Scan a Laser Through a Lens

What path does yonder laser take?


A scanned dot of laser light can show the path that light takes when it bends through a lens.



Cut the page magnifier lens in half along its longest dimension.
Mount the half lens with the binder clips so that it stands vertically over a white table surface. The cut side should touch the table.

To Do and Notice

Hold the laser in your hand and point the beam at the table.
(Optional, you can mount the laser on a metal stand with magnets and rotate it more precisely, see Magnetic Optical bench.)
Move your wrist to make a point of laser light move along the table surface.

Hold the laser above and in front of the lens and rotate the laser so that the point of light moves along the table and hits the lens in the center.
Notice how the dot of light moves toward the lens then continues through it in a straight line.
Move your wrist faster and slower to show the path of a dot of light or a line of light.

Note If the beam does not hit the table on the far side of the lens, turn the lens over. Place the top edge against the table.

Move your hand so that the dot of light moves parallel to the axis of the lens into the lens near the edge. Notice that the dot bends toward the axis when it moves through the lens.

Scan the beam through the other side of the page magnifier, note that it also bends toward the axis.

Find the point where beams through the center and two edges of the lens come together. This is the focal point of the lens.

Scan the beam through the lens half-way between the center of the lens and the edge.

Notice that the beam bends half as much as it does at the edge. The beam also passes through the focal point.

Scan the beam through the center of the lens at an angle to the axis. Notice that it does not bend regardless of the angle.

Scan the beam through the edge of the lens at an angle to the axis notice that the beam bends toward the center by the same angle reagrdless of the angle of incidence. (almost, at large angles of incidence, the fresnel lens does not work well.)

What’s Going On?

When light refracts through a lens it passes through the center of the lens without bending.

Every other point on the lens bends the light beam by some angle the angle is greater the farther the beam is from the center of the lens.

The angle the beam is bent is the same for any one point on the lens regardless of the angle of incidence.

At each point on the lens the angle the beam will be bent will send it through the focal point.


For a plano-convex lens there is only one shape that will bend the beam in exactly the right way to bring light rays parallel to the axis bend so that they come to a focus. The name of the curve is....

The curve with no name. (Sometimes it is called aspherical, not a sphere.)

Draft Activities

Scientific Explorations with Paul Doherty

© 1999

21 Feb 99