Optics is the study of the behavior of visible light and other forms of electromagnetic waves. Optics falls into two distinct categories. When electromagnetic radiation, such as visible light, interacts with objects that are large compared with its wavelength, its motion can be represented by straight lines like rays.
Ray optics is the study of such situations and includes lenses and mirrors. When electromagnetic radiation interacts with objects about the same size as the wavelength or smaller, its wave nature becomes apparent. For example, observable detail is limited by the wavelength, and so visible light can never detect individual atoms, because they are so much smaller than its wavelength. Physical or wave optics is the study of such situations and includes all wave characteristics. When you light a match you see largely orange light; when you light a gas stove you see blue light.
Why are the colors different? What other colors are present in these? Ultraviolet is also produced by atomic and molecular motions and electronic transitions. The wavelengths of ultraviolet extend from nm down to about 10 nm at its highest frequencies, which overlap with the lowest X-ray frequencies.
It was recognized as early as by Johann Ritter that the solar spectrum had an invisible component beyond the violet range. It is largely exposure to UV-B that causes skin cancer. Again, treatment is often successful if caught early. All UV radiation can damage collagen fibers, resulting in an acceleration of the aging process of skin and the formation of wrinkles. Some studies indicate a link between overexposure to the Sun when young and melanoma later in life.
The tanning response is a defense mechanism in which the body produces pigments to absorb future exposures in inert skin layers above living cells. Repeated exposure to UV-B may also lead to the formation of cataracts in the eyes—a cause of blindness among people living in the equatorial belt where medical treatment is limited.
However, treatment is easy and successful, as one replaces the lens of the eye with a plastic lens. Prevention is important. Eye protection from UV is more effective with plastic sunglasses than those made of glass. A major acute effect of extreme UV exposure is the suppression of the immune system, both locally and throughout the body.
Low-intensity ultraviolet is used to sterilize haircutting implements, implying that the energy associated with ultraviolet is deposited in a manner different from lower-frequency electromagnetic waves. Actually this is true for all electromagnetic waves with frequencies greater than visible light. Flash photography is generally not allowed of precious artworks and colored prints because the UV radiation from the flash can cause photo-degradation in the artworks.
Often artworks will have an extra-thick layer of glass in front of them, which is especially designed to absorb UV radiation. However, the layer of ozone O 3 in our upper atmosphere 10 to 50 km above the Earth protects life by absorbing most of the dangerous UV radiation.
Unfortunately, today we are observing a depletion in ozone concentrations in the upper atmosphere. The hole is more centered over the southern hemisphere, and changes with the seasons, being largest in the spring. This depletion is attributed to the breakdown of ozone molecules by refrigerant gases called chlorofluorocarbons CFCs.
For example, the reaction of CFCl 3 with a photon of light hv can be written as. Figure 9. This map of ozone concentration over Antarctica in October shows severe depletion suspected to be caused by CFCs.
Less dramatic but more general depletion has been observed over northern latitudes, suggesting the effect is global. With less ozone, more ultraviolet radiation from the Sun reaches the surface, causing more damage. A single chlorine atom could destroy ozone molecules for up to two years before being transported down to the surface. The CFCs are relatively stable and will contribute to ozone depletion for years to come.
CFCs are found in refrigerants, air conditioning systems, foams, and aerosols. However, developing-country participation is needed if worldwide production and elimination of CFCs is to be achieved. Probably the largest contributor to CFC emissions today is India. But the protocol seems to be working, as there are signs of an ozone recovery. See Figure 9.
Besides the adverse effects of ultraviolet radiation, there are also benefits of exposure in nature and uses in technology. Vitamin D production in the skin epidermis results from exposure to UVB radiation, generally from sunlight. A number of studies indicate lack of vitamin D can result in the development of a range of cancers prostate, breast, colon , so a certain amount of UV exposure is helpful.
Lack of vitamin D is also linked to osteoporosis. Exposures with no sunscreen of 10 minutes a day to arms, face, and legs might be sufficient to provide the accepted dietary level. UV radiation is used in the treatment of infantile jaundice and in some skin conditions.
It is also used in sterilizing workspaces and tools, and killing germs in a wide range of applications. It is also used as an analytical tool to identify substances. When exposed to ultraviolet, some substances, such as minerals, glow in characteristic visible wavelengths, a process called fluorescence. So-called black lights emit ultraviolet to cause posters and clothing to fluoresce in the visible.
Ultraviolet is also used in special microscopes to detect details smaller than those observable with longer-wavelength visible-light microscopes.
Figure An energetic electron strikes an atom and knocks an electron out of one of the orbits closest to the nucleus. Later, the atom captures another electron, and the energy released by its fall into a low orbit generates a high-energy EM wave called an X-ray.
X-rays can be created in a high-voltage discharge. They are emitted in the material struck by electrons in the discharge current. There are two mechanisms by which the electrons create X-rays. The first method is illustrated in Figure An electron is accelerated in an evacuated tube by a high positive voltage. The electron strikes a metal plate e. Since this is a high-voltage discharge, the electron gains sufficient energy to ionize the atom.
In the case shown, an inner-shell electron one in an orbit relatively close to and tightly bound to the nucleus is ejected. A short time later, another electron is captured and falls into the orbit in a single great plunge.
The energy released by this fall is given to an EM wave known as an X-ray. Since the orbits of the atom are unique to the type of atom, the energy of the X-ray is characteristic of the atom, hence the name characteristic X-ray. This energetic electron makes numerous collisions with electrons and atoms in a material it penetrates. An accelerated charge radiates EM waves, a second method by which X-rays are created.
