbow. three or more. Likewise, after each of these possible excitations of Astronomers use graphical spectra most of the time because they the atom, the electron could jump back down one or more steps, can get more information out of the light this way, and because emitting photons as it went. If a photon with a sufficiently large they can still plot and analyze light that is not directly visible to our energy gets absorbed, it can even cause an electron to become eyes! unbound from its nucleus, a process that is called ionization. Now we mentioned that the energy of each photon of light was We have been discussing one specific transition or “energy jump” also a basic property. It turns out that there is a simple relationship in one atom, but of course, in any physical system there are between the energy of a photon and the corresponding wavelength many atoms. In a hydrogen gas, all of the separate atoms could of that photon: be absorbing and emitting photons corresponding to the whole group of “allowed” transitions between the various energy levels, E (photon) = (constant) / (wavelength) each of which would absorb or emit at the specific wavelengths This simple equation ties together the particle and wave nature of corresponding to the energy differences between the energy light by permitting us to convert back and forth from wavelengths levels. This pattern of absorptions (or emissions) is unique to to photons and photons to their corresponding wavelengths. This hydrogen--no other element can have the same pattern—and equation is also in accord with what we said earlier...an X-ray causes a recognizable pattern of absorption (or emission) lines in a photon has a large energy (and a small wavelength) compared spectrum. with a photon of optical light. Interaction of Light with Matter: Absorption and Emission of Light It should come as no surprise to you that atoms and molecules (which are simply bound collections of two or more atoms) can absorb light (= energy!). If they did not, you could simply flick a light on and off, and then sit back while the photons continued to bounce around the room! Likewise, infrared light (= heat = energy!) Intensity would not do any good in heating up your home in the winter if it didn’t get absorbed by matter. Higher energy light photons, like X-rays, tend to want to plow through more matter before they get absorbed. (Hence, their use in medical imaging: they can pass 400 450 500 550 600 650 700 through your “soft” tissue, but are more readily absorbed in your Wavelength bones, which are denser. Well, it is time to develop another conceptual device to help us This graphic demonstrates the optical spectrum one would see understand this process. In physics, we often find it helpful to from glowing neon gas, both in colorbar and graphical formats. pretend we are looking at a single atom. Atoms are made up of As with hydrogen, discussed in the text, neon shows a specific set protons, neutrons, and electrons, and each chemical element has of spectral lines. Note how each bright colored line in the color bar a specific number of them--that is what makes them different! corresponds to an upward “spike” in the graphical format.