EM/Lenses

EM/Lenses

1. Electromagnetic Spectrum

   • 1 Radio
     • 10^3
     • Milli to Kilo metre
     • Largest wavelength
     • Uses
       • Terrestial television
       • MRI
       • Radio
         • A current quickly changing direct in an electrical circuit can generate radio waves. This is the transmitter. At the antenna, radio waves cause oscillations in an electrial conductor to produce a current
     • Dangers
       • None
   • 2 Microwave
     • Smaller wavelengths than Radio
       • And a higher frequency
     • Uses
       • Microwave ovens
         • This use high intensity microwave . The water molecules in the food absorb the microwave radiation and hence gain thermal energy cooking
       • Mobile phone (GPS)
     • Dangers
       • Cause mild internal heating of body cells
     • 10^-2
   • 3 Infrared
     • Larger wavelength than Microwaves
     • All objects radiate infrared
     • Beyond red end of spectrum
     • Uses
       • Remote Controls
       • Cooking
         • The grill glows red when hot.
         • Infrared radiation radiation to cook food
     • 10^-5
   • 4 Visible
     • 700nm - 390 nm
     • Uses
       • Endoscopes
         • A surgeon can look inside patient by sending light through a fibre and reflecting a secind image off a second fibre
       • Fibre Optics
     • Dangers
       • Causes burns
     • 0.5*10^-6
   • 5 Ultraviolet
     • Beyond Violet end of visible spectrum
     • Ritter discovered he found that silver chloride turned black faster when exposed to violet light and even faster under ultra violet light
     • Uses
       • Security marks
         • Special pens or marking can be used Under UV ink fluoresces
       • Kills bacteria
     • Dangers
       • UV can ionise
         • They remove electrons from atoms and cause them to become charged
       • Cause burns
       • Eye damage - e.g. cataracts
     • 10^-8
   • 6 X-Ray
     • Uses
       • X-rays
         • To produce image of bones to check for fractures. Denser areas like bones let fewer x-rays to pass through and appear white
       • Crack detection
     • Dangers
       • More ionising
         • Cell damage
     • 10^-10
   • 7 Gamma
     • Most energetic
     • Shortest wavelength
     • Highest frequency
     • Uses
       • Sterilisation
         • It kills bacteria. Used to kill bacteria in food and surgery tools
     • Dangers
       • Most ionising
     • 10^-12
   • The speed of light = 3*10^8 m/s

2. Reflection

   • Reflection
     • The wave does not enter the material but changes direction at the surface
   • Absorption
     • The wave enters the new material, its energy is transferred into internal energy
   • Transmission
     • The wave passes through perhaps changing speed or direction (refraction)
   • Types
     • Specular
       • A smooth surface reflects the light and it remains orders
     • Diffuse
       • Rough surface scatters the light and do not form clear image
   • Law of Reflection
     • Angle of Incidence = Angle of Reflection

3. Refraction

   • The change in direction of a wave as it enters a new material due to...
     • A change in wavespeed
     • A change in material properties
     • One end of each wave-front arriving first

4. Wavefront

   • A wave-front shows all parts of a wave at the same displacement
     • Normally at troughs or peaks
   • They are at 90* to the direction of energy transfer in transverse waves
   • The distance between wave-fronts equals the wavelength
   • When a wave enters a new material of a higher refractive index
     • Wave speed decreases
     • Wave length decreases
     • wave fronts are close
     • Frequency is constant

5. Refractive Index

   • A ratio that shows how much a material slows down light
     • n=c/v

6. Total Internal Reflection

   • Occurs when a wave no longer passes through the surface of a material it is inside instead it is reflected

7. The Critical Angle

   • TIR only occurs at a certain angle called the critical angle
   • Angle of incidence
     • Less than the critical angle will result in refraction
     • Equal results in no light leaving the material
     • Greater than the critical angle results in reflection

8. Lenses

   • Convex Lenses
     • Focuses parrallel light
   • Concave Lenses
     • Spreads out light
   • Focal Lengths
     • Convex - Distance from middle of lens to where light is focused
     • Concave - Focal length is from the middle of the lens to where the light appears to come from