# Hands On Physics

### Overview

The haze unit uses a number of different light sources and detectors. In this extension, you explore the properties of various inexpensive solid state light detectors, to gain a sense of how these detectors to respond to different colors by using sources that generate different colors of light.

### Questions to think about:

• Which detectors seem most sensitive overall?
• Do some detectors show selective color response (that is, do they detect better at some colors than at others)?
• Which detectors are linear (that is, they give double the response for double the light)?

### Materials

There are many combinations of light sources and detectors you could explore in this activity. Your instructor will probably assign you one detector or source and then set up a way of sharing so that your classmates can test out the many different combinations and share results. If you are working alone or in a small group. you should work out how to test all the different combinations.

The following light detectors can be used:
• photoresistor
• photovoltaic
• photodarlington
• photodiode
• LEDs of different colors.
(LEDs are usually used as light sources, not detectors, but, as we will see, they can detect light, too.)

For light sources use:
• red, green, blue, and infared (IR) LEDs,
• an incadesent lamp.
The lamp can be a flashlight, a flashlight bulb (with socket), or a light powered from a wall socket.
You will need a volt-ohmmeter for measuring the electrical response of the detectors. You will need a battery. If you use a battery for the incadescent lamp, be sure to match the voltage requirements of the bulb and battery. For instance, if the lamp needs 3 V, use a pair of 1.5 V batteries (A, C, or D). The simplest strategy might be to use a 9 V lamp and battery. You will need a 100 ohm and 330 ohm resistor. A breadboard would be helpful, too.

Connections for the light sources.
If your incadescent light need a battery, you can connect it directly to the batttery for which it is rated. The LEDs, on the other hand, should NOT be connected directly to a battery. If you try this, you will fry the LED. You need a resistor in series to limit the current. Try a 330 ohm resistor first. If the LED is too dim, then use a 100 ohm resistor.

Figure M5
LED Light Source

A LED will only pass current in one direction. If you cannot light your LED, dry reversing its leads. Most LEDs have one lead longer than the other. The shorter lead should be connected to the negative terminal of the battery.

Figure M6
LED
When your LED is operating, there is a voltage drop across it. Measure this voltage with your volt-ohmmeter. Record the color of the LED and the voltage drop for several LEDs. Do you see a pattern?
Connections for the Detectors.
A photoresistor changes resistance depending on the light falling on it. To use it, simply attach its leads to a volt ohmmeter and set the meter to measure ohms. A photovoltaic generates voltage in response to light. To use it, simply attach its leads to a volt-ohmmeter and set the meter to measure voltage. The photodarlington, photodiode, and LEDs all act like gates or valves that allow current to pass through.. The amount of current in one direction is related the light level. For these, you will need a battery to supply the current and a way of measuring the resulting current. The correct way of connecting each device is shown below.

Figure M6
Diode and Photodiode

(The photodarlington, not shown, is quite like the phototransistor.) If your meter can measure current, you are in luck. Otherwise create a current meter by using your volt-ohmmeter to measure the voltage across a resistor the current goes through. Ohm's law in the form I = V/R will tell you the current I through the resistor R when you measure a volatage V across it. For instance, if you use a 1 Mohm resistor, a reading of .1 V across it tells you that .1 microamp current is going through it.

Figure M7
Measuring Current

Once you have a detector, try it out on the various light sources. Be sure the detector "sees" only the light source and is not illuminated by room lighting or another source. You might do this by making your observations in a light-tight box, or by turning off room lights. Measure the response of all detectors at the same distance from each source. Then double the distance. This should result in one-quarter the amount of light falling on the detector. Does it give one-quarter the response? Look for trends. See what answers you can generate for the questions that began this section.

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