21.3. Thin Films http://www.ck12.org
21.3 Thin Films
Objectives
The student will:
- Understand how thin films recreate constructive and destructive interference.
- Solve problems involving thin films.
Vocabulary
- phase change:Occurs when a beam of light is reflected from a medium with a greater index of refraction
than the medium in which it travels.
Introduction
From our discussion of CDs, we can see that we do not need elaborate equipment to witness the effects of light
interference. In fact, everyday phenomena such as soap bubbles or a thin layer of oil on the surface of the water
provide for an array of colors reminiscent of the rainbow (seeFigure21.12 andFigure21.13). Different wavelengths
of light are subject to constructive interference and destructive interference after reflecting off the top and the bottom
of a thin layer of oil.
FIGURE 21.12
Soap bubbles.Try putting a drop of oil onto water in a bowl. Look at it from different angles and see what patterns you can make
of it.
TheFigure21.14 shows how the light reflecting from the top and bottom of a thin film (a soap bubble or a layer of
oil, for example) constructively interferes. The figure is exaggerated in order to see the effect clearly. For simplicity,
we’ll assume that all rays are essentially perpendicular (normal) to the air, oil, and water surfaces.
Compared to the distance the black reflected ray travels in air, the extra distance that the green ray travels through
the oil is, then, 2d. If the path difference 2dis equal to an integral number of wavelengths, then the reflected wave
from the water-oil interface will be in phase with the reflected ray from the air-oil interface. The wavelengths refer