Relationship Between Light Intensity of an Object and Distance From the Source of Light

The Idea

We all know that stars are intense sources of light, but they appear as faint objects in the sky because they are so far away. How does that work? If you increase the distance between yourself and a light source, does the light become one-half of its original brightness or does it drop off some other way? Check it out and see for yourself.

What You Need

• A light-sensing circuit assembled from
  • solar cell
  • insulated wire (you need two 15-inch lengths with the insulation removed)
  • wire stripper (a pen knife or a pair of scissors will do)
  • ammeter (if you have a multimeter, configure it as an ammeter)
  • soldering iron with (resin core Pb/Sn) solder
  • a third hand (or a second person to help solder)
• or a commercially available light meter, such as shown in Figure 81-1

• light bulb (not a focused source, such as a laser or flashlight)
• tape measure
• dark room

Method

Attaching wires to a solar cell to use as a sensor

1. Plug in the soldering iron. Make sure the tip is in a safe place. It gets very hot in a few minutes and should not be in contact with anybody or anything that is flammable.
2. Place the solar cell with the blue side up and locate a pad near the edge intended for wire attachment.
3. As the soldering iron gets hot, "wet" the tip by melting some of the solder on the soldering iron tip.
4. Strip about ¼ inch of the insulation from the end of each of two 15-inch lengths of wire
.
5. Position the wire on one of the contact pads on the edge of the solar cell.
6. Position the end of solder from the coil on the pad and near the wire.
7. Touch the soldering iron to the wire to get it as hot as possible. This may happen a bit faster if it is at first raised above the solar cell to avoid heat being conducted by the solar cell. You also want to avoid overheating the solar cell, which could cause it to short out if the heat is excessive. And, you should avoid having any electrical contact—whether it is the wire touching or solder—between the front pad and the back of the solar cell. This can short out the solar cell and prevent it from generating an electrical output.
8. Touch the solder to the heated wire and, as it melts, have some of the molten solder form a bridge to the solder pad.
9. Remove the soldering iron and don't move until the solder solidifies. If done properly, the solder should stick to both the solar cell pad and the wire forming a mechanical bond.
10. Similarly attach a wire anywhere to the back of the solar cell.
11. Attach the wire coming from the back of the solar cell to the positive terminal of the ammeter. Attach the wire coming from the front of the solar cell to the negative terminal of the ammeter.
12. At this point, you can (carefully) hold the solar cell or mount it by gluing or taping it to a board. Remember, solar cells are extremely fragile and will break if the slightest pressure is put on them. The solar cell may still function if fractured, but reasonable caution can avoid that.

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