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# 25: Geometric Optics

Geometrical optics describes light propagation in terms of rays, which is useful in approximating the paths along which light propagates in certain classes of circumstances. Geometrical optics does not account for certain optical effects such as diffraction and interference.

• 25.0: Prelude to Geometric Optics
When light interacts with an object that is several times as large as the light’s wavelength, its observable behavior is like that of a ray; it does not prominently display its wave characteristics. We call this part of optics “geometric optics.” This chapter will concentrate on such situations. When light interacts with smaller objects, it has very prominent wave characteristics, such as constructive and destructive interference. "Wave Optics" will concentrate on such situations.
• 25.1: The Ray Aspect of Light
A straight line that originates at some point is called a ray. The part of optics dealing with the ray aspect of light is called geometric optics. Light can travel in three ways from a source to another location: (1) directly from the source through empty space; (2) through various media; (3) after being reflected from a mirror.
• 25.2: The Law of Reflection
The angle of reflection equals the angle of incidence. A mirror has a smooth surface and reflects light at specific angles. Light is diffused when it reflects from a rough surface. Mirror images can be photographed and videotaped by instruments.
• 25.3: The Law of Refraction
The changing of a light ray’s direction when it passes through variations in matter is called refraction. The speed of light in vacuuum  $$c = 2.9972458 \times 10^{8} \sim 3.00 \times 10^{8} m/s$$ Index of refraction $$n = \frac{c}{v}$$, where $$v$$ is the speed of light in the material, $$c$$ is the speed of light in vacuum, and $$n$$ is the index of refraction. Snell’s law, the law of refraction, is stated in equation form as $$n_{1} \sin_{\theta_{1}} = n_{2} \sin_{\theta_{2}}$$.
• 25.4: Total Internal Reflection
The incident angle that produces an angle of refraction of $$90^{\circ}$$ is called critical angle. Total internal reflection is a phenomenon that occurs at the boundary between two mediums, such that if the incident angle in the first medium is greater than the critical angle, then all the light is reflected back into that medium. Fiber optics involves the transmission of light down fibers of plastic or glass, applying the principle of total internal reflection.
• 25.5: Dispersion - Rainbows and Prisms
The spreading of white light into its full spectrum of wavelengths is called dispersion. Rainbows are produced by a combination of refraction and reflection and involve the dispersion of sunlight into a continuous distribution of colors. Dispersion produces beautiful rainbows but also causes problems in certain optical systems.
• 25.6: Image Formation by Lenses
Light rays entering a converging lens parallel to its axis cross one another at a single point on the opposite side. For a converging lens, the focal point is the point at which converging light rays cross; for a diverging lens, the focal point is the point from which diverging light rays appear to originate. The distance from the center of the lens to its focal point is called the focal length $$f$$. Power $$P$$ of a lens is defined to be the inverse of its focal length, $$P = \frac{1}{f}$$.
• 25.7: Image Formation by Mirrors
Images in flat mirrors are the same size as the object and are located behind the mirror. Like lenses, mirrors can form a variety of images. For example, dental mirrors may produce a magnified image, just as makeup mirrors do. Security mirrors in shops, on the other hand, form images that are smaller than the object. We will use the law of reflection to understand how mirrors form images, and we will find that mirror images are analogous to those formed by lenses.
• 25.E: Geometric Optics (Exercises)

Thumbnail: Parallel light rays entering a diverging lens from the right seem to come from the focal point on the right.