The second method by which an energetic electron creates an X-ray when it strikes a material is illustrated in Figure The electron interacts with charges in the material as it penetrates.
These collisions transfer kinetic energy from the electron to the electrons and atoms in the material. Whenever a charge is accelerated, it radiates EM waves. Given the high energy of the electron, these EM waves can have high energy. We call them X-rays. Since the process is random, a broad spectrum of X-ray energy is emitted that is more characteristic of the electron energy than the type of material the electron encounters. In the s, scientists such as Faraday began experimenting with high-voltage electrical discharges in tubes filled with rarefied gases.
It was later found that these discharges created an invisible, penetrating form of very high frequency electromagnetic radiation. This radiation was called an X-ray , because its identity and nature were unknown. While the low-frequency end of the X-ray range overlaps with the ultraviolet, X-rays extend to much higher frequencies and energies. X-rays have adverse effects on living cells similar to those of ultraviolet radiation, and they have the additional liability of being more penetrating, affecting more than the surface layers of cells.
Cancer and genetic defects can be induced by exposure to X-rays. Because of their effect on rapidly dividing cells, X-rays can also be used to treat and even cure cancer.
The widest use of X-rays is for imaging objects that are opaque to visible light, such as the human body or aircraft parts. In humans, the risk of cell damage is weighed carefully against the benefit of the diagnostic information obtained.
However, questions have risen in recent years as to accidental overexposure of some people during CT scans—a mistake at least in part due to poor monitoring of radiation dose. The ability of X-rays to penetrate matter depends on density, and so an X-ray image can reveal very detailed density information.
The number of waves that smack into you in one second tells you the frequency of the waves. One wave per second is called 1 Hertz. A million waves per second is 1 MHz.
If the waves are long, fewer of them hit you every second, so long waves have smaller frequencies. Radio waves have long wavelengths and small frequencies. The first radio astronomer did not mean to be the first radio astronomer. Engineers there were developing the first phone system that worked across the Atlantic Ocean. When people first tried making phone calls on that system, they heard a hissing sound in the background at certain times of the day.
Bell Labs thought that noise was bad for business, so they sent Karl Jansky to find out what was causing it. He soon noticed that the hiss began when the middle of our galaxy rose in the sky and ended when it set everything in the sky rises and sets just like the Sun and Moon do. He figured out that radio waves coming from the center of the galaxy were messing up the phone connection and causing the hiss. He — and the phone — had detected radio waves from space [ 1 ].
Jansky opened up a new, invisible universe. You can see a picture of the antenna used by Karl Jansky to detect radio waves from space in Figure 2. He finished the telescope, which was 31 ft across, in and used it to look at the whole sky and see where radio waves came from. You can see visible light because the visible-light photons travel in small waves, and your eye is small.
But because radio waves are big, your eye would need to be big to detect them. So while regular telescopes are a few inches or feet across, radio telescopes are much larger. The Arecibo Telescope in the jungle in Puerto Rico is almost 1, ft across. They look like gigantic versions of satellite TV dishes, but they work like regular telescopes. To use a regular telescope, you point it at an object in space. Light from that object then hits a mirror or lens, which bounces that light to another mirror or lens, which then bounces the light again and sends it to your eye or a camera.
The surface — which may be metal with holes in it, called mesh, or solid metal, like aluminum — acts like a mirror for radio waves. This picture shows how strong the radio waves are and where they are coming from in the sky.
When astronomers look for radio waves, they see different objects and events than they see when they look for visible light. Places that seem dark to our eyes, or to regular telescopes, burn bright in radio waves. Anatomy of an Electromagnetic Wave. Retrieved [insert date - e. Science Mission Directorate. National Aeronautics and Space Administration. Tour of the Electromagnetic Spectrum Learn about this image. Anatomy of an Electromagnetic Wave Energy, a measure of the ability to do work, comes in many forms and can transform from one type to another.
Classical waves transfer energy without transporting matter through the medium. Waves in a pond do not carry the water molecules from place to place; rather the wave's energy travels through the water, leaving the water molecules in place, much like a bug bobbing on top of ripples in water.
When a balloon is rubbed against a head of hair, astatic electric charge is created causing their individual hairs to repel one another. Credit: Ginger Butcher. Electromagnetic Spectrum Series Series Homepage. Infrared Waves. Reflected Near-Infrared. Visible Light.
Ultraviolet Waves. Earth's Radiation Budget. Diagram of the Electromagnetic Spectrum. Recommended Articles. September 24, Solar Eclipse - June 10, Frequency is measured in cycles per second, or Hertz. Wavelength is measured in meters. Energy is measured in electron volts. Each of these three quantities for describing EM radiation are related to each other in a precise mathematical way.
But why have three ways of describing things, each with a different set of physical units? Comparison of wavelength, frequency and energy for the electromagnetic spectrum. The short answer is that scientists don't like to use numbers any bigger or smaller than they have to.
It is much easier to say or write "two kilometers" than "two thousand meters. Astronomers who study radio waves tend to use wavelengths or frequencies.
Most of the radio part of the EM spectrum falls in the range from about 1 cm to 1 km, which is 30 gigahertz GHz to kilohertz kHz in frequencies. The radio is a very broad part of the EM spectrum. Infrared and optical astronomers generally use wavelength. Infrared astronomers use microns millionths of a meter for wavelengths, so their part of the EM spectrum falls in the range of 1 to microns.
Optical astronomers use both angstroms 0. Using nanometers, violet, blue, green, yellow, orange, and red light have wavelengths between and nanometers.
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