34.1 The Ray Model of Light

A flashlight makes a beam of light through the night’s darkness. Sunbeams stream into a darkened room through a small hole in the shade. Laser beams are even more well defined. Our everyday experience that light travels in straight lines is the basis of the ray model of light.

The ray model is an oversimplification of reality but nonetheless is very useful within its range of validity. In particular, the ray model of light is valid as long as any apertures through which the light passes (lenses, mirrors, and holes) are very large compared to the wavelength of light. In that case, diffraction and other wave aspects of light are negligible and can be ignored. The analysis of Section 33.7 found that the crossover between wave optics and ray optics occurs for apertures ≈1 mm in diameter. Lenses and mirrors are almost always larger than 1 mm, so the ray model of light is the appropriate model for the practical optics of image formation.

To begin, let us define a light ray as a line in the direction along which light energy is flowing. A light ray is an abstract idea, not a physical entity or a “thing.” Any narrow beam of light, such as the laser beam in FIGURE 34.1, is actually a bundle of many parallel light rays. You can think of a single light ray as the limiting case of a laser beam whose diameter approaches zero. Laser beams are good approximations of light rays, but any real laser beam is a bundle of many parallel rays.

FIGURE 34.1 A laser beam or beam of sunlight is a bundle of parallel light rays.

The figure illustrates a beam of parallel light rays that point toward the upper right direction. An arrow labeled Direction of travel points along the direction of the light rays.

Chapter 33 briefly introduced the three models of light. Here we expand on the ray model, the subject of this chapter.

MODEL 34.1 Ray model of light

For use when diffraction is not significant.

  • Light rays travel in straight lines.

    • The speed of light is v=c/n, where n is the material’s index of refraction.

    • Light rays cross without interacting.

      A photograph of two intersecting light rays.

  • Light rays travel forever unless they interact with matter.

    • At an interface between two materials, rays can be reflected and/or refracted.

    • Within a material, light rays can be either scattered or absorbed.

      The figure illustrates the reflection, refraction, absorption, and the scattering of light rays.

  • An object is a source of light rays.

    • Rays originate at every point on an object.

    • Rays are sent in all directions.

      A photograph of a Christmas tree with multiple sources of light placed on it. Rays of light emerge from each source and propagates in all the directions.

  • The eye sees by focusing a diverging bundle of light rays.

    • Diverging rays enter the pupil and are focused on the retina.

    • Your brain perceives the object as being at the point from which the rays are diverging.

      The figure illustrates the image of an object inside an eye.

  • Limitations: Use the wave model if diffraction is significant. The ray model is usually valid if openings are larger than about 1 mm, while the wave model is more appropriate if openings are smaller than about 1 mm.

Workbook Exercise 3