# Come One, Come All! Explore the Effect of Light on the Speed of the Amazing Rotating Radiometer!

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## Abstract

Radiometers are fun-to-watch novelty items, but they also have a distinguished scientific history, having been studied by James Clerk Maxwell and Albert Einstein. A radiometer has a set of four vanes (like small sails) connected to a spindle that is free to rotate. When the radiometer is placed in bright light, the vanes and spindle start to spin. It looks like a magic trick, but there is a scientific explanation for this weird behavior. In this science fair project, you will experiment with this simple, but fascinating, apparatus and determine how the speed of rotation of the radiometer's vanes varies with the amount of light striking them.

## Summary

Areas of Science
Difficulty

Time Required
Average (6-10 days)
Prerequisites
This science fair project will require some creative problem solving on your part.
Material Availability
You will need to order a digital tachometer and radiometer online. See the Materials and Equipment list for details.
Cost
High (\$100 - \$150)
Safety
No issues
Credits

David B. Whyte, PhD, Science Buddies

edited by Ben Finio, PhD, Science Buddies

## 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.

 Illuminance Example 0.27 lux Full moon on a clear night 50 lux Family living room 320-500 lux Office lighting 400 lux Sunrise or sunset on a clear day 1,000 lux Outside on an overcast day 10,000-25,000 lux Indirect light on a sunny day 32,000-130,000 lux Direct sunlight

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 and Concepts

• Vane
• Spindle
• Light intensity
• Revolutions per minute (RPMs)
• Heat engine
• Sir William Crookes
• Partial vacuum
• 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?

## Materials and Equipment

To do this experiment, you will need a light source to make the radiometer spin, and a way to measure its rotation rate. There are two different ways to measure the rotation ratet: a digital tachometer, or a phone or camera with a "slow-motion" video mode. The type of light source you can use for the experiment will depend on the measurement technique you choose.

• In general, the radiometer will work best with incandescent bulbs. This is because incandescent bulbs emit more "heat" in the form of infrared radiation, so they make the radiometer's vanes spin faster. However, incandescent bulbs are being phased out in favor of more efficient bulb types like compact flourescents (CFLs) and light-emitting diodes (LEDs), so they may be difficult to find. CFL and LED bulbs will still make the radiometer spin, but it will not spin as fast, and you will have to place it closer to the bulbs. Higher-wattage bulbs will make the radiometer spin faster than lower-wattage bulbs.
• Digital tachometer, available from Amazon.com
• This tachometer works by measuring the flickering shadow of the radiometer's vanes as they rotate. Because of this, it will work best with a light source that casts a sharper shadow. Regular light bulbs may cast a soft, blurry shadow that is difficult to measure with the tachometer; as opposed to spotlights and flashlights which cast a more directional beam and thus a sharper shadow. If you want to use regular light bulbs, try doing the experiment in a dark room with no other light sources in order to get a sharper shadow.
• This tachometer will not work with compact flourescent (CFL) bulbs. CFL bulbs "flicker" 60 times per second and this will affect the tachometer reading. Certain LED bulbs (but not all of them) also flicker, so may not work with the tachometer.
• Do not use a laser tachometer. Laser tachometers send out a laser beam that must be bounced back off a reflective surface, and will not work for this project.
• In general, the tachometer has the advantages of being able to measure very high rotation rates, and giving an instantaneous reading when you do a test. Its main disadvantage is that it will not work with certain types of lights, as explained in the previous points.
• Cell phone or digital camera with "slow-motion" video mode
• Many modern smartphones and digital cameras have slow-motion video capture modes that can record video at 120, 240, or even more frames per second. You can use this video mode to record vide of the radiometer's vanes spinning, and then count the rotations when you play the video back.
• Counting the rotations may be easier in a media player that lets you step through the video one frame at a time. Tracker is a free video analysis tool for physics projects that will allow you to do this.
• In general, the advantage of using a smartphone or digital camera is that they will work with more types of lights than the tachometer. However, the disadvantage is that the maximum rotation rate you can measure will be limited by the framerate of the camera. It also takes longer to do each measurement since you have to record a video and then play it back to count the rotations manually.

Regardless of the light source and measurement tool you choose, you will also need the following materials:

• If needed, books or small box to raise level of radiometer so it lines up with the light source
• Tape measure, metric
• Light meter, such as the Mastech Professional Luxmeter, LX1010B, available from www.amazon.com
• Note: light meters may give different readings for incandescent, LED, and CFL lights, even if those lights have the same brightness rating in lumens. This is due to the different wavelengths of light emitted by different types of bulbs, and the sensitivity of the light meter to different wavelengths of light. To keep your readings consistent, make sure you use the same light source for your entire project.
• Lab notebook
• Graph paper

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## Experimental Procedure

Note: as described in the materials section, there are several different ways to do this experiment. Figure 2 shows a setup where a rechargeable spotlight casts a shadow of the radiometer's vanes, and measurements are made with a tachometer. You will need to adjust the procedure accordingly if you are using a different type of light or a camera instead of a tachometer.

### Setting Up the Spotlight and Radiometer

1. Read the directions for the tachometer (or the slow-motion video mode on your camera) and the light meter.
2. If you will be using the tachometer, 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. If the shadow is blurry, you might need to do the experiment at night or in a darker room.
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.

1. Turn on the spotlight.
2. The light from the spotlight should be parallel to the ground and pointed directly at the radiometer.
3. The radiometer should start spinning. Depending on the type and brightness of your light, you may need to adjust the distance to get the radiometer to spin. If the radiometer does not spin at all, try moving the light closer.

### 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.

### Using a Smartphone or Digital Camera to Measure the Rate of Rotation

1. Use your smartphone or digital camera to record a slow-motion video of the tachometer's vanes as they rotate. Make sure you write down the framerate of the video. Some cameras may allow you to select the framerate. Some smartphones may have a fixed framerate for slow-motion mode, like 120 or 240 frames per second (fps). You may need to look this number up in the manual for your specific model of phone, or search for it online.
2. Upload the video to a computer, play it back and count the number of rotations during a certain period of time to calculate the rotation rate.
3. Important: make sure you properly account for the ratio between the recording framerate and the playback framerate. Most video players play videos at 30 frames per second. So, for example, if you record a two-second video at 120 frames per second, it will take eight seconds to play when you play it back on your computer at 30 fps. Even though the video takes eight seconds to play, since it is slowed down, it only shows two seconds of "real world" time. If you count 40 rotations during the entire video, then the rotation rate is 80/2 = 40 rotations/second, not 80/8 = 10 rotations/second.

### 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.
8. Repeat steps 5–7 for the following distances (remember that you may have to adjust these distances depending on the type and brightness of your light; for example, for an LED flashlight, the radiometer might not spin at all at distances of several meters, so you will need to test shorter 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.

Do you have specific questions about your science project? Our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

## 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.

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General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

#### MLA Style

Science Buddies Staff. "Come One, Come All! Explore the Effect of Light on the Speed of the Amazing Rotating Radiometer!" Science Buddies, 28 Jan. 2022, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Phys_p078/physics/radiometer. Accessed 7 Feb. 2023.

#### APA Style

Science Buddies Staff. (2022, January 28). Come One, Come All! Explore the Effect of Light on the Speed of the Amazing Rotating Radiometer! Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Phys_p078/physics/radiometer

Last edit date: 2022-01-28
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