Abstract

Objective

Measure the speed of rotation of a radiometer and test how the speed varies as the light intensity varies.

Introduction

Have you ever seen a radiometer before? A radiometer is made up of a glass bulb that contains a set of vanes on a freely rotating spindle. The vanes are painted black on one side and white or silver on the other side. The spindle and vanes rotate when exposed to light. As the intensity of the light increases, the speed of rotation also increases. In bright sunlight, the vanes can spin at several thousand revolutions per minute (RPMs). There is a difference in temperature between the black and white/silver sides of the vanes, since the black sides absorb more light energy. This temperature difference causes air currents that move the vanes forward, with the black sides of the vanes trailing the white/silver sides. Because a temperature difference is used to create motion, the radiometer is a form of heat engine. Figure 1 shows a radiometer with a slowly rotating spindle.

Figure 1. Light causes the vanes of the radiometer to move. (Wikipedia, 2009.)

The radiometer was invented by Sir William Crookes in 1873. Crookes' radiometer is the result of serendipity, the act of making unexpected discoveries when looking for something else. In this case, Crookes was looking for a means to weigh very small amounts of chemicals as part of his research (the research eventually resulted in his discovery of the element thallium). To prevent drafts from affecting his scales, he enclosed them in glass and pumped out the air, creating a partial vacuum. Unexpectedly, the apparatus moved when it was struck by light. He decided to pursue the study of this effect, which he called repulsion resulting from radiation—radiation in this sense being light rays.

Crookes' original explanation for the motion of the radiometer was that the light was exerting more pressure on the dark side of each vane than on the light side. But this radiation pressure explanation was ruled out by the discovery that if more of the remaining air molecules were removed from the bulb, creating a better vacuum, the motion ceased. If radiation pressure were the correct explanation, the vanes should have spun faster in a better vacuum, since there would be less resistance from the air molecules. Light can produce a force that will push against an object, but the effect is much too small to make the radiometer move.

James Clerk Maxwell and Albert Einstein, two of the most brilliant scientists of modern times, both studied the radiometer. Their work, along with contributions from the scientist Osborne Reynolds, resulted in a description of how the flow of air molecules near the vanes results in a force that propels the vanes forward. The flow of gas molecules depends on the temperature difference between the two sides of the vanes. The temperature difference is created as the dark side absorbs more light energy than the light side. See the links in the Bibliography for more details.

The radiometer works only at certain air pressures. At normal pressure, about 1 atmosphere (atm), the air currents are not strong enough to push the vanes against the resistance of the surrounding air. At very low pressure, there are not enough air molecules to form the currents needed to move the vanes forward.

The challenge for this science fair project is to determine how fast the radiometer is spinning at a given level of light. To put it more precisely, you want to determine how the number of revolutions per minute (RPMs) vary with the illuminance (illuminance is a measure of the intensity of the incident light). You will use a digital light meter to measure the amount of light near the radiometer. The light meter gives readings of illuminance in a unit called a lux (the plural is also lux). The table below lists illuminance values for some common situations.

The lux, which is the SI-derived (International System of Units) unit of illuminance or illumination, is equal to one lumen per square meter. Lumens measure quantity of light emitted by the light source, whereas lux will tell you how many lumens you need, given the area you are trying to illuminate. The difference between the lux and the lumen is that the lux takes into account the area over which the light is spread. 100 lumens, concentrated into an area of 1 square meter (m²), lights up that square meter with an illuminance of 100 lux. The same 100 lumens, spread out over 10m², produces a dimmer illuminance of only 10 lux.

The rate of rotation (RPMs) will be measured with a digital tachometer. This device counts the number of times per minute that light striking it flickers, or changes brightness. The rate of rotation can be read by using the tachometer to measure the flicker in the radiometer's shadow.

Terms, Concepts, and Questions to Start Background Research

• Radiometer
• Vane
• Spindle
• Light intensity
• Revolutions per minute (RPMs)
• Heat engine
• Sir William Crookes
• Partial vacuum
• Radiation pressure
• James Clerk Maxwell
• Albert Einstein
• Osborne Reynolds
• Atmosphere (atm)
• Illuminance
• Digital light meter
• Lux
• Lumen
• Digital tachometer

Questions

• Who invented the radiometer, and when was it invented?
• Why is the inside of a radiometer a partial vacuum?
• Which sides of the vanes (black or white/silver) follow, and which sides lead as the vanes spin when exposed to light?
• The vanes will turn slowly if you warm the glass in your hands, even in low light. Why?
• Which way will the vanes spin if you put the radiometer in a freezer or hold an ice pack against the glass?

