Here are some things to think about and do as we start the unit on lenses. These questions can be answered by simple experiments that will only take a few minutes. Do at least 4 of them, and for the rest discuss with your group how you might do the experiment.  For each question you should discuss what you predict will be observed (if it is a situation with which you are already familiar).

There are several lenses in your kit: there are two round lenses mounted together on a handle, and there are several flat plastic rectangles which also behave as lenses. Also there are two kinds of containers (glass vial and a plastic bottle with flat sides) that have optical behavior when they are filled with water.
Warning: most of these lenses are capable of starting a fire in sunlight. We do not recommend this experiment. And of course, Don't look at the sun!

1. Fill a loaf pan or deep casserole almost full of water. Observe the appearance of a pencil stuck into the water at various angles, and from different view points.   When does it appear straight, and when does it seem to have a bend at the surface?   How does the amount of bend depend on the direction from which you view the pencil?
   Hold the pencil vertical, half in the water, and then view it from almost above, and from the side, close to the surface of the water. Does the pencil seem longer or shorter than it really is, and which view makes this effect the largest?
   The pencil is not really bending and changing length, of course; instead, light is changing direction at the surface.   Make a sketch showing a straight vertical pencil and a beam of light starting from the pencil tip and changing direction at the surface of the water, that accounts for the incorrect perception of its length.
2. Draw three straight lines that cross in the center of a piece of white paper, at an angle of about 30 degrees. Place the flat-sided plastic bottle, filled with water, on the point where the three lines meet.  Sight down each line, and compare what you see through the bottle to what you see looking over the bottle.  Can you place a ruler behind the bottle so that when you sight down a line, the rule appears to be the continuation of the line in front of the bottle?   Draw a line along the ruler up to the back side of the bottle, and then (after removing the bottle)  join it to the original line -- this will show the path taken by light inside the bottle.
3. There are two ways to use a lens as a magnifier. In either case you hold it close to the object you are studying, at the right distance (which is determined by the lens), but you can choose whether to leave the object where it is, an arm's length away, or move yourself so that your eye is right at the lens. The two methods are about equally good at making small things visible, but there are some advantages to each (for example, the arm's length approach is recommended if the object being studied is a rattlesnake). Try both, and see for yourself. Also fun is to look at the computer screen with the strongest magnifying lens, to see the little dots of light that are used to construct the picture you see.
4. Look for other lenses, or things that accidentally act like lenses. For example, little drops of water on a surface will sometimes act like lenses (a tiny drop of water on the computer screen will show this effect); thermometers have a built-in lens to help you see the thin tube of red liquid. How are these like the lenses you have studied?
5. Fill a vial with water.  You will find that it also behaves a little like a lens, when lying on its side or held a little above an object.   The glass vial has a different shape from that of the other lenses - rounded in one direction, but straight in another direction.  Because of this, it only magnifies in one direction!  This property is called astigmatism. Try using the cylinder lens to magnify the computer screen. Turn the axis of the vial various ways, and explain what you see.
6. Lenses and prisms deflect light into new directions.    The amount of deflection depends slightly on the wavelength (color) of the light.   To observe this effect, find a uniformly lit white background (a wall, a piece of paper, a cloud), hold the round plastic lens up to your eye, and inspect a pencil held horizontally at the appropriate distance to get a clear magnified view of it.  Now move the lens so that you are looking through it near the upper edge, and notice what happens to the edges of the pencil; then look near the lower edge and notice how this is similar and how it is different.  These color effects are part of the beauty of a rainbow or a diamond ring but are highly undesirable in a photographic system or a telescope.  This is why good cameras and good telescopes are expensive.

All done?