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📚 College Physics 2e
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1 Introduction: The Nature of Science and Physics2 Kinematics3 Two-Dimensional Kinematics4 Dynamics: Force and Newton's Laws of Motion5 Further Applications of Newton's Laws: Friction, Drag, and Elasticity6 Uniform Circular Motion and Gravitation7 Work, Energy, and Energy Resources8 Linear Momentum and Collisions9 Statics and Torque10 Rotational Motion and Angular Momentum11 Fluid Statics12 Fluid Dynamics and Its Biological and Medical Applications13 Temperature, Kinetic Theory, and the Gas Laws14 Heat and Heat Transfer Methods15 Thermodynamics16 Oscillatory Motion and Waves17 Physics of Hearing18 Electric Charge and Electric Field19 Electric Potential and Electric Field20 Electric Current, Resistance, and Ohm's Law21 Circuits and DC Instruments22 Magnetism23 Electromagnetic Induction, AC Circuits, and Electrical Technologies24 Electromagnetic Waves25 Geometric Optics26 Vision and Optical Instruments27 Wave Optics28 Special Relativity29 Quantum Physics30 Atomic Physics31 Radioactivity and Nuclear Physics32 Medical Applications of Nuclear Physics33 Particle Physics34 Frontiers of Physics
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25.1 The Ray Aspect of Light

Learning Objectives

By the end of this section, you will be able to:

  • List the ways by which light travels from a source to another location.

There are three ways in which light can travel from a source to another location. (See Figure 25.3.) It can come directly from the source through empty space, such as from the Sun to Earth. Or light can travel through various media, such as air and glass, to the person. Light can also arrive after being reflected, such as by a mirror. In all of these cases, light is modeled as traveling in straight lines called rays. Light may change direction when it encounters objects (such as a mirror) or in passing from one material to another (such as in passing from air to glass), but it then continues in a straight line or as a ray. The word ray comes from mathematics and here means a straight line that originates at some point. It is acceptable to visualize light rays as laser rays (or even science fiction depictions of ray guns).

This diagram shows the sun's rays reaching Earth in (a), and in (b), illustrates sunlight entering a room through a window, reflecting off a car outside. It highlights how solar energy interacts with different environments.
Figure 25.3 Three methods for light to travel from a source to another location. (a) Light reaches the upper atmosphere of Earth traveling through empty space directly from the source. (b) Light can reach a person in one of two ways. It can travel through media like air and glass. It can also reflect from an object like a mirror. In the situations shown here, light interacts with objects large enough that it travels in straight lines, like a ray.

Experiments, as well as our own experiences, show that when light interacts with objects several times as large as its wavelength, it travels in straight lines and acts like a ray. Its wave characteristics are not pronounced in such situations. Since the wavelength of light is less than a micron (a thousandth of a millimeter), it acts like a ray in the many common situations in which it encounters objects larger than a micron. For example, when light encounters anything we can observe with unaided eyes, such as a mirror, it acts like a ray, with only subtle wave characteristics. We will concentrate on the ray characteristics in this chapter.

Since light moves in straight lines, changing directions when it interacts with materials, it is described by geometry and simple trigonometry. This part of optics, where the ray aspect of light dominates, is therefore called geometric optics. There are two laws that govern how light changes direction when it interacts with matter. These are the law of reflection, for situations in which light bounces off matter, and the law of refraction, for situations in which light passes through matter.

Test Prep for AP Courses

When light from a distant object reflects off of a concave mirror and comes to a focus some distance in front of the mirror, we model light as a _____ to explain and predict the behavior of light and the formation of an image.

  1. wave
  2. particle
  3. ray
  4. all of the above

(c)

Light of wavelength 500 nm is incident on a narrow slit of width 150 nm. Which model of light most accurately predicts the behavior of the light after it passes through the slit? Explain your answer.

Section Summary

  • 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.

Problems & Exercises

Suppose a man stands in front of a mirror as shown in Figure 25.4. His eyes are 1.65 m above the floor, and the top of his head is 0.13 m higher. Find the height above the floor of the top and bottom of the smallest mirror in which he can see both the top of his head and his feet. How is this distance related to the man’s height?

A man standing in front of a mirror on a wall at a distance of several feet. The mirror’s top is at eye level, but its bottom is only waist high. Arrows illustrate how the man can see his reflection from head to toe in the mirror.
Figure 25.4 A full-length mirror is one in which you can see all of yourself. It need not be as big as you, and its size is independent of your distance from it.

Top 1.715 m from floor, bottom 0.825 m from floor. Height of mirror is 0.890 m, or precisely one-half the height of the person.

Adapted from College Physics 2e by OpenStax (openstax.org), licensed under CC BY-NC-SA 4.0. Changes were made. License: CC-BY-NC-SA-4.0.