light bulb microwave experiment

• 1 microwave oven
• 1 incandescent light bulb
• 1 microwave-safe cup

Short explanation

Long explanation.

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Content of website.

Home experiment: Harness light in your microwave

• Published: Jun. 19, 2008, 10:03 a.m.
• Shelby Wood, The Oregonian

Early advertisements touted microwave ovens as "the greatest discovery since fire." Heating food with them is, of course, commonplace, but they can regain their futuristic luster when you transform them into a physics lab for this simple experiment on microwave radiation.

What you need:

2 microwave-safe glasses

1 incandescent light bulb (60 watts works well)

1 microwave oven

Directions: Fill one glass with water. Put the light bulb in the other glass so it doesn't roll around. Put both glasses in the microwave. Turn the microwave on for three seconds (use the low-power setting if it has one).

Watch: If your microwave has a turntable, you will see the light bulb glow and dim as it travels. If your microwave doesn't have a turntable, repeat the experiment several times, moving the light bulb to different spots. Be careful, the light bulb gets hot! And don't run the microwave longer than five seconds at a time.

The science: Light bulbs glow when electrons speed across a thin metal wire called a filament, heating it to several thousand degrees. In this experiment, you harnessed the energy of the microwave radiation generated by your microwave oven.

Dr. John Essick, an associate physics professor at Reed College, hypothesizes that the powerful, changing electric field of the microwaves forces the electrons in the filament to race back and forth, creating light you can see. The light bulb converts microwave radiation into visible light radiation.

As they bounce around, the microwaves quickly settle into a stable pattern with areas of high and low energy.

"It's like plucking a guitar string," Essick says. If you pluck the center of the string, it will hum with the largest vibrations in the middle. "That's an example of a standing wave."

You could also create a standing wave where the middle of the string was stationary, and the areas between the center and the ends were vibrating the most, and so on.

"The same thing happens on the inside of a microwave (oven)," Essick says. "Microwaves are a form of light that our eyes can't see, and they reflect off shiny surfaces just like (visible) light does off a mirror."

The inside of your microwave oven is a shiny surface, which allows these standing waves to develop. The light bulb probes these standing waves, glowing only when it encounters an area of high energy.

These areas of high and low energy explain why microwaved food is sometimes heated unevenly. Turntables help by guiding your food through the hot and cold regions, averaging out the energy your food receives.

Why doesn't the metal spark? The shape of the microwaved metal plays an important role.

"There's a certain pattern of electric and magnetic fields that get set up in the microwave oven," Essick says, "and electric fields concentrate at sharp points of metal." It is only when electric fields are concentrated that they become strong enough to produce a spark. The curvy metal casing on the light bulb doesn't present pointy edges for the electric fields to gather, preventing sparks.

Why the glass of water? Microwave ovens generate lots of energy. Running a microwave without anything to absorb this energy can destroy the cavity magnetron, the expensive gadget that creates the microwave radiation. The glass of water acts as an energy sink, preventing damage.

--Carrie Nugent ; [email protected]

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Science in School

Microwave experiments at school teach article.

Author(s): Halina Stanley

Halina Stanley introduces a number of spectacular classroom experiments using microwaves.

As reported in this issue of Science in School ( Stanley, 2009 ), Israeli scientists have been using microwaves to drill holes into glass and ceramics, and to produce plasma balls. Microwave ovens are a useful resource for teachers as well as scientists. Here is a collection of fun microwave experiments that are suitable for the classroom.

Plasma balls

Using a microwave oven, you can create balls of plasma w1 at school from nanoparticles of soot. Dr Chris Schrempp, who teaches at a Californian high school, has been doing this in class for some time. He says, “This is a great demonstration that is always a sure hit with students of any level. Although the owner of the participating microwave, if present, will be absolutely sure that the appliance will be a total loss after the demonstration, it should remain surprisingly undamaged.”

• A small heatproof glass bowl
• A short wooden splint or toothpick (3-5 cm long)
• 50 ml laboratory beakers (or other similarly sized microwavable objects)
• Remove the turntable from the microwave and cover or remove the light.
• Stick the wooden splint or toothpick into the cork.
• Support a small heatproof glass bowl upside-down in the centre of the microwave using a circle of beakers. The bowl should be raised high enough that the toothpick stuck in the cork can be placed beneath it.
• Pre-program the microwave for 30 seconds at full power and turn off the lights in the room.
• Light the splint and put it into the microwave under the glass bowl.
• Close the door and turn the microwave on.

The plasma usually forms in about 10 seconds. Schrempp says, “It will make a horrific noise, sounding as though the microwave is frying from he inside out.” If a plasma ball does not form in this time, stop the microwave, relight the splint and start again.

