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Properties of Light

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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. Since Protons (and neutrons) are more massive than electrons, and so most of the lines are in the yellow and red regions of the optical we sometimes visualize an atom as a miniature solar system, with spectrum, a neon lamp appears “orange” to your eye. The presence the heavy particles at the center (the nucleus) and the electrons of this pattern of lines in the spectrum of a glowing cloud in space whizzing around in specific “orbits” like planets. (In reality, this would tell astronomers that the cloud contains neon in the gas. picture is not very accurate. Electrons are not thought to be little balls “in orbit” around a nuclear “sun.” Without delving into atomic physics and quantum mechanics too far, let us just take the following statement for granted for now: the electrons bound to any particular atom can only be found in certain, specific energy levels with respect to the atom’s nucleus. The hydrogen atom only contains one proton and one electron, and is the simplest (and most common) element in the Universe. If left undisturbed, our hydrogen atom likes to bind its electron as tightly as it can, and so we would find the electron in the lowest energy level, which is called the “ground state.” However, if our atom is immersed in a beam of light from, say, a nearby star, Intensity sooner or later the atom will encounter a photon with an energy that is just the right amount to jump the electron up to the next higher energy level. Voilà! The photon gets absorbed, and is “gone” from the beam of light coming from the star! Now our hydrogen 400 450 500 550 600 650 700 atom is in what is called an “excited” state, sort of like a kid right Wavelength before Halloween. However, as all parents know, this is not the natural state of a child, and it is not the natural state of an atom 4 Properties Library Document of Light

This diagram shows how the spectrum of neon would appear in to the boundary, the change in wavelength results in a change in the spectrum of a star. Here, the background “rainbow” comes the direction of the beam. This change of direction is known as from the atmosphere of the star, and the neon atoms in the refraction. star’s atmosphere (or outer layers) absorb the stars light, leaving The refractive quality of lenses is frequently used to manipulate dark lines. Note how the graph shows dips at each line position, light in order to change the apparent size of images. Magnifying producing the characteristic pattern of lines expected from neon. glasses, spectacles, contact lenses, microscopes and refracting Extending this somewhat, it should become clear that since every telescopes are all examples of this manipulation. chemical element has its own unique set of allowed energy levels, each element also has its own distinctive pattern of spectral absorption (and emission) lines. (See diagrams above for neon, for example.) It is this spectral “fingerprint” that astronomers use to identify the presence of the various chemical elements in Astronomical objects.

An example of refraction of light. The straw appears bent, because of Electromagnetic Spectrum refraction of light as it enters liquid from air.

