Unit I - EM Spectrum

3c (3.10-3.13)

Types of EM waves

• EM waves with different wavelengths have different properties

• 7 types (depending on wavelength) but types merge to form continuous spectrum

• All types of EM radiation are transverse waves and travel at same speed through free space

Order of EM spectrum

Radioactive Monkeys Invade Venus Using X-ray Guns

Radio waves

Microwaves

Infrared

Visible light

Ultraviolet

X-rays

Gamma rays

(in order of increasing frequency and decreasing wavelength

Visible light

ROY G BIV

Red

Orange

Yellow

Green

Blue

Indigo

Violet

(in order of increasing frequency and decreasing wavelength)

Radio wave uses

Broadcasting + communications

• Long-wave radio can be transmitted a long way because long wavelengths are bent around curved surface of Earth

• Short-wave radio signals can be received at long distances from transmitter because they’re reflected from ionosphere (layer of Earth’s atmosphere)

• Radio waves used to TV and FM radio broadcasting have very short wavelengths
  To get reception, you must be in direct sight of transmitter

Microwave uses

Cooking + satellite transmission

• Satellite communication uses microwaves

• For satellite TV, signal from transmitter is transmitted into space, where it’s picked up by satellite receiver dish orbiting Earth
  Satellite transmits signal back to Earth where it’s received by satellite dish on ground

• Mobile phone calls also travel as microwaves from phone to nearest trasmitter

• Microwaves used for cooking
  Microwaves absorbed by water molecules in food
  They penetrate a few cm into food before being absorbed
  Energy is conducted/convected to other parts of food

Infrared uses

Heating + night-vision equipment

• IR radiation is also known as heat radiation
  Electrical heaters radiate IR to keep us warm

• IR is given out by all objects - hotter object = more IR radiation

• IR radiation given out by objects can be detected in dark by night-vision equipment
  Equipment turns it into electrical signal, which is displayed on screen as picture, allowing things in dark to be seen

Visible light for optical fibres

• Can be used for communication using optical fibres - which carry data over long distances as pulses of light

• Optical fibres work by bouncing waves off the sides of very narrow core

• Pulse of light enters fibre at certain angle at one end and is reflected repeatedly until it emerges at other end

• Optical fibres are used for telephone and broadband internet cables

Visible light for photography

• Cameras use lens to focus visible light onto light-sensitive sensor

• Lens aperture controls how much light enters camera

• Shutter speed determines how long the sensor is exposed to light

• By varying aperture + shutter speed, photographer can capture as much/as little light as they want in photo

Ultraviolet uses

Fluorescent lamps

• Fluorescence is a property of certain chemicals, where UV radiation is absorbed and visible light is emitted

• Fluorescent lights use UV radiation to emit visible light
  Safe to use because nearly all UV radiation is absorbed by phosphor coating on inside of glass which emits visible light instead

• Fluorescent lights are more energy-efficient than filament bulbs

X-ray uses

Observing internal structure of objects/materials

• Including bodies - which is why they’re useful in medicine

• To make X-ray image, X-rays are directed through object/body onto detector plate
  Brighter bits are where fewer X-rays get through

• Radiographers in hospitals take X-ray photos to help doctors diagnose broken bones

  X-rays easily pass through flesh but not denser material like bones/metal

Gamma uses

Sterilising medical equipment:

• Sterilise by killing all microbes

• This is better than boiling plastic instruments, which may be damaged by high temp

Sterilising food:

• Sterilise by killing all microbes

• Keeps food fresh for longer, without freezing or cooking to preserve

• Food is not radioactive afterwards, so is safe to eat

Higher frequency EM radiation is…

more dangerous (usually)

because it has more energy

Microwave dangers

• Have similar frequency to vibrations of many molecules - so can increase these vibrations

• Resulting in internal heating - the heating of molecules inside things

• Microwaves heat human body tissue internally

• Microwave ovens need shielding to prevent microwaves from reaching user

Infrared dangers

• IR frequency range can make surface molecules of any substance vibrate - like microwaves, this has heating effect

• IR has higher frequency, so carries more energy than microwave

• If human body is exposed to too much infrared, can cause skin burns

• Protect yourself using insulating materials to reduce amount of IR reaching skin

Ultraviolet dangers

• Can damage surface cells and cause blindness

• Some frequencies of UV are ‘ionising’ - carry enough energy to knock electrons off atoms
  Can cause cell mutation/destruction and cancer

• Protect yourself by wearing sunscreen with UV filters when out in sun, and stay out of strong sunlight to protect skin from UV radiation

Gamma dangers

• Very high-frequency waves (such as gamma) are also ionising, and carry more energy than UV rays

• So can be much more damaging and can penetrate further into body

• Ionising radiation so can cause cell mutation, leading to tissue damage/cancer

• Radioactive sources of gamma rays should be kept in lead-lined boxes when not in use
  When people need to be exposed to them, e.g. medical treatment, exposure time should be as short as possible