Gamma Rays

Detection: Resonance in similar special electrical circuits (also klystron, magnetron).

Uses: Greatly used in air-borne communications, for example with mobile phones. The shorter the wavelength the greater the bandwidth. Microwaves are also used in point to point "beaming" of energy, it has been suggested that orbiting power satellites could simply "beam" the energy produced to earth based stations via microwave links. However this is still only theory since security precautions would have to be considered to prevent these satellites from being hijacked and turned against civilian areas for the purpose of frying people.

Comments: Note the size of these wavelengths - the longer microwaves' wavelengths are literally measured in centimeters. Contrast this with gamma rays, whose wavelengths are sub-atomic in size. Many stars are microwave emitters.

Alpha particles:

• consists of 2 protons and 2 neutrons (Helium nucleus)
• highly ionizing therefore they strongly attract electrons
• have a charge of +2
• bent by electric and magnetic fields but not as much as beta due to larger mass
• relatively heavy and slow moving
• absorbed by skin / 5cm air / card therefore it has poor penetrating power

Beta Particles:

• a fast moving electron
• in beta radiation, a neutron turns into a proton and an electron and the electron is ejected
• very light (little mass)
• weak ionizing ability
• move at 9/10 the speed of light
• bent strongly by electric and magnetic fields
• fast moving, moves up to 30-50cm in air, absorbed by 5mm Al. Can ionize an air molecule

Gamma Rays:

• electromagnetic rays
• moves at the speed of light
• high energy radiation with shorter wavelength than X rays
• never fully absorbed therefore has a very long range
• very very weakly ionizing because it has no charge or mass
• not affected by electric and magnetic fields
• kills living cells
• only emitted with alpha and beta particles
• strength halved by thick concrete, thick lead (25mm)

For nuclear equations:

• When alpha decay occurs, mass no. decreases by 4 and atomic no. decreases by 2.
• For beta decay, mass number does not change, atomic number increases by 1.
• Gamma decay, a gamma ray is released (energy), no change to mass & atomic number.

Radio Waves
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Wavelength: 10-1m to 104m

Sources: Transmitters (using inductance and capacitance), sparks (eg, from brushes of unsuppressed motors).

Detection: Receivers containing inductance and capacitance which are set into resonance by the wave.

Uses: Medium range air-borne communication.

Comments: There are five main classes of radio waves, they are:

Ultra High Frequency (UHF) - 10-1m to 100m - TV, radar, airport control

Very High Frequency (VHF) - 100m to 101m - FM radio

Short Wave Radio (Ultra High Frequency) - 101m to 102m - Amateur radio, police, ambulances

Medium Wave Radio (Ultra High Frequency) - 102m to 103m - AM radio (national stations)

Long Wave Radio (Ultra High Frequency) - 103m to 104m - AM radio (international stations)

Reflection, Refraction, Total internal Reflection
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Total internal reflection

Waves going from a slower to a faster medium speed up and bend at the boundary, e.g. light going from glass to air. Beyond a certain angle (the critical angle) all the waves bounce back into the glass - they are totally internally reflected.

All waves that hit the surface beyond this critical angle are effectively trapped. The critical angle for most glass is about 42o.

An optical fibre is a thin rod of high-quality glass. Very little light is absorbed in the glass. Light getting in at one end is totally internally reflected, even when the fibre is bent.

Optical fibres can carry enormous amounts of information in light pulses trapped inside them.

Reflection
When waves reflect, they always do it regularly.

Remember
i = r
The angle of incidence = the angle of reflection

Rough surfaces
Each bit of the surface obeys this law, but the overall effect of the jagged surface is to scatter the light diffusely.
The reflected waves head off in all directions, e.g. sunlight on a piece of paper.

Smooth surfaces
These act as mirrors. The rays are reflected uniformly and can form images. They can:

Check what your syllabus needs you to know in detail about mirrors and images.

The radiation from all these sources - whether they are part of planet Earth or caused by man - is called our background radiation.

Refraction
If a surface is transparent to waves, some or most of the waves hitting the surface will pass through. The rest get internally reflected.

The speed of waves usually changes when they cross a boundary. This also changes their direction. (Imagine a car driving off a smooth, hard road into a muddy field.)

The bending follows a regular pattern known as Snell's Law. Most syllabuses don't test this in detail, but check in case yours does.

In refraction, the bigger the change in speed, the bigger the change in direction.

Spectrum of light
In a vacuum, light of all colours travels at the same speed. In any transparent medium (e.g. glass), the different colours travel at different speeds, so they bend by different amounts. This is why light is split by a prism. Violet light is the most violently bent.

Diffraction

When waves meet a gap in a barrier, they carry on through the gap. This may seem obvious, but what happens on the far side of the gap isn't so straightforward. There is never a perfect cut-off between the waves that get through and the non-wavy surroundings. The waves always 'leak' to some extent into the shadow area beyond the gap. This is diffraction - the spreading-out of waves when they go through a gap.

The extent of the spreading depends on how the width of the gap compares to the wavelength of the waves.

Remember
The bigger the width of the gap compared with the wavelength of the wave, the less the diffraction.

Some examples of diffraction:

• Water 

Waves spreading into a harbour; narrower gap, wider spread

• Sound 

Sound through a doorway
Lower pitched sounds travel better into the space behind the door than high-pitched sounds.
Low sound - a long wavelength compared with the gap - large spread.
High sound - a short wavelength compared with the gap - little spread.

