CHAPTER 22

Radiating Energy

SUMMARY

KEY TERMS

emission

radiation

light ray

white light

reflection

transmission

transparent

opaque

absorption

light wave

wavelength

frequency

speed of light

electromagnetic radiation

radio wave

microwave radiation

infrared radiation

visible light

ultraviolet radiation

x-ray

gamma ray

Planck’s constant

photon model

light detector

spectrometer

spectroscopy

The hot gases emitted from the reaction of wood and oxygen in a fireplace transfer thermal energy by conduction when they collide with your body. Upon collision, the faster-moving gas molecules slow down, the atoms moving more slowly in your body speed up, and you sense warmth. Conduction is one way that a fire transfers thermal energy, but this is not the only way that energy is transferred.

You feel the warmth of the Sun even though its light travels great distances through nearly empty space to reach Earth. This occurs by the emission of electromagnetic radiation. We perceive energy transfer from the Sun by radiation as visible light with our eyes, as the warmth due to infrared radiation, and, sometimes, as the sunburn caused by ultraviolet radiation. The color of a fire’s flames and much of its warmth are due to energy transfer by the radiation emitted by the hot gas molecules produced by the combustion reaction.

There are a number of models that lend understanding to the nature of light. The ray model suggests that light travels in straight lines until it is reflected, transmitted, or absorbed when it strikes an object. If white light is partially absorbed, the reflected or transmitted light has a color. The wave model is used to explain that different colors are waves all traveling at the speed of light but of varying wavelength and frequency. The photon model suggests that energy is transferred in small units, called photons. Photon energy increases with frequency.

Electromagnetic radiation exists with a huge range of wavelengths and frequencies. The wavelength regions include radio waves, microwave radiation, infrared radiation, visible light, ultraviolet radiation, x-rays, and gamma rays. Instruments, called spectrometers, use various wavelengths of electromagnetic radiation to probe matter. In essence, spectroscopy extends our ability to see light that we would otherwise not be able to detect with only our eyes.