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Converging Lenses - Rules of Refraction
Light refracts at any boundary - including those that bound a lens material - according to Snell's law. For converging lenses, some generalizations can be made to simplify ray construction. They are:
- An incident ray traveling parallel to the principal axis will refract and pass through the focal point on the opposite side of the lens.
- An incident ray traveling through the focal point on the way to the lens will refract and travel parallel to the principal axis.
- An incident ray traveling through the exact center of the lens will refract upon entering the lens and upon leaving the lens and continue traveling in its original direction.
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This diagram shows a ray of light that is heading toward the focal point on the opposite side of a converging lens. Many student incorrectly pick the answer "refract and pass parallel to the principal axis." If the lens were diverging, this is exactly how it would behave. But don't be fooled! This is a converging lens and there is no simple rule predicting how such a ray will behave for a converging lens. Review your rules of refraction in the Know the Law section.
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Certain rays of light refract through a converging lens in a rather predictable manner. The behavior of these rays - known as principal rays - are of vital importance. You should quickly become familiar with the rules for how these rays refract. Give attention to details as you ponder the rules. See Know the Law section.
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