#### Main PageLearning Track

##### Introduction to Light What is Light? Light as a wave Light as a particle Properties of Light Speed of Light Polarization Color What is color? Emitted light Reflected light Mixing colors Optics What is optics? Reflection Refraction Mirrors Lenses
Experiments

Questions and Answers

Glossary

References

Related Sites

Send Us Mail!

# Lenses

Lenses are basically panes of glass whose entrance and exit surfaces are not parallel. This causes the direction of the light beam that enters them to change. Lenses can be divided into two basic groups: those that cause light rays to diverge, and those that cause them to converge; if they are thicker in the middle, they are converging, if they are thicker on the sides, they are diverging. All lenses have two focal points; one on each side. In a converging lens, light coming in parallel to the optic axis is focused at a point on the other side called the principal focal point. The distance from this focal point to the center of the lens is called the focal length. To find the focal point on the opposite, just shine the light in the other direction. On a diverging lens, the principal focal point is found on the same side from which the light was shown; that is, the point where the rays on the other side seem to come from. In a lens, the focal point that is not the principal one is referred to as the "other" one.

Finding the image of objects through a lens is very similar to that of the finding the image involving mirrors. Once again, three rays are drawn, and where they meet (or their extensions meet), the image is formed. First, draw a ray from your point on the object through the center of the lens, and just continue it. Secondly, draw a ray coming from the point going parallel to the optic axis toward the lens; it should go through (or from, in some instances) the principal focal point. Finally, draw a ray through (sometimes, towards) the other focal point that is parallel to the optic axis.

On a converging lens, if the object is outside of the focal point, the image that appears will be real. If it is inside one of the focal points, the image that appears will be virtual, and acutally behind the object. On a diverging lens, images created are always virtual.

[Previous Page: Mirrors]

This ThinkQuest Project last updated 7/31/97 by

Stan Seibert, Brett Bennett, and Jur Jang