Electromagnetic Waves

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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?)

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 electron f = frequency of emitted EM radiation

The Electromagnetic Spectrum
Zero to over 1020Hz Interactive EM spectrum

Vibrating electrons e.g. AC current Microwaves Excited semiconductors or vibrating electrons Infra-red Electrons transitions between energy levels Visible Ultraviolet X Ray Emitted when decelerate rapidly electrons e.g. when they hit a metal target Gamma Emitted by nuclei after a nuclear reaction Type Source Radio Microwaves Infra-red Visible Ultraviolet X Ray Gamma

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.

This is also why raindrops create rainbows...
... and why lenses cause different colours to focus at different points....

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? NOT because of the ocean!!
- 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 0.15°C temp increase
- 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

What is special about laser light?
Lasers IMPORTANT VOCAB!!!! What is special about laser light? The light is monochromatic. This means it is of just one frequency. It is directional. This means it is concentrated in one direction. It is coherent. This means all waves produced are in phase with each other. non - coherent coherent