Physics P1b

P1B

RADIATION AND

THE UNIVERSE

Electromagnetic waves carry energy, not matter

All waves have Wavelength, frequency, Amplitude and speed

Wavelength - the distance from one peak to the next

Frequency - the amount of complete waves per second (measured in Hz)

Amplitude - the height of the wave (from the midline to the peak)

Speed - how fast it goes

Spectrum of EM waves:

Radiowaves, Microwaves, Infrared, Visible light, Ultra violet, X-rays, Gamma rays.

They all travel at the same speed in a vacuum.

Higher frequency=lower wavelength

Wavespeed=frequency x wavelength

EM radiation can be absorbed, reflected or transmitted.

Radiation could be transmitted (pass through) a substance like glass

It could be reflected (bounce back) from a mirror

It could be absorbed (energy is transferred to the matter)

More than one of these things can happen at a time and what happens to the radiation depends on the wavelength of it, what the substance is like and the surface of the substance.

If EM radiation is absorbed it can have 2 effects: 

 - It could heat the substance absorbing it

 - It could set up a tiny alternating current with the same frequency as the EM wave

Some EM radiation is harmful

 - Most types pass through tissue but some can be absorbed and cause heating of    the cells.

 - Some can cause cancerous changes in the cells and some can destroy cells.

Radiowaves

 - Radiowaves are EM waves with wavelengths longer than 10cm

 - Long wave radio (1-10km) can bend around surface of the earth and get around    hills and tunnels

 - Radio waves used for TV and radio are short (10cm-10m) and to get reception      you must be within direct sight of the transmitter

 - Short wave radio signals (10m -100m) can go long distances because they are reflected by the ionosphere.

Microwaves are used for satellite communication

 - For satellite TV, the signal from the transmitter is transmitted into space where        it's picked up by the satellite receiver dish orbiting the earth. It then transmits it        back down to earth where it is received by a satellite dish on the ground.

 - Mobile phone calls travel as microwaves from the phone to nearest transmitter

 - Microwaves are used by remote sensing satellites to 'see' through clouds and        monitor oil spills, track the movement of icebergs and see how much the                  rainforest has changed.

Optical fibres

 - Optical fibres carry information over long distances as pulses of light or IR               radiation.

 - They work by bouncing off the inner core of glass/plastic until it reaches the             other end of the fibre.

 - They work because of total internal reflection. This can only happen when a             wave travels through a dense substance to a less dense substance.

 - If the angle of incidence is....

 - Less than the critical angle then some of the light passes out but only a little bit      is internally reflected.

 - Equal to the critical angle the ray comes out along the surface and there is              internal reflection.

 - Greater than the critical angle then no light comes out it is all internally reflected

Optical fibres can be bent but not sharply.

Higher frequency EM radiation is usually more dangerous

 - The energy of any EM waves is directly proportional to its frequency and so             because the more energy the more damage then the higher the frequency, the       more the energy.

 - Visible light isn't harmful unless it's really bright.

 - Infra-red can cause burns or heatstroke but both are easily avoidable. 

 - Ultraviolet can cause skin cancer because the tissue absorbs radiation and is      made cancerous. Dark skin gives some protection as it absorbs UV radiation

 - X-rays are used in hospitals but can be pretty dangerous. X-rays pass through      flesh but not bones.

 - They can cause mutations which lead to cancer and so people doing them have     to wear lead aprons and stand behind a lead screen

 - Pregnant women shouldn't have x-rays as it could harm the baby.

 - Information is converted into signals before being transmitted. It is then sent long    distances down telephone wires or carried on EM waves.

 - The amplitude and frequency of an analogue signal vary continuously. They can     take any value in a particular range.

 - Examples of analogue devices - dimmer switches, thermometers and                       speedometers.

 - Digital signals are coded pulses that can only have 2 values (on/off    1/2)

 - Both digital and analogue signals weaken as they travel so need amplification on    their route.

 - They can also pick up interference or noise from electrical disturbances.

 - Digital signals are far better quality

An advantage of digital signals is several signals can be submitted at once using just one cable/EM wave.

Although the nucleus holds nearly the entire weight of the atom it is TINY.

Electrons are negatively charged and their paths give the atom its overall size.

Isotopes - Atoms with the same amount of protons but a different number of neutrons.

Usually each element only has one or two stable isotopes.

