4. The properties, uses and dangers of microwaves

– microwave radiation for mobile phones and satellite television communication

The sources and properties of microwave radiation

Microwaves, like radio waves, are produced by an oscillating electric and magnetic field in an electric circuit (no details required at GCSE level).

The explanation of how microwave transmissions work is the same as for radio waves (so need to repeat the notes here)

 

Uses of microwave radiation

TV transmitters/receiver sets use microwave signals via satellite communication ('satellite TV dishes').

Mobile phones ('satellite/cell phone') can use satellite communication systems too, as well as local transmitting/receiving 'mobile phone' masts that are now appearing everywhere!

Microwave mobile phone call signals are usually picked by, or transmitted by, the nearest 'mobile phone mast' receiver/transmitter.

The path between the microwave transmitter or receiver usually needs to be a straight line with no large buildings or other obstruction blocking the signal. That's why you see so many tall mobile phone relay masts are all over the place!

For some applications, the microwave signal must have a wavelength to allow it to penetrate clouds (water droplets) and water vapour and through the ionosphere to communicate with satellites in orbit thousands of km above the Earth's surface.

Reflection/refraction of radio waves

Radio waves are refracted by some layers of the atmosphere, so cannot be used for satellite communications. However, microwaves of shorter wavelength, are not refracted in the atmosphere and can be used for satellite communications.

The lower atmosphere is not electrically charged and does not inhibit the transmission of radio waves or microwaves.

However, part of the upper atmosphere, called the ionosphere, contains electrically charged particles (ions) that interfere with radio waves, but not microwave radiation.

The longer wavelength radio waves are either reflected or refracted by the ionosphere, but the microwaves can pass through the upper atmosphere to satellites orbiting high above the Earth's surface.

The microwave signal (telephone or TV) is transmitted through the Earth's atmosphere into space where the satellite receiver dish (many miles above Earth's surface) picks up the signal.

The signal is then re-transmitted back to a receiver on the Earth's surface eg TV satellite dish and detector.

e.g. telephone signals between the UK and USA can be sent by microwaves via satellites.

There is a small time delay between the transmission and reception of the signal because of the long distance it travels and the operation of the electronics.

We can detect microwave radiation from around the Universe using huge radio telescope dishes - so microwaves are used by radio-astronomers to investigate objects in the universe.

Satellite microwave transmission-reflection-receiving can be used to monitor certain geophysical aspects of the Earth's surface eg rainforest versus deforestation, ice sheet cover and icebergs in arctic areas.

It has already pointed out that is important for communications that the microwave frequency waves can pass through any moisture in the Earth's atmosphere. However, its different for cooking, where you want the opposite effect.

Microwave oven cookers use microwave EM radiation to heat up food and pretty rapidly as the high frequency microwaves readily penetrates the food.

The microwave frequency used in cookers is slightly different to that used in mobile phones and is absorbed by water and fat molecules e.g. in food.

The microwaves can penetrate right into the food several cm as it is absorbed by water molecules.

The water and fat molecules become excited, increasing their kinetic energy of vibration.

On 'relaxing' to their normal energy level, the molecules rapidly transfer this excess energy to the food, increasing its thermal kinetic energy store and rapidly cooking it.

The thermal energy is further dispersed throughout the food by conduction.

If your skin is exposed to too much microwave radiation you can damage cells and suffer burns, which is why microwave cookers cannot operate with the door open!

Technically, the high frequency radiation is absorbed by water molecules which spin round faster than normal. When the water molecules 'relax' and return to their normal energy state, the energy is transferred and released as heat to whatever you are cooking. The heat energy of the water is distributed throughout the food increasing the thermal energy store of your pizza!

Cooking note comparing use of microwaves and infrared:

When food is grilled, initially only the surface is cooked, because infrared is not very penetrating and deeper inside the food will be less cooked - perhaps not sufficiently for health and safety. However, in microwave cooking, the radiation can penetrate deep into the food and quite quickly too (often just a few minutes), so the food is more thoroughly cooked, but not necessarily as tasty, since we like 'fried' food.

Microwave radar scanners

Microwave radar emission and detection systems can be used as echolocation detectors e.g.

(i) Military application: Microwave pulses can be sent out by a transmitter mounted on one aircraft and monitoring the reflected echo to detect another aircraft. The computer will work out the range and direction of target.

(ii) Civilian application: Airliners use microwave radar to detect a hazard e.g. another aircraft that is too close

(iii) Microwave scanning is used by meteorologists to monitor precipitation of rain or snow to help with short-term weather forecasting.

Example of simple calculation (speed of electromagnetic radiation)

Q Suppose an airliner sends out a microwave radar signal of wavelength of 1.20 cm.

The microwave reflects off another aircraft and the echo is detected after a time lapse of 6.0 µs.

The speed of electromagnetic radiation = 3.00 x 10⁸ m/s.

(a) What is the frequency of the microwave beam?

speed = wavelength x frequency

f = v ÷ λ = 3.00 x 10⁸ ÷ (1.20 / 100) = 2.50 x 10¹⁰ Hz

(b) What is the distance between the two aircraft?

s = d / t,  d = s x t = 3.00 x 10⁸ x 6.0 x 10^-6 = 1800 m (total distance including echo)

distance between aircraft = 1800 ÷ 2 = 900 m

(µ is micro = 10^-6, and total distance is halved because it involves 'there and back')

 

Dangers of microwave radiation

Direct exposure to high intensity microwave radiation can cause burns - it has a similar effect to infrared radiation. It can also damage your eyes - which is why microwave ovens can only work with the door closed.

The inside metal walls of a microwave cooker reflect the microwaves around and stop them exiting the cooker.

The door is fitted with special glass that also reflect microwaves, so the cooker is designed to internally retain any potentially harmful microwave radiation.

Dangers of using mobile phones?

There has been some controversy about the use of mobile phones.

Water molecules readily absorb certain microwave frequencies and become heated (this is how a microwave cooker works!).

So, potentially, since you contain a lot of water, microwaves heat you up by being near a mobile phone mast or excessive use of your mobile phone BUT there is no real evidence (as far as I know?) to support the notion that there is a danger to you.

However, there are strict limits on the amount of energy a mobile can transfer when emitting a signal.

In the UK the legal limit is 2 watts, that is 2 joules of energy per second.

This should cause no harm and it is distributed in ALL directions.