When light strikes a soap bubble, some of the light is reflected
from the first surface of the soap film and some continues through
the soap film and is reflected at the second surface. These two
light waves are certainly coherent. As the thickness of the soap
film varies these two light waves will interfere constructively
and provide a bright area or interfere destructively and provide
a dark area. This is illustrated in Figure 20.17. Their interference
also depends upon the angle and upon the wavelength of the light
. This means one area of a soap film may provide bright, constructive
interference for one color of light while another area provides
bright, constructive interference for another color of light.
This provides the many different colors that we see. Colors change,
too, with the thickness of the film.
Destructive interference from thin films is a very useful phenomenon in the design of high quality photographic lenses. You already know that you can see yourself in an ordinary glass. Five to ten percent of the light striking a piece of glass from air is reflected. In a camera lens-which may have five to ten or even more different elements in it-a loss of light of a few percent at each surface can mean a dim image. Reflected light can cause other problems as it bounces around the lens. Therefore, manufacturers of photo lenses coat the lens elements with a thin film to make them non-reflecting. They can not be non-reflecting for all wavelengths and all angles so the non-reflecting coating is usually optimized for yellow light, near the middle of the visible spectrum and near the eye's most sensitive region, and for the primary angle of the instrument's use.
Non-reflecting coatings are also used on glass for picture frames to reduce glare or reflections and, thus, make the picture behind the glass easier to see. By carefully controlling the thickness of a layer of aluminum oxide on a piece of aluminum, the color of the aluminum can be varied-almost chosen at will. This process is known as anodizing aluminum.