Light Wavelength and Radiation

Electromagnetic radiation is energy given off by matter, traveling in the form of waves or particles. Electromagnetic energy exists in a wide range of energy values, of which visible light is one small part of the total spectrum. The source of radiation may be the hot and therefore highly energized atoms of the Sun, pouring out radiation across a wide range of energy values, including of course visible light, and they may also be unstable (radioactive) elements giving off radiation as they decay.

Radiation is called "electromagnetic" because it moves as interlocked waves of electrical and magnetic fields. A wave is a disturbance traveling through space, transferring energy from one point to the next. In a vacuum, all electromagnetic radiation travels at the speed of light, 983,319,262 feet per second (299,792,458 m/sec, often approximated as 300,000,000 m/sec). Depending on the type of radiation, the waves have different wavelengths, energies, and frequencies (see the following figure). The wavelength is the distance between individual waves, from one peak to another. The frequency is the number of waves that pass a stationary point each second. Notice in the graphic how the wave undulates up and down from peaks to valleys to peaks. The time from one peak to the next peak is called one cycle. A single unit of frequency is equal to one cycle per second. Scientists refer to a single cycle as one hertz, which commemorates 19th-century German physicist Heinrich Hertz, whose discovery of electromagnetic waves led to the development of radio. The frequency of a wave is related to its energy: The higher the frequency of a wave, the higher its energy, though its speed in a vacuum does not change.

The smallest wavelength, highest energy and frequency electromagnetic waves are cosmic rays, then as wavelength increases and energy

Electromagnetic Waves

Electromagnetic Waves

Each electromagnetic wave and frequency decrease, come gamma rays, then X-rays, then ultravi-

has a measurable wavelength olet light, then visible light (moving from violet through indigo, blue, andfrequency. green, yellow, orange, and red), then infrared (divided into near, meaning near to visible, mid-, and far infrared), then microwaves, and then radio waves, which have the longest wavelengths and the lowest energy and frequency.The electromagnetic spectrum is shown in the accompanying figure and table.

As a wave travels and vibrates up and down with its characteristic wavelength, it can be imagined as vibrating up and down in a single plane, such as the plane of this sheet of paper in the case of the simple example in the figure here showing polarization. In nature, some waves change their polarization constantly so that their polarization sweeps through all angles, and they are said to be circularly polarized. In ordinary visible light, the waves are vibrating up and down in numerous random planes. Light can be shone through a special filter called a polarizing filter that blocks out all the light except that polarized in a certain direction, and the light that shines out the other side of the filter is then called polarized light.

Polarization is important in wireless communications systems such as radios, cell phones, and non-cable television.The orientation of the transmitting antenna creates the polarization of the radio waves transmitted by that antenna: A vertical antenna emits vertically polarized waves, and a horizontal antenna emits horizontally polarized waves. Similarly, a horizontal antenna is best at receiving horizontally polar-

ized waves and a vertical antenna at vertically polarized waves. The best communications are obtained when the source and receiver antennas have the same polarization. This is why, when trying to adjust television antennas to get a better signal, having the two antennae at right angles to each other can maximize the chances of receiving a signal.

The human eye stops being able to detect radiation at wavelengths between 3,000 and 4,000 angstroms, which is deep violet—also the

Electromagnetic Spectrum

108-

10"

1016

1018-

Frequency (hertz)

Power-line emissions

AM radio

Radio

FM radio infrared

Light

Ultraviolet

X-rays

Medical X-rays

Wavelength (meters)

X-rays

Medical X-rays

3 X 104

3 X 102

3x10

r 14

Electromagnetic spectrum ranges from cosmic rays at the shortest wavelengths to radiowaves at the longest wavelengths.

Polarization

/ /

k

1

f\ ;1

k

\ \

Plane polarization \

V

s. i

\l

V

Waves can be thought of as rough limit on transmissions through the atmosphere (see the table plane or circularly polarized. "Wavelengths and Frequencies of Visible Light"). (Three thousand to

4,000 angstroms is the same as 300—400 nm because an angstrom is 10-9 m, while the prefix nano- or n means 10-10; for more, see appendix 1, "Units and Measurements.") Of visible light, the colors red, orange, yellow, green, blue, indigo, and violet are listed in order from longest wavelength and lowest energy to shortest wavelength and highest energy. Sir Isaac Newton, the spectacular English physicist and mathematician, first found that a glass prism split sunlight into a rainbow of colors. He named this a "spectrum," after the Latin word for ghost.

