Physics 1b

electro magnetic waves carry energy but not matter

wavelength- is the distance from one peak to another

Frequency- how many complete waves per second measured in hertz

Amplitude-height of the wave

Speed-how fast it goes.

em waves with different wavelengths have different properties

all types of electromagnetic wave travel at the same speed in a vacuum space

Em waves with higher frequencies have lower wavelengths

wave speed = frequency x wavelength  speed = frequency x wavelength

Em radiation can be absorbed, reflected or transmitted

the radiation might be transmitted-pass through the substance

could be reflected-bounce back, like light reflected from a mirror

could be absorbed - energy of wave is transferred to the matter

when em radiation enters living tissue its harmless  --------some radiation passes through soft tissue without being absorbed-radio waves      other types are absorbed and cause heating of the cells -microwaves        some radiations cause cancerous changes in living cells - uv can cause skin cancer                          some radition can destroy cells ------radio waves are used mainly for communications long waves can bend around curved surfaces of the Earth ------ radio waves used for TV and radio  have very short wavelengths and be recieved across long distances because they are reflected from th ionosphere

Microwaves are used for satellite communications pass easily through the Earth's atmosphere .--------- satellite tv the signal from a transmitter is transmitted into space when it is recieved by the satellite dish orbiting above the Earth. The satellite transmits the signal back to Earth where it is recieved  by a satellite disch on the ground.

mobile phone calls travel as microwaves from the phone to the nearest transmitter.

Optical fibres use visible light or infrared ----- optical fibres can carry data over long distancesas pulses of light or infra red. Work by bouncing off the sides of a thinner inner core of glass or plastic. The waves enters the fibre and is reflected repeatidly until it emerges at the other end

optical fibres work because of total internal reflection, which can only happen when a wave travels through a dense substance like glass, water, perspex towards a less dense substance - air 

If the angle of incidence is less than the critical angle most of the light passes out  but a little of it is internally reflected

equal to critical angle the emerging ray comes out along the surface, a small amount of internal reflection

greater than the angle of incidence no light comes out its all internally reflected, total internal reflection.

The main types of electromagnetic radiation

frequencytype of electromagnetic radiationtypical usewavelength highest gamma radiation killing cancer cells shortest X-rays medical images of bones ultraviolet detecting forged bank notes by fluorescence visible light seeing infrared optical fibre communication microwaves cooking lowest radio waves television signals longest

Gamma waves have a very high frequency. Gamma radiation cannot be seen or felt. It mostly passes through skin and soft tissue, but some of it is absorbed by cells.

Gamma radiation is used, among other things, for the following purposes.

• to sterilise surgical instruments
• to kill harmful bacteria in food
• to kill cancer cells (note that lower doses of gamma radiation could lead to cells becoming cancerous)

X-rays have a lower frequency than gamma radiation. Like gamma rays, they cannot be seen or felt. X-rays mostly pass through skin and soft tissue, but they do not easily pass through bone or metal.

X-rays are used to produce photographs of bones to check for damage such as fractures. They are also used in industry to check metal components and welds for cracks or other damage.

Lower doses of X-rays can cause cells to become cancerous, so precautions are taken in hospitals to limit the dose received by patients and staff when X-ray photographs are taken.

Ultraviolet radiation is found naturally in sunlight. We cannot see or feel ultraviolet radiation, but our skin responds to it by turning darker. This happens in an attempt to reduce the amount of ultraviolet radiation that reaches deeper skin tissues. Darker skins absorb more ultraviolet light, so less ultraviolet radiation reaches the deeper tissues. This is important because ultraviolet radiation can cause normal cells to become cancerous.

Ultraviolet radiation is used in:

• sun beds
• security pens
• fluorescent lights (coatings inside the tube or bulb absorb the ultraviolet light and re-emit it as visible light)

Infrared radiation is absorbed by the skin and we feel it as heat. It is used in heaters, toasters and grills. It is also used for television remote controls and in optical fibre communications.

• analogue vary, the amplitude and frequency of an analogue signal vary continuosly take any value
• dimmer switches,thermometers --analogue

digital on or off are coded pulses can only take two values

• weaken as they travel, need to be amplifide along route
• pick up interference or noise from electrical disturbances
• noise doesn't affect digital signals
• digital dignales much higher quality information recieved is same as original
• digital signals easy to process using computers
• digital can transmit several signals at once so can send more information than analogue signals