How a convex lens refracts light

 

 

We just observed how a convex lens bundles the light rays from the sun into a single bright spot. This means that a convex lens refracts light in a very special way, as this graphic will illustrate. It shows a lens from the side, and light rays coming in from the left. Rays near the middle of the lens are not changing direction very much. But rays that hit the lens closer to its rim are bent much more, such that all rays meet in one point, the focal point of the lens. The distance between the lens and the focal point is called the focal length of the lens. As we have shown in our experiment in the sun, a thicker lens has a shorter focal length than a thinner lens.

Footnote: To understand in more depth why light rays that hit the lens closer to its rim are bent more than rays hitting in the middle, consider the angle of incidence near the rim, vs. in the middle. You might want to go back to Snell's Law on page 4 to remind yourself how the angle of incidence is measured: relative to the normal of the glass surface. So, closer to the rim, the angle of incidence is greater than in the middle of the lens. The same goes for the angle of refraction. And that's why light rays are bent more when hitting the lens near its rim!

 

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