Using lenses constructed from broken pieces of clear Christmas ornaments we determine focal points and focal lengths of two concave and two convex lenses.

You will receive two small plastic concave lenses.

Using a permanent marker or some other means, place two small dots on each lens, with one dot about 2 mm above the center and the other 2 mm below the center (maybe 3 mm for the larger lenses).  You will end up with a top and a bottom dot, separated by about 4-6 mm.

Support the lens by the handle (e.g. use two blocks or books to sandwich the handle) in such a way that it lies in a vertical plane, perpendicular to your table top.

We begin by determining the focal length of each of the convex lenses.

• First estimate the focal length of each lens by placing the screen 'behind' the lens and moving the laser back and forth in front of the lens, as you did with the circular lens in the preceding experiment.

• The focal distance is the screen distance at which, if you manage to keep the direction of the pointer consistent, the dot on the screen remains stationary.
• Measure the distance of this point behind the lens.

• Now determine the focal distance using the two dots on each lens.

• Place the screen at about twice the distance behind the lens as your estimated focal point; measure and note this distance.
• Keeping the laser pointer in the horizontal direction, direct the beam through the top dot on the lens and onto the ruler on the screen; note as accurately as possible the vertical position of the beam on the ruler.
• Repeat for the bottom dot, again being sure to keep the pointer horizontal so the incoming beams for both measurement are parallal.
• Measure the vertical distance between the dots, then using this distance and other observed distances construct an accurate picture of the paths of the beams.
• From your picture determine the distance from the lens at which the beams cross.
• This distance should be the focal distance.

• To check the focal distance, place the screen at this focal distance.

• Determine whether horizontal rays through the two dots on the lens strike the screen at the same vertical position.
• If this is the case, as it should be, then you have determined the focal distance.

• Now place a burning candle as far away as possible from the lens.  The candle flame should be at the same height as the lens, which might require that you elevate the lens slightly.  The room should be dark.  Move the screen behind the lens until a sharp image of the candle flame appears on the screen.  As long as the candle is 2 meters or more from the screen, the distance between the lens and the image will be within experimental error of the focal distance.  Compare this distance to the focal distance you determined earlier.

• You can also determine the focal distance on a sunny day by forming a concentrated image of the sun on your screen.  This part is optional and you don't need to report it but it's worth a couple of minutes, so some time when the sun is out do it.

• The image of the sun will shrink to a very small and sharply defined disc at the focal point.
• Do not attempt to look at the sun through this lens. If you do, and if your eye is anywhere near the focal point, you will burn out parts of your retina and see dark blotches for the rest of your life.
• Don't leave the point image of the sun on its target for long or you risk setting it on fire.
• Resist the temptation to start fires or to focus the sun on small defenseless insects.

• Look through the lens at your finger.

• Move your finger close to the lens, then further from the lens. Note and describe what you see.
• What does the image of your finger look like when your finger is at the focal distance?
• What happens to the image of your finger if you move back and forth across the focal point?