Figure 3.14 shows two glass slides illuminated by monochromatic light incident perpendicularly. The top slide touches the bottom slide at one end and rests on a 0.100-mm-diameter hair at the other end, forming a wedge of air. (a) How far apart are the dark bands, if the slides are \(7.50 \mathrm{cm}\) long and 589 -nm light is used? (b) Is there any difference if the slides are made from crown or flint glass? Explain.

Monochromatic light plays a crucial role in the field of optical physics. With a meaning derived from the Greek words 'mono', meaning single, and 'chroma', meaning color, monochromatic light consists of a single wavelength and, consequently, a single color. This purity of color is essential for creating clear and predictable interference patterns, which are a core aspect of many optical experiments and technologies.

In the context of the given exercise, the use of monochromatic light ensures that the interference pattern observed is due to the air wedge between the glass slides and not due to any variations in the wavelength of the light. This simplification allows for precise calculations of the interference effects, enabling students to isolate and understand the influence of other variables, such as the angle of the wedge or the material of the glass.

The wavelength of light is the distance over which the light's wave shape repeats. It is one of the most important properties when discussing light in a physical context. The wavelength is closely tied to the frequency of the light wave, with higher frequencies corresponding to shorter wavelengths. Visible light wavelengths range approximately from 400 nanometers (violet) to 700 nanometers (red).

In the exercise, we work with a specific wavelength, 589 nanometers, which typically corresponds to the sodium D-line, often used in lab settings for its distinct yellow hue. This parameter is key to understanding the behavior of light as it interacts with the air wedge, since the wavelength directly influences the interference pattern that develops between the glass slides.

An interference pattern occurs when two or more waves superimpose to form a new wave pattern. In optics, this is often observed when coherent light waves from a monochromatic source, such as a laser, meet after traveling different paths. The result can be a series of bright and dark bands known as an interference pattern. This pattern is a direct visualization of the constructive (bright bands) and destructive (dark bands) interference between the light waves.

The pattern's details—like the distance between the dark bands in the exercise—are determined by factors such as the wavelength of the light and the angle at which waves intersect. Understanding how these dark and bright bands form and how their spacing can be calculated is essential for any student studying wave optics.