5.7 - Microwaves & Infrared

Microwaves carry signals to and from satellites

• Microwaves are used to send signals between the ground and satellites orbiting high above the Earth.
• Microwaves pass easily through the Earth's watery atmosphere, whereas some lower-frequency radio waves would be reflected back by the upper atmosphere.

The path of a satellite TV signal

1. A transmitter on the ground sends the signal up into space.
2. A satellite orbiting thousands of kilometres above the Earth picks up the signal.
3. The satellite transmits it back down to Earth, in a different direction.
4. A satellite dish on the ground receives it.

Because the signal travels such a long distance up and back, there's a slight time delay between it being sent and received. Mobile phone calls, satellite phones, and satellite navigation (GPS) signals also travel as microwaves.

Microwave ovens cook food

Microwave ovens cook food using microwaves, which are absorbed by the water molecules in the food.

How the heating happens

1. The microwaves penetrate up to a few centimetres into the food before being absorbed.
2. The energy they carry is transferred to the water molecules, making them heat up.
3. These hot water molecules transfer energy to the rest of the food by heating, which quickly cooks it.

The microwaves used in ovens have a different wavelength from those used for satellite communication.

Infrared is used to monitor and to raise temperature

Infrared (IR) radiation is given out by all objects, and the hotter an object is, the more infrared it gives out.

Monitoring temperature

• An infrared camera detects the infrared coming from an object and turns it into an electrical signal, shown on a screen as a picture.
• The hotter a part is, the brighter it appears - different colours represent different amounts of infrared. This is called thermal imaging.
• Uses include:
  • Police spotting suspects hiding in the dark
  • Showing where a building is losing heat
  • Infrared security sensors that set off an alarm or switch on a light when they detect a change in infrared

Raising temperature

• When an object absorbs infrared, it gets hotter.
• Cooking food - Its temperature rises as it absorbs infrared, e.g. from the heating element of a toaster or grill.
• Electric heaters - A long wire heats up as current flows through it and emits infrared (plus a little visible light, so it glows), which transfers energy to the surroundings.