Bibliography

• HowStuffWorks.com. (2000). How does a Crooke's radiometer work? Retrieved November 11, 2009, from, http://science.howstuffworks.com/question239.htm
• Gibbs, P. (1996, July). How does a light-mill work? Retrieved November 12, 2009, from http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html
• Wikipedia Contributors. (2009, October 5). Crookes Radiometer. Wikipedia: The Free Encyclopedia. Retrieved November 12, 2009, from http://en.wikipedia.org/w/index.php?title=Crookes_radiometer&oldid=317999172
• Wikipedia Contributors. (2009, October 27). Lux. Wikipedia: The Free Encyclopedia. Retrieved November 12, 2009, from http://en.wikipedia.org/w/index.php?title=Lux&oldid=322436757
• Nave, R. (n.d.). Lux. Retrieved November 12, 2009, from http://hyperphysics.phy-astr.gsu.edu/hbase/vision/Areance.html#c1

Materials and Equipment

• Radiometer, Crookes'; available from www.amazon.com
• Books or small box to raise level of radiometer, if needed
• Tape measure, metric
• Rechargeable spotlight; available at most department and hardware stores
• Digital tachometer; available from online suppliers such as Tower Hobbies at www.towerhobbies.com, model # L5PT3102
• Note: Do not use a laser tachometer. The beam is bent by the glass housing of the radiometer.
• Lab notebook
• Light meter, such as the Mastech Professional Luxmeter, LX1010B, available from www.amazon.com
• Graph paper

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Experimental Procedure

Note: It's important to keep in mind that the overall goal is to have the spotlight shine onto the radiometer and have the shadow of the radiometer visible on a vertical surface, such as a wall. The rate at which the radiometer is spinning will be measured by analyzing the shadow of the radiometer with the tachometer. The tachometer senses how quickly the shadow is flickering, which is a measure of the rate of rotation.

Setting Up the Spotlight and Radiometer

1. Read the directions for the tachometer, the light meter, and the rechargeable spotlight.
1. The spotlight may need to be charged overnight.
2. Place the radiometer on a flat surface near to a wall (about 10 centimeters [cm] away from the wall).
1. The shadow of the radiometer should be projected onto the wall.
2. Any vertical surface will work, as long as it is large enough to see the shadow of the radiometer.
3. Keep the radiometer out of direct sunlight. The level of ambient light should be low enough that the radiometer is not spinning.
4. Place the radiometer on some books or a box to raise it into the beam of the spotlight, if necessary.
3. Place the spotlight 25 cm from the radiometer, so that the shadow of the radiometer falls on the wall or other vertical surface.

Figure 2. An example of an experimental setup for measuring the rate of rotation of the vanes of a radiometer at various levels of light.

4. Turn on the spotlight.
5. The light from the spotlight should be parallel to the ground and pointed directly at the radiometer.
6. The radiometer should start spinning.
1. Turn the spotlight off when not in use to preserve the batteries.

Using the Tachometer to Measure the Rate of Rotation

1. Point the tachometer at the shadow of the radiometer and turn it on.
2. Set the tachometer to "2 blades."
1. Later, you will have to divide the readings by two, since the radiometer has four vanes and the tachometer is set to read RPMs for a two-blade propeller.
2. If your tachometer has a four-blade setting, use that setting to get the correct RPM value.
3. Point the tachometer at the edge of the shadow of the vanes, rather than at the middle.
1. In the middle of the shadow, the tachometer will read 2X RPMs, since each blade will produce a shadow twice, once while near the light and once while near the wall.
4. Record the value from the tachometer output in your lab notebook.
5. It may require some practice to get an accurate reading.
6. Take at least three readings from the edge of the shadow.

Measuring the Light Level

1. Use the light meter to measure the level of light at the radiometer.
2. Hold the probe of the light meter next to the radiometer.
3. Record the light level in your lab notebook, next to the associated number for the RPMs from the tachometer.
4. Repeat the light level readings two more times. Record your results.
5. Now move the spotlight so that it is 50 cm away from the radiometer.
6. Repeat the entire procedure above, taking three readings for the rate of rotation and three readings for the light level.
7. Record your results in your lab notebook.
8. Repeat steps 5–7 for the following distances:
1. 1 m
2. 2 m
3. 4 m

Analyzing and Graphing Your Results

1. Calculate the actual RPMs of the radiometer by dividing the readings from the tachometer by 2 (if the tachometer was set to read for "2 blades").
2. Calculate the average rate of rotation and average light level for each distance.
3. Graph the rate of rotation on the y-axis and the level of light on the x-axis.

Variations

• Devise a procedure to test the how the color of light affects its ability to power a radiometer.
• Measure the rate of flicker in the middle of the shadow. Is it twice the rate on the edge?
• Does reflected light from the vertical surface affect the speed?
• Change the angle at which the light strikes the vanes. For example, what if the light shines directly down on the vanes? Graph RPM vs. angle, keeping the illuminance (lux) constant.
• The radiometer is powered by the temperature difference between the dark and light sides of the vanes. Devise a way to determine the temperature difference for various levels of illuminance. Hint: An infrared thermometer might come in handy!
• How does the rate of rotation depend on the level of vacuum? You will need a supervised physics lab to perform these experiments.

• For more science project ideas in this area of science, see Physics Project Ideas.

Credits

David B. Whyte, PhD, Science Buddies

Last edit date: 2011-01-31 12:00:00