Wave Optics
78 Introduction to Wave Optics
Learning Objectives
- Describe the evidence that demonstrates the wave nature of light.
- Distinguish between geometric optics and wave optics.
- Identify everyday phenomena that arise from interference, diffraction, and other wave effects.
Have you ever noticed the brilliant rainbow colors reflected from a compact disc, a soap bubble, or a thin layer of oil floating on water? These colors are not produced by pigments. Instead, they arise because light behaves as a wave. As light waves interact with structures whose dimensions are comparable to their wavelength, they interfere, diffract, and combine in ways that produce striking visual effects.
Earlier chapters treated light primarily as rays that travel in straight lines. This approach, known as geometric optics, successfully explains image formation by mirrors and lenses and describes the operation of cameras, microscopes, telescopes, and the human eye. However, geometric optics cannot explain many familiar optical phenomena, including the shimmering colors of soap bubbles, the iridescence of butterfly wings, or the colorful reflections from optical storage media such as CDs and DVDs.
To understand these phenomena, we must treat light as an electromagnetic wave. The branch of physics that studies these behaviors is known as wave optics (also called physical optics). Wave optics explains how light waves interfere with one another, bend around obstacles, spread after passing through narrow openings, and separate into different colors when interacting with periodic structures.
Explore at Home
A compact disc provides an inexpensive diffraction grating that you can investigate at home.
Observe the disc under different light sources, such as:
- a candle flame,
- an incandescent light bulb,
- an LED light bulb,
- a fluorescent lamp, and
- daylight.
Notice how the reflected colors change as you tilt the disc. Compare the spectra produced by each light source. Continuous sources, such as incandescent bulbs, produce smooth rainbows, whereas fluorescent and LED lights often produce distinct colored bands because they emit light at specific wavelengths.
Knowing that the tracks on a compact disc are separated by approximately 1.6 μm, you can later use the equations of diffraction to estimate the spacing between adjacent tracks and even relate that spacing to the amount of digital information that can be stored on the disc.
Wave optics has applications far beyond astronomy and photography. The same principles are used in medical imaging, optical fibers, laser surgery, microscopy, spectroscopy, anti-reflective coatings for eyeglasses, and modern semiconductor manufacturing. Throughout this chapter, you will discover how treating light as a wave explains phenomena that cannot be understood using rays alone and provides the foundation for many of today’s optical technologies.
