# Hands On Physics

## Haze Messing Around A LIGHT DETECTOR

### Overview

The Visible Haze Sensor requires a light detector to measure the intensity of the sunlight which gets through the haze. There are many combinations of light sources and detectors you could explore. This suggests the possibility of an extension to study detectors in detail. Here you will consider only one; a photoresistor. In this activity, you will gain a sense it works, and its light detecting capability.You should use LED's of different colors, and an incandescent (flashligh) bulb to investigate the properties of this detector.

• What does a light detector do?
• Does this detector show selective color response (that is, do they detect better at some colors than at others)?
• Is this detector linear (that is, does it give double the response for double the light)?

### Materials

• Photoresistor
• LED's of sevral colors
• Incandescent bulb
• Multimeter
• Battery
• Resistors, 330 ohm and 100 ohm.

You will need the volt-ohmmeter for measuring the electrical response of the detector. 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 needs 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, try 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 Detector.
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.

#### Testing the Detector

Try out your detector 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 the detector at the same distance from each source. Then double the distance. This should (if the detector has a linear response) result in one-quarter the amount of light falling on the detector. Does it give one-quarter the response? Pool the observations from the entire class and look for trends. See what answers you can generate for the questions that began this section.

### Reporting

Keep a journal as scientists do, even when messing around. Record all your questions and observations in a bound notebook. Don't erase, just cross out errors. Write enough so someone else could reproduce your experiments. Diagrams can save lots of words and help make your ideas clear. If you make measurements, record your data, with units. Tables are useful for repeated similar measurements. To make the logic of any calcuations clear, first record the formula you used, then show the numbers used to evaluate the formula, and finally your answer.

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