Safety note:

 The microwave should only be allowed to run for about 20-30 seconds, otherwise the glassware might overheat and break. Be sure not to let the toothpick burn right down and set fire to the cork.

The inverted glass bowl serves to contain the plasma so that it can be viewed through the window easily. The demo can be performed without the bowl, but the fireball will then rise to the top of the microwave, so you have to bend down and look up into the window to see it.

The only negative effect of the demonstration is a smoky smell in the microwave. Schrempp says he has never had any real damage to the oven, just some sooty marks, but suggests that an older oven be used just in case.

Schrempp’s demonstration of this and lots of other dramatic experiments can be seen on the Exploscience website w2 .

Plasma balls can also be created using grapes, as described in Schrempp’s e-book Bangs, Flashes, and Explosions – An Illustrated Guide of Chemistry Demonstrations w3 :

• Cut a grape almost completely in half along its length, retaining a small piece of the skin on one side to keep the two halves connected.
• Place the grape on a dish, cut side up, and put it into the microwave.
• When the oven is turned on, plasma will be emitted from the section of skin connecting the two halves.

A video of the grape plasma can also be found online w4 .

Soap sculpture

When microwaved on full power for about a minute, a bar of soap grows into a strange volcanic lava, or something that looks like horrible fungus. The deformation is caused by tiny pockets of water in the soap vaporising, or by air in the soap expanding as it heats up.

The soap sculpture may leave the microwave oven (and the classroom) smelling quite strongly, so try to find non-perfumed soap and avoid doing this in a microwave that is used to prepare food. This demonstration has the added benefit that the teacher can leave the microwaved soap lying around the science preparation lab at school to worry colleagues, or the students can take it home to perturb members of their family.

This and other experiments can be found on the physics.org website w5 .

Eggy explosions

If demonstrations are good, explosions are unforgettable. My children will never let me forget the night my son’s boiled egg had a rather runny white and I said, “a few seconds in the microwave will just finish it off nicely”! A hen’s egg, even with the top cut off, will explode dramatically when heated in a microwave. You can try it in a lesson, but only if you’re prepared to clean the inside of the microwave afterwards!

A US TV programme, Brainiac Science Abuse, has taken this experiment to the logical limit by microwaving an ostrich egg. This is probably not an experiment that you will want (or be able) to do yourself, but there are many versions of it on YouTube w6 . I strongly suspect that the experiment was rigged in some way (they call it science abuse ), but you could use the video to wake up any class.

Light bulbs

Another classic demonstration is to put a light bulb in a microwave oven. An incandescent light bulb (whether or not it is still functional) will light up when irradiated with microwaves, provided the glass is intact. Depending on the type of bulb, you can get different colours. Remember that the bulb will heat up very quickly; 10 seconds is probably long enough before allowing it to cool down again.

Fluorescent tubes will also light up, and the effect can be used to test for microwave leakage around the doors of microwave ovens. Switch on the microwave and hold a fluorescent tube against the edges of the oven door. If the microwave leaks, it will make the bulb glow. (Switch off the lights in the room so that you can see the glow.) This works much better if the oven is empty, but if you’re testing an older (pre-1980s) oven, you might want to include a glass of water. Note that this method only shows the larger leaks.

This and other facts, myths and experiments about or with microwaves are collected on William Beaty’s website w7 .

Measure the speed of light with bread and margarine

The ‘naked scientists’ Chris Smith and Dave Ansell describe a very nice demonstration using standing waves to calculate the speed of light microwaves in their book Crisp Packet Fireworks and on their website w8 , where you will also find further microwave and other experiments.

Having been taught all about the really difficult historical experiments to measure the speed of light, students think it is great to use this easy method. The only drawback of this demonstration is a rather strong smell of toast. This experiment can also be used to reinforce the notion that all waves in the electromagnetic spectrum travel at the speed of light.

• A plate (and possibly a bowl)
• Four pieces of toast
• A buttering knife
• Remove the turntable from the microwave.
• Arrange four pieces of toast in a square shape on a plate.
• Cover them completely with margarine, making sure to include the joints where the pieces meet.
• You need to ensure the plate won’t turn when you switch on the microwave. If there is a central pillar supporting the turntable, you may cover it with a bowl turned upside down and balance the plate on top of it.
• Switch on the microwave at full power for 15 – 20 seconds until the margarine just begins to melt. Powerful microwaves may need less time, so check every 5 seconds. Be very careful not to microwave for too long.
• You should see a series of parallel melted patches or lines separated by unmelted patches. Take out the plate.
• Measure the distance in centimetres between two of these patches with a ruler. Multiply by two and note down the value: this is the wavelength of the microwaves produced by your oven – it should be around 12–12.5 cm.
• Now you need to find out the frequency of the microwaves. You should be able to find it on a sticker, usually at the back or door lip of the microwave. If you can’t find the specific value of your microwave, use 2450 MHz (2.45 GHz) as a standard value.
• Multiply the wavelength (about 12 cm) by the frequency. If you are using MHz, you’ll need to multiply the result by one million, with GHz by one billion.
• The result will be the speed of light in centimetres per second. Divide it by 100 to convert it to metres per second. Your answer should be about 300 million metres per second.

