MadSci Network: Physics |
Question: Can a plastic that is transparent to visible light reflect
microwaves?
Question ID Number: 1268845253.Ph
References:
1. NASA Electromagnetic Spectrum Web Site
ht
tp://imagine.gsfc.nasa.gov/docs/science/know_l2/emspectrum.html
2. Mad Science Archives Re: Microwaves plastics, and ceramics
http://www.madsci.org/posts/archives/2000-
05/959035838.Ch.r.html
3. Mad Science Archives Re: What causes the reflection of light?
http://www.madsci.org/posts/archives/1997-05/863034503.Ph.r.html
4. Mad Science Archives Re: measuring the characteristics of photons
in
confined spaces
http://www.madsci.org/posts/archives/2003-12/1072272975.Ph.r.html
5. Detecting microwave oven leaks.
http://www.madsci.org/posts/archives/2001-
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
side.
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
Archives.
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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