Light or visible light is electromagnetic radiation that is visible to Light sources the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometers to There are many sources of light. The most common light sources about 740 nm, with a frequency range of about 405 THz to 790 are thermal: a body at a given temperature emits a characteristic THz. In physics, the term light sometimes refers to electromagnetic spectrum of black body radiation. Examples include sunlight (the radiation of any wavelength, whether visible or not. radiation emitted by the chromosphere of the Sun at around 6,000 Kelvin peaks in the visible region of the electromagnetic spectrum Primary properties of light are intensity, propagation direction, when plotted in wavelength units and roughly 40% of sunlight frequency or wavelength spectrum, and polarization, while its is visible), incandescent light bulbs (which emit only around 10% speed in a vacuum, 299,792,458 meters per second (about 300,000 of their energy as visible light and the remainder as infrared), kilometers per second), is one of the fundamental constants of and glowing solid particles in flames. The peak of the black body nature. spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter Light, which is emitted and absorbed in tiny “packets” called wavelengths, producing first a red glow, then a white one, and photons, exhibits properties of both waves and particles. This finally a blue color as the peak moves out of the visible part of the property is referred to as the wave–particle duality. The study of spectrum and into the ultraviolet. These colors can be seen when light, known as optics, is an important research area in modern metal is heated to “red hot” or “white hot”. Blue thermal emission physics. Generally, EM radiation (the designation ‘radiation’ is not often seen. The commonly seen blue color in a gas flame or excludes static electric and magnetic and near fields) is classified a welder’s torch is in fact due to molecular emission, notably by CH by wavelength into radio, microwave, visible region we perceive radicals (emitting a wavelength band around 425 nm). as light, ultraviolet, X-rays and rays. The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter Atoms emit and absorb light at characteristic energies. This wavelengths, and lower frequencies have longer wavelengths. produces “emission lines” in the spectrum of each atom. Emission When EM radiation interacts with single atoms and molecules, can be spontaneous, as in light-emitting diodes, gas discharge its behaviour depends on the amount of energy per quantum it lamps (such as neon lamps and neon signs, mercury-vapor lamps, carries. etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission Refraction can also be stimulated, as in a laser or a microwave maser. Refraction is the bending of light rays when passing through Deceleration of a free charged particle, such as an electron, a surface between one transparent material and another. It is can produce visible radiation: cyclotron radiation, synchrotron described by Snell’s Law where 01 is the angle between the ray and radiation, and bremsstrahlung radiation are all examples of this. the surface normal in the first medium, 02 is the angle between Particles moving through a medium faster than the speed of light the ray and the surface normal in the second medium, and n1 and in that medium can produce visible Cherenkov radiation. n2 are the indices of refraction, n = 1 in a n > 1 in a transparent substance: Certain chemicals produce visible radiation by chemoluminescence. n1 sin 0₁ = n2 sin 02 In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through When a beam of light crosses the boundary between a vacuum water can disturb plankton which produces a glowing wake. and another medium, or between two different media, the wavelength of the light changes, but the frequency remains Certain substances produce light when they are illuminated by constant. If the beam of light is not orthogonal (or rather normal) more energetic radiation, a process known as fluorescence. Some Library Document Properties 5 of Light substances emit light slowly after excitation by more energetic Color temperature is important in the fields of image projection radiation. This is known as phosphorescence. and photography where a color temperature of approximately 5,600K is required to match “daylight” film emulsions. In Phosphorescent materials can also be excited by bombarding them astronomy, the stellar classification of stars and their place on the with subatomic particles. Cathodoluminescence is one example. Hertzsprung–Russell diagram are based, in part, upon their surface This mechanism is used in cathode ray tube television sets and temperature, known as effective temperature. The photosphere of computer monitors. the Sun, for instance, has an effective temperature of 5,778K. Certain other mechanisms can produce light:

– scintillation Color Temperature Example of Source

– electroluminescence 1,900K Candle light or sunlight at sunrise or sunset – sonoluminescence Often used as accent lighting to blend in with fluo- 2,000K - 2,700K rescent 2700K applications. – triboluminescence 3,000K - 3,200K Used as a primary light source for retail applications. – Cherenkov radiation Coated lamps. Used where a “softer” metal halide 3,700K When the concept of light is intended to include very-high-energy light source is desired. photons (gamma rays), additional generation mechanisms include: Used in general lighting; factories: parking lots, 4,000K warehouses

Daylight lamps: horticulture, aquariums, high color 5,000K - 5,500K definition. 5,600K Nominal sunlight (mid day during mid summer) Starts to get a blue tint like some automotive 6,000K headlights

EM Spectrum Properties

The kelvin is often used in the measure of the color temperature of light sources. Color temperature is based upon the principle that a black body radiator emits light whose color depends on the temperature of the radiator. Black bodies with temperatures below about 4,000K appear reddish whereas those above about 7,500K appear bluish.

10,000

9,000

8,000 Daylight Metal Halide 7,000 5,500K Cool White Fluorescent 4,200K 6,000 Standard Clear Metal Halide 4,000K 5,000 Standard Warm Metal Halide 3,200K Halogen 4,000 3,000K Standard Incandescent 3,000 2,700K High Pressure Sodium 2,200K 2,000 Candle Light 2,000K 1,000 Kingspan Light + Air North America - California Operation 401 East Goetz Avenue, Santa Ana, California 92707 Toll Free: +1 (800) 854 8618 T: +1 (714) 540 8950 F: +1 (714) 540 5415 E: [email protected] www.kingspan.com/us

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Kingspan_Properties_of_Light_Library_Document_US_EN_R1

02/2019