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The Electromagnetic Spectrum, Vision, Color, and Light. A. Electromagnetic Spectrum: The Electromagnetic Spectrum is the continuum of all electromagnetic waves according to frequency, wavelength, or energy.
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The Electromagnetic Spectrum, Vision, Color, and Light • A. Electromagnetic Spectrum: • The Electromagnetic Spectrum is the continuum of all electromagnetic waves according to frequency, wavelength, or energy. • All electromagnetic waves travel at the speed of light, regardless of their frequency. The speed of light in a vacuum is c=3.00 x 108 m/sec, c is lower for all other mediums. • Visible light is a very narrow band, with wavelengths in the 400-700 nanometer range. Sources http://hyperphysics.phy-astr.gsu.edu/hbase/hph.htmlhttp://www1.sura.org/2000/SURA_Electromagnetic_Spectrum_Full_Chart.jpg http://mynasadata.larc.nasa.gov/images/EM_Spectrum3-new.jpg
B. VISION i. The Retina The retina is a light-sensitive layer at the back of the eye that covers about 65 percent of its interior surface. Photosensitive cells called rods and cones in the retina convert incident light energy into signals that are carried to the brain by the optic nerve. The roughly 125 million rods and cones are intermingled non-uniformly over the retina. The ensemble of rods (each about 0.002 mm) has the characteristics of a high-speed, black and white film. It is exceedingly sensitive, performing in light too dim for the cones to respond to, yet it is unable to distinquish color, and the images it relays are not well defined. "In contrast, the ensemble of 6 or 7 million cones (each about 0.006 mm in diameter) can be imagined as a separate, but overlapping, low-speed color film. It performs in bright light, giving detailed colored views, but is fairly insensitive at low light levels.
ii. The Color-Sensitive Cones In 1965 came experimental confirmation of a long expected result - there are three types of color-sensitive cones in the retina of the human eye, corresponding roughly to red, green, and blue sensitive detectors. Painstaking experiments have yielded response curves for three different kind of cones in the retina of the human eye. The "green" and "red" cones are mostly packed into the fovea centralis. By population, about 64% of the cones are red-sensitive, about 32% green sensitive, and about 2% are blue sensitive. The "blue" cones have the highest sensitivity and are mostly found outside the fovea. In the final visual perception, the three types seem to be comparable, but the detailed process of achieving this is not known.
iii. Color • It is common practice to define pure colors in terms of the wavelengths of light as shown. This works well for spectral colors but it is found that many different combinations of light wavelengths can produce the same perception of color. • This progression from left to right is from long wavelength to short wavelength, and from low frequency to high frequency light. The wavelengths are commonly expressed in nanometers (1 nm = 10-9 m). The visible spectrum is roughly from 700 nm (red end) to 400 nm (violet end). The letter I in the sequence above is for indigo - no longer commonly used as a color name. It is included above strictly for the reason of making the sequence easier to say as a mnemonic, like a person's name: Roy G. Biv - a tradition in the discussion of color.
iv. Additive Color Mixing Additive color mixing is the kind of mixing you get if you overlap spotlights in a dark room, as illustrated at left. The commonly used additive primary colors are red, green and blue, and if you overlap all three in effectively equal mixture, you get white light as shown at the center. Additive color mixing is conceptually simpler than the subtractive color mixing, since you are just adding light energy in different ranges of the visible spectrum.
v. Subtractive Color Mixing Subtractive color mixing is the kind of mixing you get if you illuminate colored filters with white light from behind, as illustrated at left. The commonly used subtractive primary colors are cyan, magenta and yellow, and if you overlap all three in effectively equal mixture, all the light is subtracted giving black. Subtractive color mixing is more complex than the additive color mixing you get with colored spotlights.
C. Reflection (See Mirrors Handout) • Light incident upon a surface will in general be partially reflected and partially transmitted as a refracted ray. The angle relationships for both reflection and refraction can be derived from Fermat's principle. The fact that the angle of incidence is equal to the angle of reflection is sometimes called the "law of reflection". • Reflection is responsible for the images formed in mirrors. Typical types are planar, convex, and concave.