Ultrasound
Very short wavelength compared with most gaps - little spreading. This makes sharp focusing of ultrasound easier, which is good for medical scanning or surveillance.

• Light
  Very short wavelength compared with most everyday gaps - windows, pupil in eye, lens in camera etc. - so there is little obvious diffraction and sharp shadows. Try looking at a bright light through the narrow gap between your fingers and squeezing the gap smaller - strange diffraction patterns appear.
• Radio
  Long-wave radio signals are much less affected by buildings, tunnels, etc. than those of short-wave or vhf radio.

The detailed patterns of diffraction are complicated, especially when lots of different wavelengths (e.g. white light) go together through complex patterns of gaps (e.g. in a grid of fibres). Try looking at a bright white light at night through an umbrella. Diffraction lies behind the colourful reflected patterns from CDs and oil films, the colour of dragon-flies' wings, and helps us find out what stars are made from.

• All waves in the electromagnetic spectrum are transverse.
• The waves all travel at 3 x 108ms-2 known commonly as the speed of light.
• The following table puts waves in order of increasing wavelength and decreasing frequency.

Ultraviolet Light
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Wavelength: 10-9m to 3.5 × 10-7m

Sources: Ultra hot bodies, mercury vapor lamp, electric arcs (sparks).

The mercury vapor lamp works by photoelectric effect (exciting e- in the mercury and thus releasing photons of the right frequency).

Detection: Photographic plates, fluorescence of certain chemicals, photocells, photoelectric devices.

Uses: UV light produces vitamins (in particular Vitamin D) in the skin and causes sun-tans. Note, though, that UV light is harmful even in modestly large doses. The shorter the wavelength the more dangerous the UV light is. It is used in bacteriology to kill some cells.

Comments: UV light was found shortly after infrared (early 1800s). Much of the UV light emitted by the sun is absorbed by the ozone layer in the Earth's atmosphere. Since our eyes are especially sensitive to UV light, a UV lamp should never be viewed directly. Snow-blindness, which is what skiiers suffer from when skiing on sunny areas, is caused by UV. Manufacturers of washing powders often add fluorescent powders to their products to live up to the claim that their product washes whiter than white, since these powders will absorb UV light and reradiate it as bright visible light.

Visual Light
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Wavelength: 3.5 × 10-7m to 7.5 × 10-7m

Sources: Very hot bodies (progressively red-hot, yellow-hot and then white-hot), discharge lamps (eg, most bulbs), phosphorence and fluorescence of other types of electromagnetic radiation into visible light.

Detection: Photographic plates, photocells, the human eye.

Uses: Cathode ray tubes, which emit light, are used for televisions, computer monitors and the like. LED displays are used for cheap low resolution visual information. LCDs use the reflection of light for a similar goal. Bulbs are used for lighting which human beings and other animals then use as an aid for (amongst other things) location resolving, navigation, communication, and peripheral/accessory movement (eg, lifting cups of tea).

Apart from all the everyday types of visual communication there are a few other less obvious uses for visible light (which do not depend on it's visible to humans property). One is for high-speed fibre optic links, where red light lasers, green light lasers and (in the future) blue light lasers can carry digital data across long distances at very high speed. With the emergence of the internet these high-bandwidth solutions hold the key to global information sharing since the current infrastructure is not be capable of sustaining the emerging traffic.

Comments: The label "Visible" light demonstrates the ego-centricity of the human race. The short side of infrared and the long side of ultraviolet are separated by an extremely short band (relatively speaking) of radiation which is detectable by the human eye. It is unlikely that another race of intelligent beings, if it had a different natively "visible" section, would highlight the small part between the infrared and ultraviolet as being important.

X-Rays
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Wavelength: 10-11m to 10-9m

Sources: The bombardment of targets of heavy atoms (typically tungsten) by fast moving electrons causes energy levels in the target to change. When the target atoms' excited electrons drop back to their original level, they release fixed quanta of electromagnetic energy. (This is called the photoelectric effect). Basically, X-rays are produced whenever electrons are rapidly brought to rest by matter, however only < 0.5% of the electron's kinetic energy gets converted into these X-rays.

Detection: Photographic plates, fluorescence of certain chemicals (eg, barium platnocyanide), ionization chambers (similar to geiger counters but at a higher pressure).

Uses: The most well known use of X-rays is for medical scans. These are commonly known as “x-rays”, this, of course, is incorrect since this is the name of the wave not the method. The method is really called radiography or X-ray photography. This form of detection uses it's fluorescence property.

Another use of X-rays in the medical profession is the use of short wavelength X-rays in a fashion similar to gamma rays for the killing of cancerous cells.

Radiography is also used in industry for the examining of potentially damaged machinery to ascertain the cause of any difficulties, or to verify castings or welded joints prior to distribution.

X-Rays are also used with Bragg diffraction.

Comments: There is no difference between the longest wavelength gamma rays and shortest wavelength X-rays (10-11m). Which name is used usually depends on source and use. X-rays were so called because at first their nature was unknown, for some reason the name stuck once it's nature had been discovered. Short wavelength X-rays are called hard X-rays, long wavelength X-rays are called soft X-rays. X-rays -- just like all eletromagnetic rays -- are not deflected by electric or magnetic fields, and it can thus be deduced that they do not carry a charge.

When X-rays come into contact with atoms they may ionize them (this is cause by the electromagnetic wave's strong electric field). This is why X-rays can be detected in ionization chambers.