If visible light strikes molecules of gas as it passes through the atmosphere, it may get absorbed as energy by the molecule. After a short amount of time, the molecule releases the light, most probably in a different direction. The color that is radiated is the same color that was absorbed. All the colors of visible light can be absorbed by atmospheric molecules, but the higher energy blue light is absorbed more often than the lower energy red light. This process is called

WAVELENGTHS AND FREQUENCIES OF VISIBLE LIGHT

Visible light color Wavelength (in A, angstroms) Frequency (times 1014 Hz)

violet

4,000-4,600

7.5-6.5

indigo

4,600-4,750

6.5-6.3

blue

4,750-4,900

6.3-6.1

green

4,900-5,650

6.1-5.3

yellow

5,650-5,750

5.3-5.2

orange

5,750-6,000

5.2-5.0

red

6,000-8,000

5.0-3.7

WAVELENGTHS AND FREQUENCIES OF

THE ELECTROMAGNETIC SPECTRUM

Energy

Frequency in hertz (Hz)

Wavelength in meters

cosmic rays

everything higher in

everything lower in

energy than gamma rays

wavelength than gamma rays

gamma rays

I020 to I024

less than 10H2 m

X-rays

I017 to I020

I nm to I pm

ultraviolet

I015 to I017

400 nm to I nm

visible

4 x I0I4 to 7.5 x I0I4

750 nm to 400 nm

near-infrared

I x I0I4 to 4 x I0I4

2.5 ^m to 750 nm

infrared

I0I3 to I0I4

25 ^m to 2.5 ^m

microwaves

3 x 10" to I0I3

I mm to 25 ^m

radio waves

less than 3 x I0II

more than I mm

COMMON USES FOR RADIO WAVES

User

Approximate frequency

AM radio

0.535 x I06 to 1.7 x l06Hz

baby monitors

49 x I06Hz

cordless phones

49 x I06Hz

900 x I06Hz

2,400 x I06Hz

television channels 2 through 6

54 x I06 to 88 x I06Hz

radio-controlled planes

72 x I06Hz

radio-controlled cars

75 x I06Hz

FM radio

88 x I06 to I08 x I06Hz

television channels 7 through 13

I74 x I06 to 220 x I06Hz

wildlife tracking collars

2I5 x I06Hz

cell phones

800 x I06Hz

2,400 x I06Hz

air traffic control radar

960 x I06Hz

I,2I5 x I06Hz

global positioning systems

I,227 x I06Hz

I,575 x I06Hz

deep space radio

2,300 x I06Hz

Rayleigh scattering (named after Lord John Rayleigh, an English physicist who first described it in the 1870s).

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through: The air affects little of the red, orange, and yellow light. The gas molecules absorb much of the shorter wavelength blue light. The absorbed blue light is then radiated in different directions and is scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue. Note also that there is a very different kind of scattering, in which the light is simply bounced off larger objects like pieces of dust and water droplets, rather than being absorbed by a molecule of gas in the atmosphere and then reemitted.This bouncing kind of scattering is responsible for red sunrises and sunsets.

Until the end of the 18th century, people thought that visible light was the only kind of light. The amazing amateur astronomer Frederick William Herschel (the discoverer of Uranus) discovered the first non-visible light, the infrared. He thought that each color of visible light had a different temperature and devised an experiment to measure the temperature of each color of light. The temperatures went up as the colors progressed from violet through red, and then Herschel decided to measure past red, where he found the highest temperature yet. This was the first demonstration that there was a kind of radiation that could not be seen by the human eye. Herschel originally named this range of radiation "calorific rays," but the name was later changed to infrared, meaning "below red." Infrared radiation has become an important way of sensing solar system objects and is also used in night-vision goggles and various other practical purposes.

