Radiation and Climate

The Electromagnetic Spectrum and Solar Radiation

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Earth’s Atmosphere

• Without Earth’s atmosphere, the midday sunshine would heat a rock hot enough to fry an egg, and the Sun’s ultraviolet rays would burn exposed skin quickly; nights would be so cold that carbon dioxide gas would freeze to a solid.

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Because Earth’s moon lacks an atmosphere, the extreme conditions mentioned on Slide 2 are not science fiction; they exist there now.

Since the Moon lacks an atmosphere, it has very different temperatures on its “Sun-facing” side than it does on the other side.

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http://www.lpi.usra.edu/resources/lunar_orbiter/

Earth’s Climate

• The Sun’s radiant energy and the gases that make up Earth’s atmosphere combine to maintain a hospitable climate for life on this planet.– Along with its vital role as an abundant reservoir for

both oxygen gas and carbon dioxide gas, Earth’s atmosphere provides protection from some of the Sun’s ultraviolet radiation.

– The atmosphere helps moderate Earth’s temperature by controlling how much solar radiation is trapped close to Earth’s surface.

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Nuclear Fusion

• The enormous quantity of energy produced by the Sun, which is Earth’s main external source of energy, is a result of the fusion of hydrogen nuclei to form helium.– Nuclear fusion is the combining of two nuclei to

form a new, heavier nucleus, with the accompanying release of energy.

– Nuclear fusion occurs at high temperature and pressure, powering the Sun and other stars.

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The Sun is powered by nuclear fusion—a process that releases huge quantities of energy. This energy is used by growing plants, drives the hydrologic cycle, and warms the atmosphere on Earth.

6http://www.universetoday.com/16338/the-sun/

Electromagnetic Radiation

• Some energy liberated through nuclear fusion in the Sun is transmitted to Earth as electromagnetic radiation, which consists of a broad range of energetic emissions.– It is useful to identify particular types of

electromagnetic radiation, with each type representing a particular energy range, that together make up the electromagnetic spectrum.

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The Electromagnetic Spectrum

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http://www.enotes.com/topic/Electromagnetic_radiation

Radio waves and microwaves are at the low-energy end of the spectrum; X-rays and gamma rays are at the high-energy end of the spectrum. Between these extremes are infrared and ultraviolet radiation and the familiar visible spectrum, which is the only type that is visible to the unaided human eye.

Movement of Electromagnetic Radiation

• Electromagnetic radiation is composed of photons, or bundles of energy. – Photons travel as waves, and as you might

expect, move at the speed of light.– Unlike sound waves or ocean waves,

electromagnetic waves do not require a medium, or substance, to support their movement.

– Electromagnetic radiation can move through a vacuum as well as through air and other media.

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Satellite dishes atop roofs in a French village receive electromagnetic signals

from a satellite above Earth.

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http://ipodmp3accessories.info/antenna-rooftop/

Frequency• Photons of electromagnetic radiation have

energy changes characteristic of the type of radiation involved (such as visible, ultraviolet, and gamma radiation).– All waves, including photon waves of EM radiation,

involve oscillation.– The rate of oscillation, or the number of waves that

pass by a given reference point per second, is called frequency.

– The frequency of a wave is directly proportional to its energy; high-frequency radiation is also high-energy radiation.

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Wavelength

• Another characteristic of waves is wavelength.– The distance between the tops (or any

corresponding part) of successive waves equals the wavelength.

– The wavelength and energy of a photon are inversely proportional; radiation with longer wavelengths is less energetic than is radiation with shorter wavelengths.

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Parts of a Wave

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http://www.catie.org.uk/facesofscience/invisible_waves/default.htm

Photon Interaction with Matter

• Photons can transfer their energy as they collide and interact with matter.

• The photon’s energy, and thus, either its wavelength or frequency, largely determines its effect on living things and other types of matter.

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Solar Radiation

• Most of the radiant energy emitted by the Sun is spread over a large portion of the electromagnetic spectrum:– About 45% is infrared (IR)– About 46% is visible– About 9% is ultraviolet (UV)

• Less than 1% of solar radiation falls outside these three regions.

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The Solar Spectrum

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http://www.mpoweruk.com/solar_power.htm

Intensity, plotted on the y-axis, is an expression of the quantity of radiation at a given wavelength.

Infrared Radiation

• IR radiation causes certain bonded atoms to vibrate more energetically, which is observed as an increase in the material’s temperature; IR is “heat” radiation.– Most of the IR radiation from the Sun cannot reach the

Earth’s surface because it is absorbed by CO2 and gaseous H2O molecules in the atmosphere (demonstrated by John Tyndall in the 1860s).

– Some of the shorter wavelength (higher energy) solar radiation is transformed and reradiated as infrared energy.• This reradiated energy is reflected back and retained by the

atmosphere, a fact that will come into play later when we consider the role of CO2 and certain other gases in the atmosphere.

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Visible Radiation• On a clear day, more than 90% of the visible region of

the solar radiation directed toward Earth travels down to Earth’s surface.– The scattering of this portion of the Sun’s radiation by

water, air, and dust is the cause of red sunsets and blue skies.

– Visible radiation can energize electrons in some chemical bonds.• An example is the interaction of visible light with electrons in

chlorophyll molecules, which provides the energy for photosynthesis reactions.

• Photon-electron interactions also occur in the double bonds of certain molecules within your eyes, making it possible for you to see.

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Some visible radiation from the Sun is scattered by Earth’s atmosphere. This scattering creates the colors we see in the sky.

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http://www.freefoto.com/preview/9907-09-1?ffid=9907-09-1

http://blog.thomaslaupstad.com/2007/12/18/my-personal-top-5-favorite-sunset-pictures-from-the-summer-2008-in-northern-norway/

Ultraviolet Radiation• There are three subcategories of UV radiation, all of which possess greater

energy than does visible light.• Of the three, UV-A radiation has the longest wavelengths, and thus, the

lowest energy.• UV-B radiation has more energy; it can cause sunburn; and, with long-

term exposure, it is linked to skin cancer.• UV-C radiation, the most energetic form of UV radiation, is useful for

sterilizing materials because it can kill bacteria and destroy viruses.– This is due to the fact that UV-C photons have enough energy to break

covalent bonds.– As a result, chemical changes can occur in materials exposed to some UV

radiation, including damage to tissues of living organisms.• UV-C consists of UV radiation with wavelengths shorter than 280 nm, UV-

B wavelengths range from 280 to 320 nm, and UV-A radiation has wavelengths longer than 320 nm.

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UV Absorption

• UV-C radiation is absorbed in the stratosphere before reaching Earth’s surface.

• Most UV-B radiation, and much UV-A radiation, does not reach Earth’s surface; it is absorbed by the stratospheric ozone layer, which you will learn more about later in this unit.

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UV Damage

• If all the UV radiation reaching the atmosphere actually reached the Earth’s surface, it is likely that most life on Earth would be destroyed.

• UV radiation, however, is not all bad.– Humans and animals must have some exposure to

it because Vitamin D is produced when the skin receives moderate doses of UV radiation included in sunlight.

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HOMEWORK

1) Why is visible light useful in plant photosynthesis while other forms of EM radiation are not?

2) A. List the main types of EM radiation in order of increasing energy.B. Describe how each type of radiation listed in 2A affects living things.

3) Ultraviolet light is often used to sterilize chemistry lab goggles. Why is UV light effective for this use, while visible light is not effective?

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