Learning Objectives

Electromagnetic Spectrum

The Sun emits many forms of radiation including cosmic rays, x-rays, ultraviolet, visible, infrared, microwave and radio waves. These emissions are included in what is referred to as the electromagnetic spectrum. The following chart shows the types of radiation found in the electromagnetic spectrum.

The colors we see in the visible spectrum represent only a tiny portion of the electromagnetic spectrum.

Radiation, although a wave of light, should be thought of as a form of energy. Radiation energy is not really continuous, but is quantized into bundles of energy, which are called photons.

The following diagram illustrates the concept of wavelength. Wavelength can be measured from node to node, as shown, or from crest to crest. The points of zero amplitude are called nodes and always occur at intervals of l/2.

Radio Waves

Radio waves have wavelengths that range from less than a centimeter to hundreds of meters. A frequency modulated wave, FM, that occurs at 100 on the radio dial (100 megahertz) would have a wavelength of about 3 meters. An amplitude modulated wave, AM, that occurs at 750 on the radio dial (750 kilohertz) would have a wavelength of about 400 meters. The medical field uses radio waves in the important diagnostic tool called magnetic resonance imaging.

Microwaves

Microwaves range from about 1 millimeter to about 30 centimeters. Microwave ovens are tuned to the rotation frequency of two water molecules joined by a hydrogen bond. When the microwave radiation is absorbed by the water, the water will warm up. The container does not absorb microwave radiation at that wavelength and remains much cooler, except for the heat transfer from the hot water in the food that was warmed up.

Infrared Radiation

Infrared radiation has an intermediate energy and extends from about 1 millimeter in wavelength to the visible region. Infrared radiation has the same energy as molecular vibrations. Chemical bonds are analogous to springs and can either be bent or stretched. The stretching and bending vibrations of bonds between different atoms all have different energies. When infrared radiation matches the specific stretching or bending energy of a particular bond, it will be absorbed if there is a dipole associated with the bond.

When a molecule absorbs infrared radiation, the amplitude of vibration is increased and the molecule heats up. This heat can be lost in either of two ways. The molecule can come into direct contact with another body and directly transfer the heat or the molecule can re-emit infrared radiation.

Warm objects can lose heat to the surroundings by direct transfer to air molecules or objects with which they are actually in contact. Why on a day when the temperature is below freezing do you experience wind-chill.?

How do those plastic aluminized survival blankets keep you warm?

How does a thermos bottle keep coffee warm?

Planets warm up when they absorb infrared radiation from suns. The only way that they can cool down is to emit infrared radiation. Infrared active molecules such as carbon dioxide and water contribute to a phenomenon called "Global Warming".

Ultraviolet Radiation

We are constantly irradiated with radio and television waves yet we never have an adverse effect from this radiation because a photon of radio waves has a very low energy of about 10^-26 joule. The energy of one photon of ultraviolet radiation is about 10^-19 joule which is ten million times more energy than a radio wave. Although one photon of ultraviolet radiation has a very small amount of energy, one must consider that if we are in the sun we are receiving vast numbers of photons.

The energy of ultraviolet radiation, when calculated on the basis of a mole of photons, is strong enough to break typical chemical bonds. Biological molecules such as DNA (deoxyribonucleic acid) can be changed by interaction with ultraviolet light. Cancer can occur when DNA is altered and this is one reason that ultraviolet radiation is harmful. Ultraviolet radiation also promotes the formation of cataracts and impairs the immune system.

Most of the ultraviolet radiation emitted by the sun is blocked by the presence of oxygen and ozone in the Earth's atmosphere. Oxygen absorbs ultraviolet radiation in the region below 242 nm (A nanometer is 10^-9 meter). Ultraviolet radiation with a wavelength longer than 242 nm is not absorbed by oxygen but is absorbed by ozone. Without the ozone shield, we would be more susceptible to skin cancer, cataracts and impaired immune systems.

The amount of ozone in the upper atmosphere has been steadily decreasing and was first noticed as a hole in the ozone layer above Antarctica. Although this phenomenon is very serious for countries like Chile, it is also serious for everyone since the amount of ozone depletion is global. It is just more pronounced over Antarctica because of less air circulation over Antarctica.

The following picture shows the ozone hole over Antarctica. The units are Dobson units and are equal to 2.7 x 10¹⁶ molecules per cubic centimeter.

Although many of the conditions are similar, there has not been the same severe problem over the North Pole because the warmer temperatures there promote better circulation in the upper atmosphere.

One of the main causes of ozone depletion has been the chemical compounds called chlorofluorocarbons.

Problems

1. What are the three separate phases of free radical reactions?

2. What form of electromagnetic radiation from the sun does the ozone layer partially filter out?

3. What is the major health consequence resulting from depletion of the ozone layer.

4. Which of the following compounds is most responsible for depletion of the ozone layer?

SO₂

CHClF₂

CO₂

N₂

5. Which of the following compounds are "Greenhouse Gases"?

CO₂

H₂O

O₂

Cl₂

He

CF₄

CHClF₂

SO₂

NO₂

N₂

Ne

6. Which of the following compounds is most responsible for global warming?

SO₂
CHClF₂
CO₂
N₂

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