ELECTROMAGNETIC WAVES SECONDARY 3 PHYSICS

WHAT ARE EM WAVES? Electromagnetic waves (EM waves for short) are waves that can travel in a vacuum. These waves are created by the vibration of an electric charge.

EM radiation is a wave that is produced as follows: When charges accelerate, they produce a changing magnetic field. This changing magnetic field creates a changing electric field at 90° to it. The electric field now causes a changing magnetic field at right angles to it. The magnetic and electric fields are able to generate each other without any decrease in strength if the fields move at 3  10 8 m.s -1.

Electromagnetic Spectrum It is a group of different electromagnetic waves. There are 7 components in the spectrum.

Electromagnetic Spectrum Gamma rays X – rays Ultraviolet Visible light Infra-red Microwaves Radio waves Highest frequency Shortest wavelength Lowest frequency Longest wavelength

Uses of Gamma Rays Kill cancer cells Study the nucleus in atoms

Uses of X-rays Take ‘pictures’ of bones in the body Study the crystal structure of crystalline substances Check for cracks in metal plates

Uses of Ultraviolet Detect counterfeit notes Gives the clothes a ‘glow’ effect in discotheques Sun-tanning Sterilise medical equipment

Uses of Visible Light Enable us to see things Photosynthesis in plants

Uses of Infra-red Heating Haze photography

Uses of Microwaves Radar communication Analysis of the molecular and atomic structure Telephone communications

Uses of Radio Waves Radar communications TV and radio broadcasting

Common Properties of EM Waves All transverse waves All travel at the speed of light. (3 x 10 8 m/s) Can travel through solid, liquid, gas and vacuum Obey the laws of reflection and refraction All can be absorbed and emitted by matter The wave equation is applicable to all

Penetrating ability of electromagnetic radiation The ability of EM radiation to go through (penetrate) bone, glass or concrete depends on the energy of the radiation and then also on the frequency of the radiation. Radio waves have the lowest frequency of EM radiation and thereby the lowest energy. They are not able to travel through the ground, therefore we cannot receive a radio signal underground. The radio waves are strongly diffracted (bent) around objects, while the shorter radio waves are reflected by the charged upper atmosphere fluctuate because of the changing reflecting ability of this layer.

Microwaves have a low penetrative ability. Cell phone masts that use microwaves must have no obstructions in between them. Infrared radiation has low energy. It is able to warm the skin but cannot penetrate through the skin. Ultraviolet radiation has a frequency higher than violet light in the visible spectrum. The energy is such that it is harmful to the eyes and can cause the skin to tan.

X-rays have a high energy and a high penetrative ability. They can penetrate the soft tissue in the body but cannot go through the bone. Gamma rays can pass through a few centimetres of lead or concrete. They have a large amount of energy

Energy carried by electromagnetic radiation Energy  frequency We calculate the energy of the radiation using: E = h  f Where E is the energy of the radiation measured in Joules (J) h is plancks constant and has a value of 6,63  J  s f is the frequency of the radiation measured in hertz (Hz)

Since f = c/ To calculate the energy of the photon, we use the following equation: E = hc/ Where E = Energy in joules (J) h = planks constant (6,6  J.s) f = frequency in hertz (Hz) = wavelength in metres (m) c = speed of light (3  10 8 m.s -1 )