14.3 - 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.

Infrared and light transfer information

Both infrared and visible light are used to carry information, over short distances and over very long ones.

Short-range links

• Information is encoded as pulses of infrared sent between an emitter and a receiver.
• The devices must be fairly close and the receiver must be in the emitter's line of sight.
• Examples: a TV remote control (a small bulb emits infrared the TV detects) and sending files wirelessly between phones or laptops.

Optical fibres

• For long distances, data is sent as pulses of infrared or light through optical fibres - thin strands of glass or plastic.

• A pulse enters at a shallow angle and is reflected again and again off the boundary between the narrow core and the cladding around it, until it emerges at the far end. This is called total internal reflection. It happens because the angle of incidence at the boundary is greater than the critical angle.

• A single wavelength is usually used so the pulses stay sharp and information isn't lost.

• Optical fibres carry telephone and broadband internet signals.

• In medicine, they're used in endoscopes to see inside the body through a small hole, avoiding major surgery.

  • An endoscope uses non-ionising visible light, whereas a CT scan builds a 3D image using ionising X-rays - so an endoscopic examination exposes the patient to less harmful radiation than a CT scan.

Geostationary and geosynchronous satellites

Communication satellites relay microwave signals around the world, and the type of orbit decides how a satellite is used.

Geosynchronous and geostationary orbits

• Geosynchronous orbit - The satellite takes 24 hours to make one full orbit, the same time the Earth takes to spin once.
• Geostationary satellite - A special geosynchronous satellite that orbits directly above the equator. It travels in the same direction and at the same rate as the Earth's spin, so it stays fixed above the same point on the ground.
• This allows a receiving dish to be pointed once and never moved.
• Satellite TV works this way - data is beamed up to a geostationary satellite, which transmits it straight down to home dishes.

Low orbit satellites

• Travel relatively close to the Earth (usually within about 1,000 km) and move quickly, often orbiting in less than two hours.
• Each covers only a small area at a time, so several are needed working together for constant coverage.
• Satellite phones use both low orbit and geostationary satellites; satellite TV uses geostationary satellites only.

Relaying over long distances

• A single satellite only "sees" part of the Earth at once, so a signal may be bounced between satellites and ground stations to get around the Earth's curve.
• A signal sent up and back to the same point travels twice the orbit height.
• For a satellite 36,000 km up: 36,000 km × 2 = 72,000 km.

Uses of visible light

Visible light is the only part of the electromagnetic spectrum human eyes can detect.

• Vision - We see the world because visible light enters our eyes; the eye is the detector for this band.
• Photography - Cameras form images using visible light, recording a scene on a sensor or on film.
• Illumination - Lamps and other light sources give out visible light to illuminate surroundings.