Electromagnetic Waves

G1 – The nature of EM waves and light sources

The Nature of EM Waves

Demo: Move a charged balloon near to some tissue paper / someone's hair.

Observation: The tissue or hair may also move.

A charge has its own electric field and thus moving it creates a disturbance in the electric field existing in the room. It can also be shown that a moving charge creates a magnetic field perpendicular to its electric field (just like a moving electron in a wire - remember the right hand thumb rule?)

EM Waves

When a charge moves, a ripple is created in Earth’s electric and magnetic fields. If the charge vibrates according to SHM (e.g. an AC current), a sinusoidal variation in the two fields is created: this is how a typical low frequency electromagnetic wave is created (e.g. radio waves).

Animation - link

Note: All EM waves are transverse and travel at the speed of light.

Light Waves

Visible light waves have a far higher frequency (approx 1015 Hertz) and so cannot be produced by oscillating electrons. As we have already seen, light waves / photons are emitted when electrons in atoms fall to lower energy levels.

E = hf

E = change in energy of the electronf = frequency of emitted EM radiation

The Electromagnetic Spectrum

- Zero to over 1020Hz

-

Interactive EM spectrum

Type Source

Radio

Microwaves

Infra-red

Visible

Ultraviolet

X Ray

Gamma

Sources of EM radiation

Type Source

Radio Vibrating electrons e.g. AC current

Microwaves Excited semiconductors or vibrating electrons

Infra-red Electrons transitions between energy levels

Visible Electrons transitions between energy levels

Ultraviolet Electrons transitions between energy levels

X Ray Emitted when decelerate rapidly electrons e.g. when they hit a metal target

Gamma Emitted by nuclei after a nuclear reaction

Refraction of EM Waves

EM radiation moves at different speeds in different mediums. According to v = f λ this means that if frequency is unchanged, wavelength must change. This gives rise to refraction.

Dispersion of EM waves

The shorter the wavelength the greater the change in velocity and so the greater the degree of refraction i.e. The greater the refractive index.

Thus violet diffracts more than red. When white light is shone through a prism this gives rise to the spreading out of the different colours, known as dispersion of visible light.

... and why lenses cause different colours to focus at different points....

This is also why raindrops create rainbows...

The discovery of Infrared (1800)

William Herschel found that a thermometer placed just beyond the red end of a dispersion spectrum from sunlight showed an increase in temperature!

Prism

Prism

Transmission, Absorption and Scattering

Objects have a tendency to selectively absorb, scatter, reflect or transmit light of different wavelengths.

E.g. A green wall reflects frequencies of visible light in the green range but absorbs others as internal energy. A blue glass may transmit blue frequencies while absorbing others.

Examples

Explain...

- Why is the sky blue?

- Why are sunsets red?

- Why do glaciers sometimes look blue?

Why is the sky blue?

- The air molecules selectively absorb shorter (blue) wavelengths of EM radiation. This energy is then reemitted in all directions in the sky (scattered). Other wavelengths are not absorbed and are transmitted straight through the atmosphere. This also explains why the Sun looks yellower on Earth than in space (i.e. The blue wavelengths have been absorbed and scattered).

Why are sunsets red?

- When the Sun is on the horizon its light must travel further to reach you. This means that more of its radiation is scattered and only the longest wavelengths (red) are not. Thus the light that reaches you is mainly red.

Health Hazards

- Radio waves have no proven dangers.

- Microwaves cause heating of water molecules.

- IR can cause burns.

- UV damages skin (similar to burns)

- X and Gamma rays cause ionisation and cancers.

0.15°C temp increase

Lasers

What is special about laser light?

1. The light is monochromatic. This means it is of just one frequency.

2. It is directional. This means it is concentrated in one direction.

3. It is coherent. This means all waves produced are in phase with each other.

coherent non - coherent

Link – how stuff works

How Ruby Lasers Work

1. Population inversion: Normally electrons in the ruby occupy the lowest energy levels. A flash of light excites many of them to a higher energy level.

2. Over a period of time these electrons fall back, releasing photons of light with identical frequency.

3. These photons stimulate the remaining excited atoms to de-excite, emitting more coherent photons.

4. Mirrors at both ends amplify the effect. Light can escape through one which is partially transparent.

Hence... “Light Amplification by Stimulated Emission of Radiation”, LASER.

Uses of Lasers

- Reading and writing CDs.

- Communications (e.g. Laser light in fibre optics)

- Distance and level finding (e.g. on building sites)

- Barcode reading

- Eye surgery

- Metal cutting and welding

The last two are made possible by constructive interference of two parts of the laser beam creating a region of highly localised energy.

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