ge of one volt (V). Thus, an electron moving from a negative metal plate to a positive metal plate when cur- rent flows in a common 1.5-V C battery would result in 1.5 eV of energy. Returning to the use of electronvolts, visible light in the form of pho- tons has an energy range of 1.8 to 3.1 eV; these electronvolts are visible as that energy range triggers the photo receptors in a human eye. We can also note visual perception in terms of wavelength. Thus, the electromag- netic spectrum that can be seen by the naked human eye (macroscopical- ly) ranges from approximately 4.3 X 1014 Hz for red to 7.5 X 1014 Hz for vio- let. Lower energies that have longer wavelengths are invisible to the naked human eye but can be detected by special sensors or even by the human ear. For example, infrared, television, and radio waves can be detected by equipment, while sound frequencies of up to 20 kHz can be detected by the human ear. Electromagnetic waves with energies above 3.1 eV and shorter wavelengths are also undetected by the unaided human eye; how- ever, they can be detected by different types of equipment. For example, X rays can be detected by lead plates or photographic film. Chapter Two Figure 2.6 The general relation- ship between attenu- ation and the wave- length of light results in three "windows" where a particular group of wave- lengths encounter Window Window Window the least amount of resistance. Wavelength (nm) ENERGY AND WAVELENGTH As the energy of photons increases, the wavelength of these photons decreases; thus, the energy of light is inversely proportional to its wavelength. For example, red-light pho- tons have an energy level of approximately 1.8 eV and a wavelength of 700 nm, while violet has an energy level of 3.1 eV and a wavelength of 400 nm. We can also note that the energy of photons is then pro- portional to the light frequency, such as E = hf, where h represents a famous constant that we will note shortly. This equation was first pro- posed by Max Planck when he investigated the emission of rad