MadSci Network: Physics

Re: Can a plastic that is transparent to visable light reflect microwave?

Date: Wed Mar 17 16:23:06 2010
Posted By: Adrian E. Popa, Retired Laboratory Director
Area of science: Physics
ID: 1268845253.Ph

Question: Can a plastic that is transparent to visible light reflect microwaves?

Question ID Number: 1268845253.Ph


1. NASA Electromagnetic Spectrum Web Site

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2. Mad Science Archives Re: Microwaves plastics, and ceramics 05/959035838.Ch.r.html

3. Mad Science Archives Re: What causes the reflection of light?

4. Mad Science Archives Re: measuring the characteristics of photons in
confined spaces

5. Detecting microwave oven leaks. 11/1006881965.Eg.q.html

Yes there are several ways that plastics can transmit visible light and
reflect or absorb microwaves. However, we first must discuss how materials
behave when illuminated by electromagnetic (EM) waves. Both light and
microwaves are (EM) waves and they obey the same laws of physics. However
the properties of materials can be different at different EM wavelengths.

Electromagnetic Waves

The electromagnetic (EM) spectrum ranges from radio waves to microwaves
to X-rays and beyond (See Reference 1.). All EM waves travel at the speed of
light which is about 300 million meters per second (186, 280 miles per
second). The voltage in an EM wave alternates back and forth between plus and
minus many times per second and we call this the frequency of the wave
in cycles per second. Scientists have named frequency Hertz (abbreviated Hz)
after Heinrich Hertz, a German scientist, who succeeded in transmitting the
first radio waves across a room in 1888. Thus when you hear a radio wave has a
frequency of one megahertz ( 1 MHz) it means 1,000,000 cycles per second.
A radio frequency of 100 megahertz (100 MHz) means 100 million
(100,000,000) cycles per second.

The microwave region of the electromagnetic spectrum begins around 1000 MHz
which is also called 1 Gigahertz (1 GHz). Microwaves are similar to radio waves
with wavelengths between about 30 cm (11.8 inches) to 3 millimeters (0.12 inches).
Most microwave ovens operate at a frequency of 2.45 GHz (2,450,000,000 cycles
per second) (a wavelength of 12 cm (4.75 inches)).

Electromagnetic wave propagation in matter

In general there are two types of matter (substances) in the universe that
affect EM waves, conductors and insulators. These properties are determined by
the atomic structure of the atoms and molecules. Insulators are called dielectrics
by scientists. Most, but not all, conductors are metals, such as copper, aluminum,
silver and gold. Most, but not all, dielectrics are non metals. Examples of
dielectrics are paper, plastic, Teflon, glass, ceramic, and wood. Very pure water
is a good dielectric substance. Salt water is a rather poor conductor!

Reflection, Transmission and Absorption of Microwaves

To help understand the effects of materials on microwaves I will compare them to
light waves. However, not all materials behave the same way at both light
frequencies and radio frequencies. For example wood is transparent to radio
waves and is opaque to (blocks) light waves. Light waves have a frequency around
500,000,000,000,000 cycles per second (500 Terahertz or 500 THz).

When a microwave hits a material some of the power is reflected at the
surface and some of the power is transmitted into and possibly through
the material. If the material is a metal, almost all of the microwave power is
reflected within the first few atoms of the material. A small amount of
power is absorbed by the metal atoms and converted to heat.

Example: a silvered mirror reflects about 95 % of light power and about
95% of radio power and it absorbs about 5 % of light and microwave power as heat.

If the material is a dielectric, some of the power is reflected at the
front and back surfaces and some of the power travels through the material.

Example: Some light reflects from the two surfaces of clear glass and some light
travels through the glass. The same is also true for radio waves in pure glass.

As the radio wave travels through a dielectric material some of the power is
reflected from the surfaces and some power is absorbed in the material, generating
heat, and some of the power travels through the material and comes out of the other

Example: Light traveling through sunglasses has a few percent of the power
reflected at each surface and between 10% and 90% of the light power is absorbed
in heating inside the darkened glass and a few percent of the power comes out the
other side of the glass and into our eyes. Depending on the absorbing material in
the glass, the same is true for microwaves. However, the light absorbing material
in the glass is usually different than microwave absorbing material deposited in
special glasses used to absorb or reflect microwaves (See references).

This power absorption in a dielectric is called the Attenuation Coefficient
or the Loss Tangent of the material. How much power travels through a
dielectric depends on the surface reflection losses, and both the thickness of the
material and its attenuation coefficient. All dielectrics have some loss except
for a vacuum which is the only perfect dielectric!

Dielectrics such as cardboard, paper, clear glass, Teflon, some plastics,
pure water, and many building materials have low attenuation coefficients
and a small amount of radio power reflects from them while most of the power
passes through them (For example, microwave cooking containers).

Most foods contain water and many different salts that absorb radio waves
and they are easily heated in a microwave oven (For example salt water).

Because metals reflect most of the microwave power, microwave ovens are enclosed
in metal boxes to shield us from the 1,000 watts of microwave power generated for
cooking dielectric material within the oven. Metal objects placed in the oven will
cause short circuiting and arcing and may damage the oven. There are a number of
questions and answers about arcing within microwave ovens in the Mad Science

Answers to Your Question.

The reflection of microwaves is dependent on the Index of Refraction of the plastic
at microwave wavelengths and the absorption of the plastic materials depends on the
material's Attenuation Coefficient or the Loss Tangent at microwave frequencies.
The plastic material will have reflection from the first surface of the plastic and
also from the second surface.

Answer 1.

If the thickness of the plastic is adjusted to be the length of the microwave
wavelength, the back reflections from both surfaces will add and less energy will
be transmitted through the material. If you have a stack of a number of plastic
sheets of similar dimensions, all of the microwave back reflections will add and
much less energy will be transmitted. The number of sheets used will determine how
little microwave energy will be transmitted through the stack.In cameras they use
many thin coatings to make antireflection coated optics. You can also use
different thickness of coatings to make better reflectors. Lasers use this type of
coating to improve the efficiency of reflection from silvered mirrors.

Answer 2

As noted above, some of the microwave energy will be absorbed in each plastic sheet.
It is also possible to machine one-half-wavelength-deep groves in the first surface
of the plastic making it a microwave mirror, while transmitting visible light.
Once again the efficiency of the mirrored surface will determine how little microwave
energy is transmitted. This type of plastic microwave mirror has no metal and is
transparent to light waves.

Answer 3

Small holes in the metal screen in the door of a microwave oven transmit some
microwave energy and reflect most of the energy. However, the microwave energy
transmitted through the screen is very, very small. I discuss this problem in
Reference 5. Instead of screen there are also special glasses that have very small
metal particles distributed in them so that they reflect microwaves and transmit
light. These glasses usually are copper colored and also reflect infrared heat waves.
I assume you could also suspend the same metal particles in plastic when it is
manufactured. These glasses are used in windows to prevent microwave spying through
the windows to monitoring things going on inside of buildings etc.

Thank you for an interesting question.

Best regards, Your Mad Scientist
Adrian Popa

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