D. Refraction (See Lens Handout and Lens Laboratory) • Refraction is the bending of a wave when it enters a medium where it's speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. The amount of bending depends on the indices of refraction of the two media and is described quantitatively by Snell's Law. • As the speed of light is reduced in the slower medium, the wavelength is shortened proportionately. The frequency is unchanged; it is a characteristic of the source of the light and unaffected by medium changes. • Refraction is responsible for image formation by lenses and the eye.
E. Wave-Particle Duality Does light consist of particles or waves? When one focuses upon many of the phenomena observed for light, a strong case can be built for a wave picture. But the photoelectric effect suggests a particle nature for light: Phenomenon Can be explained Can be explained in terms of waves. in terms of particles. Reflection Refraction Interference Diffraction Polarization Photoelectric Effect
F. Photons: The Quanta of Light • According to the Planck hypothesis, all electromagnetic radiation is quantized and occurs in finite "bundles" of energy which we call photons. The quantum of energy for a photon is not Planck's constant h itself, but the product of h and the frequency. The quantization implies that a photon of blue light of given frequency or wavelength will always have the same size quantum of energy. For example, a photon of blue light of wavelength 450 nm will always have 2.76 eV of energy. It occurs in quantized chunks of 2.76 eV, and you can't have half a photon of blue light - it always occurs in precisely the same sized energy chunks. • But the frequency available is continuous and has no upper or lower bound, so there is no finite lower limit or upper limit on the possible energy of a photon. On the upper side, there are practical limits because you have limited mechanisms for creating really high energy photons. Low energy photons abound, but when you get below radio frequencies, the photon energies are so tiny compared to room temperature thermal energy that you really never see them as distinct quantized entities - they are swamped in the background. Another way to say it is that in the low frequency limits, things just blend in with the classical treatment of things and a quantum treatment is not necessary.
Photoelectric Effect (what Einstein actually received his Nobel Prize in Physics for) - Data showed that the energy of the ejected electrons was proportional to the frequency of the illuminating light. This showed that whatever was knocking the electrons out had an energy proportional to light frequency. The fact that the ejection energy was independent of the total energy of illumination showed that the interaction must be like that of a particle which gave all of its energy to the electron!
G. Other Topics (See Light Topics Laboratory) i. Dispersion • Chromatic dispersion is the change of index of refraction with wavelength. Generally the index decreases as wavelength increases. Hence, blue light travels more slowly in the material than red light and correspondingly undergoes more bend • Dispersion is the phenomenon which gives you the separation of colors in a prism. The blue light travels the slowest through the prism while the red light travels fastest. This results in the blue/red light bending more/less as it passes through the prism. The associated spreading of the light in the visible spectrum is called dispersion.
ii. Interference • When water waves are set into motion from two nearby sources or there is a single wave source approaching two narrow inlets into a reservoir, locations of constructive and destructive interference can be seen. • When light from a source far away shines on two slits, an interference pattern will occur on the screen behind the slits. The distance between the peak intensity bands (constructive interference) is dependent upon the frequency of the light, the distance between the slits, and the distance of the screen behind the slits.
iii. Diffraction When there is a need to separate light of different wavelengths with high resolution, then a diffraction grating is most often the tool of choice. This "super prism" aspect of the diffraction grating leads to application for measuring atomic spectra in both laboratory instruments and telescopes. A large number of parallel, closely spaced slits constitutes a diffraction grating. The condition for maximum intensity is the same as that for the double slit or multiple slits, but with a large number of slits the intensity maximum is very sharp and narrow, providing the high resolution for spectroscopic applications. The peak intensities are also much higher for the grating than for the double slit. The tracks of a compact disc act as a diffraction grating, producing a separation of the colors of white light .
iv. Polarization Light emitted by the sun, by a lamp in the classroom, or by a candle flame is unpolarized light. Such light waves are created by electric charges which vibrate in a variety of directions, thus creating an electromagnetic wave which vibrates in a variety of directions. A simpler model of polarized light is a wave that vibrates both vertically and horizontally. An ideal polarized filter produces linearly polarized light from unpolarized light, by only allowing light that is oriented in one direction through (known as the transmission orientation. Two ideal polarized filters could eliminate all light if their transmission directions are placed at right angles.