At lower energies and longer wavelengths than the visible and infrared, microwaves are commonly used to transmit energy to food in microwave ovens, as well as for some communications, though radio waves are more common in this use.There is a wide range of frequencies in the radio spectrum, and they are used in many ways, as shown in the table "Common Uses for Radio Waves," including television, radio, and cell phone transmissions. Note that the frequency units are given in terms of 106 Hz, without correcting for each coefficient's additional factors of 10.This is because 106 Hz corresponds to the unit of megahertz (MHz), which is a commonly used unit of frequency.

Cosmic rays, gamma rays, and X-rays, the three highest-energy radiations, are known as ionizing radiation because they contain enough energy that, when they hit an atom, they may knock an electron off of it or otherwise change the atom's weight or structure. These ionizing radiations, then, are particularly dangerous to living things; for example, they can damage DNA molecules (though good use is made of them as well, to see into bodies with X-rays and to kill cancer cells with gamma rays). Luckily the atmosphere stops most ionizing radiation, but not all of it. Cosmic rays created by the Sun in solar flares, or sent off as a part of the solar wind, are relatively low energy.There are far more energetic cosmic rays, though, that come from distant stars through interstellar space. These are energetic enough to penetrate into an asteroid as deeply as a meter and can often make it through the atmosphere.

When an atom of a radioisotope decays, it gives off some of its excess energy as radiation in the form of X-rays, gamma rays, or fast-moving subatomic particles: alpha particles (two protons and two neutrons, bound together as an atomic nucleus), or beta particles (fast-moving electrons), or a combination of two or more of these products. If it decays with emission of an alpha or beta particle, it becomes a new element. These decay products can be described as gamma, beta, and alpha radiation. By decaying, the atom is progressing in one or more steps toward a stable state where it is no longer radioactive.

RADIOACTIVITY OF SELECTED OBJECTS AND MATERIALS

Object or material

1 adult human (100 Bq/kg) 1 kg coffee

1 kg high-phosphate fertilizer 1 household smoke detector (with the element americium) radioisotope source for cancer therapy 1 kg 50-year-old vitrified high-level nuclear waste 1 kg uranium ore (Canadian ore, 15% uranium) 1 kg uranium ore (Australian ore, 0.3% uranium) 1 kg granite

Radioactivity

100 million million Bq

10 million million Bq

25 million Bq

500,000 Bq

1,000 Bq

The X-rays and gamma rays from decaying atoms are identical to those from other natural sources. Like other ionizing radiation, they can damage living tissue but can be blocked by lead sheets or by thick concrete. Alpha particles are much larger and can be blocked more quickly by other material; a sheet of paper or the outer layer of skin on your hand will stop them. If the atom that produces them is taken inside the body, however, such as when a person breathes in radon gas, the alpha particle can do damage to the lungs. Beta particles are more energetic and smaller and can penetrate a couple of centimeters into a person's body.

But why can both radioactive decay that is formed of subatomic particles and heat that travels as a wave of energy be considered radiation? One of Albert Einstein's great discoveries is called the photoelectric effect: Subatomic particles can all behave as either a wave or a particle. The smaller the particle, the more wavelike it is. The best example of this is light itself, which behaves almost entirely as a wave, but there is the particle equivalent for light, the massless photon. Even alpha particles, the largest decay product discussed here, can act like a wave, though their wavelike properties are much harder to detect.

The amount of radioactive material is given in becquerel (Bq), a measure that enables us to compare the typical radioactivity of some natural and other materials. A becquerel is one atomic decay per second. Radioactivity is still sometimes measured using a unit called a Curie; a Becquerel is 27 X 10-12 Curies. There are materials made mainly of radioactive elements, like uranium, but most materials are made mainly of stable atoms. Even materials made mainly of stable atoms, however, almost always have trace amounts of radioactive elements in them, and so even common objects give off some level of radiation, as shown in the following table.

Background radiation is all around us all the time. Naturally occurring radioactive elements are more common in some kinds of rocks than others; for example, granite carries more radioactive elements than does sandstone; therefore a person working in a bank built of granite will receive more radiation than someone who works in a wooden building. Similarly, the atmosphere absorbs cosmic rays, but the higher the elevation, the more cosmic-ray exposure there is.A person living in Denver or in the mountains of Tibet is exposed to more cosmic rays than someone living in Boston or in the Netherlands.

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