Light, including microwaves, is a wave consisting of a series of peaks and troughs. The wavelength is the distance from one peak or trough to the next. The frequency is the number of waves per second. To know how fast a wave is travelling, you need both values.

A microwave oven produces waves on one side of the oven, which are reflected on the opposite side and return to where they started. The reflected waves will encounter the original waves, cancelling each other out in some places, while adding up in others: the waves bouncing about in the oven interfere with each other, creating a standing wave with positions of high amplitude (antinodes) where there will be strong heating, and positions where the amplitude is close to zero (nodes) where there will be little heating. The distance between two hot spots is half a wavelength – the distance from one antinode to the next. In these hot spots, the margarine will melt first.

• Stanley H (2009) Plasma balls: creating the 4th state of matter with microwaves. Science in School 12 : 24-29. www.scienceinschool.org/2009/issue12/fireballs
• Smith C, Ansell D (2008) Crisp Packet Fireworks. London, UK: New Holland Publishers

Web References

• The Internet Plasma Physics Education Experience website: http://ippex.pppl.gov/fusion/fusion3.htm
• The glossary on the Southwest Research Institute website: http://pluto.space.swri.edu/image/glossary/plasma.html
• The Fusion Energy Division of the Oak Ridge National Laboratory: www.ornl.gov/sci/fed/Theory/tt/ttmcp/plasma.htm
• The FusEdWeb (fusion energy education) website: http://fusedweb.llnl.gov/cpep/Chart_Pages/5.Plasma4StateMatter.html
• w2 – The Exploscience website has many videos of dramatic experiments: www.exploscience.com/ChemTV_Page_5.html
• w3 – Chris Schrempp’s e-book, Bangs, Flashes, and Explosions – An Illustrated Guide of Chemistry Demonstrations , a manual with over 170 chemistry demonstrations and activities, can be ordered here: http://exploscience.com/Book.html
• w4 – The grape plasma can also be seen on the Naked Scientists website ( www.thenakedscientists.com ) or here: http://tinyurl.com/mklx73
• w5 – The physics.org website has many fun experiments and games: www.physics.org/interact-wide-template.asp
• w6 – For a dramatic demonstration of microwaving an ostrich egg, see: http://www.youtube.com/watch?v=Wgy1Yhgk_BY
• w7 – William Beaty’s ‘Unwise microwave oven experiments: high voltage in the kitchen’ website contains facts, myths and experiments about or with microwaves: http://amasci.com/weird/microwave/voltage3.html
• w8 – You can find the margarine experiment, and much more, on the Naked Scientists website ( www.thenakedscientists.com ) or here: http://tinyurl.com/lhdk7r
• The UK’s Institute of Physics describes a number of experiments involving microwaves on its website: www.iop.org/activity/education/Projects
• Mobile phones transmit and receive using microwave radiation – either 900 MHz or 1800 MHz – similar to the frequency of the radiation in a microwave oven (2450 MHz). The UK’s Science Enhancement Programme has some very useful documents on radiation in the environment, including background information and student activities. See: www.sep.org.uk/teacher/view_resource.asp?resource_id=20

Halina Stanley is a physicist by training. She spent ten years as a research scientist in industry and academia using neutron and X-ray scattering techniques to characterise materials before joining the American School of Grenoble, France, where she teaches physics, chemistry and mathematics to secondary-school children.

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Microwaving Light Bulbs Is Genuinely Useful (And Entertaining)

I love finding new things to put in the microwave. Say, for instance, an incandescent light bulb, which can test your microwave for a number of issues, besides just looking awesome. Even a non-functioning bulb can work, so dig out the dusty old bulbs you have at the back of the closet.

First, find a short juice glass and add some water to it. Put the bulb in the glass like it’s a flower in a vase. If the water doesn’t cover the metal casing of the bulb, add more water until it does. The water serves two purposes. First, it’s very bad for a microwave to be working with nothing, or only very little, inside it. The water soaks up a little of the energy that’s going to be bouncing around inside the microwave. The water also covers the metal casing of the light bulb, preventing sparks.

