Teaching Notes

You will probably have to help some students correctly orient the convex lens and screen to see a real image. You can refer the students to the drawing in the Student Book on page 301. Encourage them to make an image of an object like a tree, so they can observe that the image is upside-down.

Keep the groups well separated.

SAFETY PRECAUTION: Students are not to look at a bright light through the lens. Of course, students should not look at the sun under any circumstances. If the students make images of 120-V bulbs, caution them not to remove the bulbs from the sockets. And if the sockets have screw-terminals underneath, fasten the sockets to a piece of wood so these terminals are inaccessible.

Students maintain a remarkable variety of ideas about light. They usually can understand a light bulb as a source of light radiating out in all directions. They have much more difficulty with the concept that an illuminated object is a source of reflected light radiating out in many directions. Consequently, it is also difficult for students to visualize a lens intercepting certain of this light and bringing it to a focus. A useful exercise is to have a student volunteer walk around the room and view an object. Then trace with the class the path of the light that entered the student’s eye. Ask how the light must be reflected if the student can see the object from many different places. You could even tie many long pieces of yarn to an object and then tie the other ends around the room to represent reflected light rays.

Sample Data

Di (cm)	Do (cm)
13	2500
14	142
18	91
20	50
56	25
549	15

The above data was taken with an inexpensive plastic magnifier. Note that the first pair of numbers (object distance = 25 m) defines the focal length. As the object distance approaches this focal length, the image moves far away from the lens. Notice that at each object distance, the sum (1/d_i + 1/d_o) is approximately equal.