Other isotopes tend to be radioactive (nucleus is unstable and decays).

Radioactive decay

The nuclei of unstable isotopes break down at random. Completely unaffected by temperature or chemical bonding.

When the nucleus decays it spits out one or more of the three types of radiation - alpha, beta and gamma. 

In the process the nucleus also changes into a new element.

Nuclear radiation causes ionisation by bashing into atoms and knocking electrons off of them. Atoms (no overall charge) are turned into ions (charged).

The further the radiation can penetrate before hitting an atom and getting stopped, the less damage it will do and so the less ionising it is.

Alpha particles are Helium nuclei. They're big, heavy and slow moving. They don't penetrate far and get stopped quickly. They are positively charged and so are deflected by electric and magnetic fields. 

Beta particles are electrons. They come from when a neutron turns into a proton and an electron. For every Beta particle emitted the number of protons go up by 1. They are fast moving and small.They penetrate moderately before colliding and moderately ionising too. They are negatively charged and deflected by electric and magnetic fields.

Gamma rays are very short wavelength EM waves. They're the opposite of alpha particles. No mass, just energy. They penetrate a long way and are weakly ionising. They have no charge so aren't deflected by electric or magnetic fields. 

The radioactivity decreases over time. Each time an unstable nucleus decays and emits radiation it means one more unstable nucleus isn't there to decay later.

The older a radioactive source is, the less radiation it emits.

How quickly it the activity decreases varies a lot. Some isotopes take a few hours before its nearly all decayed, whereas some take billions of years.

Activity never reaches zero which is why measures are in half-life.

Half life is the time taken for half of the present nuclei to decay.

You can work it out by monitoring its count rate (number of atoms which decay per minute).

If you know the half life you know how old the source is.

Sterilising machines use gamma radiation to kill bacteria. It has a long half life so lasts a long time.

Smoke detectors use a weak source of alpha radiation to ionise the air between two electrodes making charged particles with a current. Smoke absorbs radiation so the current stops and the alarm sounds.

Medical tracers use beta or gamma radiation. They penetrate the skin and so are suitable for medical tracers.

It can be injected or swallowed and detected externally. A radioactive source has to have a short half life so less can be used but a reading is still present.

An alpha source is worse than useless as a MT because it would be stopped by the tissues.

Gamma radiation can be used to detect a leak in pipes. It penetrates through but some gets absorbed. If there's a crack then it would be detected. The isotope must be a gamma emitter so it can be detected. It also needs a short half life.

Beta and gamma can penetrate the skin and reach delicate organs which is hazardous.

Alpha can't penetrate the skin but if it's in the body it can cause a lot of damage.

Beta and gamma are less dangerous as they're not as ionising. High doses of radiation can kill cells causing radiation sickness.

Cancer patients are given radiotherapy to kill cells.

Source should be kept in a lead box.

Nuclear workers wear full protective suits to prevent tiny particles being inhaled or lodged in the skin.  Lead suits or barriers are used. They sometimes have to use robot arms.

Light from other galaxies is red shifted. This is because frequency is lower than it should be.

The doppler effect is like when a car moves away and it seems as though the wavelengths are longer near you so the frequency of the sound seems to be lower.

The further away a galaxy is, the greater the red-shift. This means they're moving away from us faster than nearer galaxies. This provides evidence that the whole universe is expanding.

The big bang theory states that all the matter and energy in the universe was compressed into a tiny space then exploded and is continuously expanding.

Age of universe - 13.7 billion years.

Space telescopes have a clearer view than those on earth. This is because the earth's atmosphere gets in the way of normal ones and absorbs all the light.

Pollution (light and air) can reflect and absorb light coming from space so the best place is the top of a mountain where there's less atmosphere.

The best thing to do is put it in space as they can see objects about a billion times fainter than can be seen on earth. Hubble is an optical telescope and has a mirror. Earth based telescopes are however easier to maintain and cheaper.

 - To see a faint, distant object, a big telescope (or lots of small ones) are needed.   - Astronomers like good resolution so the longer the EM wave, the bigger the           telescope needed.                                                                                                   - Radio telescopes need to be large like the large wave.                                        - Optical telescopes detect visible light and look at objects close by and in other        galaxies.                                                                                                                 - X-ray telescopes see violent, high temp events in space like exploding stars but only work in space since the earth's atmosphere absorbs x-rays.