http://www.ck12.org Chapter 21. Physical Optics
21.1 Dispersion
Objectives
The student will:
- Understand the definition of dispersion and its causes.
- Understand the dispersion of light through a prism.
- Solve problems involving dispersion.
Vocabulary
- dispersion:The spreading of light through a prism.
Introduction
A rainbow is one of the oldest mysteries of nature. We know that white light has many different wavelengths, but
what is the mechanism that causes those different wavelengths to separate?
FIGURE 21.2
The principle of refraction is that light and other waves change direction when they change speed at an angle, similar
to how a car may change direction when it starts to drive over sand. In geometric optics like thin lenses, we assume
that all light travels at the same speed. However, the speed of light in a medium like glass really depends on its
wavelength. Some wavelengths of light slow down more, and some slow down less.
This is most clearly seen through a prism, which is a glass with a triangular cross-section. The triangular shape
means that refraction continues as light exits. When white light passes through glass prism, a rainbow of colors,
called the visible spectrum, exits the prism. The spreading of light in this manner is calleddispersion, seeFigure
21.3.
Dispersion reveals three very important attributes of light: