http://www.ck12.org Chapter 20. Geometric Optics
20.1 Light as a Ray and the Law of Reflection
Objectives
The student will:
- Explain the ray model of light.
- Describe the Law of Reflection.
- Explain how images are formed from flat mirrors.
Vocabulary
- angle of incidence:
- angle of reflection:
- incident ray:Makes an angleθiwith the normal,N.
- light ray:Light traveling in straight lines.
- normal:
- reflected ray:Makes an angleθrwith the normal,N.
- virtual image:An illusion created by the brain because light travels in straight lines.
Introduction
Among other things, the sun provides a source of light for us, as does the moon,Figure20.2. The sun sends light
that is generated by thermonuclear fusion in its core. The moon shines only by the light it reflects from the sun.
Nearly all the objects we see in daily life are reflecting light that shines on them, rather than producing their own
light. This concept covers the case of true reflection.
Light as a Ray
Today, we know that light is a transverse wave that passes by the periodic motion of electric charge. The vibrations
are so fast and the wavelengths so small, however, that to our eyes, light functions like a ray traveling in a straight
line. Sunlight shining through clouds, for example, often gives this impression, as in the Figure20.3. Indeed, our
entire visual orientation is predicated on the notion that light travels in straight lines. When light does not travel in a
straight line, this produces an image to our eyes in a different place than where the true object is.
The study of geometrical optics depends upon light traveling in straight lines, calledlight rays.