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 3 108 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 bodyStudy the crystal structure of crystalline substancesCheck for cracks in metal plates

Uses of Ultraviolet

Detect counterfeit notesGives the clothes a ‘glow’ effect in discothequesSun-tanningSterilise medical equipment

Uses of Visible Light

Enable us to see thingsPhotosynthesis in plants

Uses of Infra-red

HeatingHaze photography

Uses of Microwaves

Radar communication

Analysis of the molecular and atomic structure

Telephone communications

Uses of Radio Waves

Radar communicationsTV and radio broadcasting

Common Properties of EM Waves

All transverse waves

All travel at the speed of light. (3 x 108 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 radiationThe 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 10-34 Js

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 10-34 J.s)

f = frequency in hertz (Hz)

= wavelength in metres (m)

c = speed of light (3 108 m.s-1)