I suggest a very brief test run, during which you put the glass and bulb on the tray, and watch it rotate for about five to ten seconds. (If you’re feeling confident, you can keep it rotating and slowly increase the time. The maximum time suggested anywhere is 45 seconds.)

As the bulb goes round, it will flicker, lighting up, and dying down. The waves inside a microwave interfere with each other, sometimes adding up together and creating areas of high intensity, and canceling each other out and creating areas of very low intensity. High intensity areas cause high intensity light. Low intensity areas cause no light.

The light is generated in light bulbs by electrons racing along the filament, heating it up and making it glow. Put the light bulb in a lamp, and the outlet the lamp is plugged into will yank electrons back and forth. Put the bulb in a microwave, and the electromagnetic waves will also pull the electrons back and forth. Enough of this, and the filament will light up.

Too much of this, though, and the entire light bulb will slowly heat up and explode, so be careful. If you’ve managed the rotating bulb successfully, try putting the light bulb at different spots in your microwave to see where it lights up and where it stays dark. If you ever have to take the tray out to microwave food, you’ll know where to put the food to heat it up, and at what spots the food will stay cold no matter how long you keep it in the microwave. Nuke the light bulb in bursts of no more than five seconds, giving the light bulb plenty of time to cool down in between tests. If the water is heating up, make sure you change it.

One final light bulb microwave test: the gas in a fluorescent bulb will also be excited by a microwave. If you suspect your microwave isn’t fully sealed, and is “leaking” radiation, turn the lights off in the room, run the microwave (remember to have a large bowl of water inside the microwave so you’re not running it empty) and hold the bulb close to various parts of the microwave. If the bulb lights up, then yes, your microwave needs to be fixed or replaced.

[Via Science in School , Steve Spangler Science , Oregon Live ]

Experiment Microwaves Physics Science

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So what happens when you put a light bulb in the microwave?

The next experiment in our series ( What can you do with $25 and a few hours ) of microwaving household items was a common everyday light bulb. In this experiment we wanted to see what would happen since there was metal, gases, glass, and who knows what else inside of a bulb.

We started the bulb by adding 6 minutes as a speed setting to see what might happen. In just 4 seconds, it started lighting up randomly with some brilliant colors. Mind you that this isn’t connected to any electricity source, and Uncle Fester from the Adams Family came to mind since he was always putting a light bulb in his mouth and it lit up.

The light bulb continued to ignite with brilliant colors for about the first 30 seconds, and then it appeared that just the spent element glowed softly after that. So after about 2 minutes and 15 seconds we decided to put it aside.

We were surprised by a couple of things:

• The glass didn’t break – Glass on a light bulb is very thin. Think how easy it is to break if you squeeze it or step on it, or even drop it. So add some heat and arcing going on within it and it could break.
• The metallic base didn’t spark – I’ve always been told not to put metal in a microwave because it would ruin the microwave. Yet this base just sat there and provided no entertainment value.
• The filament didn’t glow continually – This may have been caused by the bulb rotating around inside the microwave, but if there is a consistent flow of microwaves, why didn’t it stay lit up? What caused it to glow and then cool, and then repeat the cycle?
• The gases inside the bulb – Our guess is that some of these gases reacted to the microwaves which is what game the brilliant spectrum of colors in the beginning, but as the burned off only the filament lit up.

If you have other thoughts or expectations from the video below, please share them. I would be interested to hear what you have to say and also to gather some ideas to test for the next time.

This looks like a fun adventure. Thank you for sharing.

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Physics and Physicists

ZapperZ's physics blog on the world of Physics and Physicists.

Thursday, June 19, 2008

Physics experiment with your microwave.

2 microwave-safe glasses 1 incandescent light bulb (60 watts works well) 1 microwave oven Directions: Fill one glass with water. Put the light bulb in the other glass so it doesn't roll around. Put both glasses in the microwave. Turn the microwave on for three seconds (use the low-power setting if it has one). Watch: If your microwave has a turntable, you will see the light bulb glow and dim as it travels. If your microwave doesn't have a turntable, repeat the experiment several times, moving the light bulb to different spots. Be careful, the light bulb gets hot! And don't run the microwave longer than five seconds at a time. The science: Light bulbs glow when electrons speed across a thin metal wire called a filament, heating it to several thousand degrees. In this experiment, you harnessed the energy of the microwave radiation generated by your microwave oven.

The wording of the article implies that the glowing & dimming is due to the turntable moving the filament through constructive & deconstructive interference parts of the microwave standing wave. By guess of what is causing it is that when you turn the "power" down on a microwave, it is only the duty cycle that changes. That is, the microwave goes at it's full power on and off by different amounts. Although if the author saw the dimming & glowing repetition rate change with the light bulb at different radii on the turntable, then I am wrong...

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