Energy Radiation

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Radiation is the transport of energy by photons of light migrating from a hotter to a colder surface. Unlike conduction and convection, both of which require a material medium to transport heat energy, radiation transports energy via electromagnetic waves of different wavelengths , even in a vacuum (Figure 1). Wavelength is the distance between two successive crests or troughs in a wave; the shorter the wavelength, the higher its energy is. No matter what the wavelength is, they all travel at 300,000 kilometers per second, the speed of light.

 Electromagnetic wave spectrum
Figure 1: Electromagnetic wave spectrum

The most energetic radiations are gamma rays and x-rays. Gamma rays come from the nuclei of certain atoms with wavelengths smaller than the size of the atom; x-rays come from the innermost orbits of electrons. The least energetic radiations are from radars and radio waves, with wavelengths that can exceed several meters or kilometers. Radiation at wavelengths shorter than 0.3 microns (high intensity ultraviolet) is dangerous to humans. Its effects can range from simple sunburn to cancer and death. Photons at these short wavelengths are sufficiently energetic to break bonds in molecules of living matter. Fortunately, most are filtered out by earth’s atmosphere. Wavelengths between 0.3 and 0.4 microns (near ultraviolet) are weakly absorbed by clouds and dust in the atmosphere, while the rest reach the earth’s surface. Microwaves have frequencies close to the resonance frequency of water molecules, and therefore, readily absorbed by water molecules, a feature exploited in microwave ovens for the rapid heating of food. Radar, TV, and radio waves have very long wavelengths (from many meters to kilometers) and thus are of very low energy; they are mainly of interest in communications. The atmosphere is largely transparent to rainbow colors, which make up visible light and cover wavelengths in a very small range from only 0.4 microns for deep violet to 0.7 microns for bright red. Although earth’s atmosphere is transparent to visible radiation, it is virtually opaque to infrared radiation, which has wavelengths longer than 0.7 microns. As we will see in the next chapter, this is the main property responsible for the atmosphere’s behavior as a greenhouse; letting solar radiation in while trapping terrestrial infrared radiation results in the global warming phenomenon.

Whether a body emits energy at one wavelength or another depends on its temperature and surface properties. An object at room temperature, say 20°C, emits nearly all its energy in infrared. The human body at ordinary temperatures also emits in the infrared region. In fact, 98% of the radiation from a bare human body ranges from 4.78 to 75 microns in wavelength. Using a special infrared camera it is possible to “see” a human body or a passing car in total darkness. The image will not look like what we see with our single lens cameras, but will be a contour map of constant temperature regions. An infrared (thermal) image of a man wearing a tee-shirt is shown in Figure 2.

Figure 2: Thermal image of a man. The hotter regions (bare skin) are lighter in color, and cooler spots (tee-shirt, arm pits) are darker grey. The choice of shades of grey is arbitrary and does not imply a physical significance.

Question: If human beings emit infrared, why do we see them in “visible” light?

Answer: What we see is not emitted light, but the reflection of light from other sources (sunlight, fluorescent light, etc.). This is why in the absence of a light source (darkness) there is no light reflected to the eye and a person cannot see or be seen.

Unlike conduction and convection losses that increase with temperature differences between cold and hot objects, radiation losses increase as the differences of the temperatures to the fourth power (T4hot - T4cold), and become dominant only for very high and very low temperatures. In buildings, radiative losses are most significant when the surrounding terrain is either much colder or warmer than inside. Roofs can radiate a substantial amount of energy to the cold night sky. They also provide a low-resistance path to solar heat during the summer months. Window glass is much colder than adjacent walls during the winter, causing internal heat, such as heat released by the occupants or heaters, to migrate toward windows. This results in a larger temperature difference across the glass layer and causes more heat to escape through windows. Double-glazing the windows, closing the curtains, and adding additional insulation in the walls and attic can significantly reduce these losses.

Question: You have probably experienced colder room temperatures in the winter when drapes are not closed, even though the thermostat records air temperature that should be comfortable. Why?

Answer: Glass is transparent to radiation in the visible range, but is opaque to infrared. The radiation from cold glass windows to you is much less than yours to the window.

Question: You can easily feel the heat from the sun through a glass window, but behind a sheet of glass you do not feel much heat from a fireplace. Why?

Answer: Common window glass is transparent to the wavelengths of radiation between 0.3-2.5 μm. A large portion of solar radiation falls in this range, allowing both sunlight and solar heat to pass through. Flames, however, emit in wavelengths in excess of 2.5 μm, the region where window glasses are practically opaque.


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