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Answers to Readers' Questions on Physical Science:

EM Waves

The following 73 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Ultrasonic waves and electromagnetic spectrum
• Is there a EMF device to check freshness of fruits?
• What changes when light goes into a vacuum?
• Who discovered electromagnetic wave theory?
• How can light be both a wave and a particle?
• Why don't electromagnetic waves require a material?
• How are electromagnetic wave created?
• Converting signals from optical to radio
• How do electromagnet waves travel in empty space?
• Why can't microwaves travel through oven screen?

Next 10

Ultrasonic waves and electromagnetic spectrum

Question

November 1, 2009

Which part of the electromagnetic spectrum does the Ultrasonic wave falls into?

biswajit - India

18605

Answer

Ultrasonic is a high frequency sound wave ("sonic" = sound). It is not part of the electromagnetic spectrum.

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Is there a EMF device to check freshness of fruits?

Question

July 28, 2009

1) Is there a EMF device to check freshness of fruits and vegetables?

2) Can EMF apply to testing physical condition?

Quan - USA

18073

Answer

There is a device that used a small electrical current to measure freshness of foods, called EFresh. See:
http://www.ipgdx.com/EFresh-Measures-Food-Freshness.html

Usually, people are concerned about EMF radiation causing harm to them. I don't know of any EMF devices that can measure the physical condition of a person.

Microwaves are sometimes used to measure physical faults in structures.

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What changes when light goes into a vacuum?

Question

June 11, 2009

Hello sir,
I was wondering if you can give me answer of the following question:

Which of the following property of Electromagnetic waves does not change in vacuum?
A. Speed B. Wavelength C.Frequency D. All

Waiting for your reply,
Thanks,

Najeeb - Pakistan

17892

Answer

Once an electromagnetic wave in in a vacuum, nothing changes. But I think the question meant to ask: "What changes when an electromagnetic wave passes from a material into a vacuum (like light passing through glass into a vacuum)?"

The speed or velocity and wavelength changes, with the frequency remaining constant. Note that speed of light = wavelength times frequency.

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Who discovered electromagnetic wave theory?

Question

May 4, 2009

what are the names of the two men that dicoverd the electromagnetic wave theory?

and also

how are electromagnetic waves differnt from mechanical waves.

thanks so much :]

Mary Ellen - USA

17696

Answer

Although many people were involved, James Clerk Maxwell and Heinrich Hertz are often given the most credit.

Mechanical waves are from the vibration of matter, while electromagnetic waves are from the vibration of electrical and magnetic fields.

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How can light be both a wave and a particle?

Question

March 12, 2009

Hellow Sir,

I have a question to ask you. My teacher says that light is both a wave & a particle. How can an element of nature have two charecters? How is this possible?

Thank you.

UPOMA - Bangladesh

17432

Answer

Light is usually thought of as a wave, but in some cases it behaves like a particle, called the photon. Likewise, electrons usually behave as particle, but sometimes they act as a waveform.

This is called the Wave-Particle Duality Principle of Quantum Mechanics. Some of the theories in Quantum Mechanics are strange.

Another viewpoint is that many electrons or many photons could act as waves, similar to water molecules in a water wave.

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Why don't electromagnetic waves require a material?

Question

March 1, 2009

mechanical waves require material medium while E.M. wave do not for its prorogation. why?

akash - India

17354

Answer

Electrical fields, magnetic fields, and gravity can create forces that act at a distance and seem to go through empty space. At one time, scientists believed that space was made up of what they called aether, but they no longer believe it.

Electromagnetic waves consist of an oscillation of electrical and magnetic fields. But it is really not clear what they are or how they travel through space.

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How are electromagnetic wave created?

Question

December 25, 2008

How does electromagnetic wave arise?

Mujib Rabiu - Nigeria

17003

Answer

Electromagnetic waves include visible light, radio waves and x-rays. Visible light electromagnetic waves are usually created from heating material to high temperatures. Radio waves come from the rapid movement of electrons. X-rays come from high energy electrons striking a metal target.

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Converting signals from optical to radio

Question

July 26, 2008

Some of my friends convert signals e.g. optical to radio etc. and i was just wondering if you convert a signal too many times e.g. radio to optical to x ray, can it become useless and unusable?

Thank you for your time and help,

Alice - England

16282

Answer

Typically, signals can be converted from one wavelength region such as optical to another region like radio waves by first converting the optical to electric current and then converting that to the radio wave. Apparently, there is a company in the UK that has a device that can convert light to radio waves directly in an optical fibre.

The main thing in this issue concerns the information in the signal. Since visible light has a much shorter wavelength than radio waves, there can be a greater amount of modulation with the shorter wavelength. Thus it is possible there is more information in the light wave than is possible in the radio wave.

another problem is that any time you make a conversion, some information will be lost and some noise will be added. Too many conversions may result in a weaker signal and too much noise to make it useful for communication.

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How do electromagnet waves travel in empty space?

Question

July 2, 2008

How does an EM wave travels through space where there is no medium?If it acts as both particle as well as wave ,does it got any mass?

Rutvij - India

16170

Answer

Electromagnetic waves, magnetism and gravity are examples of forces or waves that travel through space. One theory for electromagnetic waves is that they travel as groups of particles called photons, in a waveform. Another theory is that empty space consists of a material called aether, and the waves are vibrations in that medium. Neither theory is complete, so at this time scientists really don't have the whole answer.

The photon has a momentum, and that implies it has a mass as long as it is moving. Apparently, once the photon stops moving, its mass goes to zero.

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Why can't microwaves travel through oven screen?

Question

June 7, 2008

What is the height of EM waves? For example, the holes in a microwave oven door lets visible light out, but not microwaves...so microwaves must be "taller" than vis light.....i think.
Why don't the M waves come out the door if this is not so?
I know Microwave wavelengths are longer than vis light, but cant find any info on this

thanks

John - USA

16060

Answer

First of all, neither microwaves nor visible light can pass through a solid sheet of metal. Also, electromagnetic radiation cannot readily pass through a hole in the metal that is smaller than its wavelength.

Since the wavelength used in a microwave oven is about 12 cm, the waves cannot pass through the small holes in the metal screen and are reflected back into the oven. Light has a wavelength of about 40/1,000,000 cm, so it can easily pass through the holes.

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