Chapter 24, Problem 85RPP

Thin-film window coatings for energy conservation and comfort Thin-film coatings are applied to eyeglasses, computer screens, and automobile instrument panels to reduce glare and to binocular and camera lenses to increase the amount of light transmitted. Thin-film coatings are also used to add color to architectural glass, to reduce surface friction, and to improve the energy efficiency of windows. Consider this latter application.

Figure 24.30 shows that over 50% of the radiation reaching Earth from the Sun is long-wavelength infrared radiation, which we perceive as warmth, not light. Thin-film coatings on windows serve two purposes. (1) On warm days, the coatings reflect almost all of the infrared radiation from outside while allowing almost all of the visible light through the window (the glass absorbs most of the ultraviolet). The rooms inside stay cooler and require less air conditioning (2) On cold days, the same thin-film coating allows visible light to enter the room and reflects the thermal infrared radiation from the interior back into the room, keeping it warm. The reflectance of the thin-film window coating as a function of wavelength is shown in Figure 24.31.

The actual thin film built for windows reflects about what percent of the incident infrared radiation at 1000 nm?

a. 10

b. 30

c. 50

d. 70

e. 90