EM Radiation and Energy

EM Radiation and Energy

1. EM Radiation is emitted from a source

   • Any object that emits radiation is called a source
     • The Sun
     • Radio transmitters
     • Mobile phones
   • The frequency of thermal (heat) radiation emitted from an object increases with temperature
   • Once emitted, all types of EM Radiation can travel through space - a vacuum
     • In a vacuum, they travel at the same speed
       • The speed of light
         • Speed o light in vacuum is 300,000 km/s

2. It can be transmitted, reflected or absorbed by something else

   • Transmitted
     • It just keeps going
       • EG. light passes through glass
   • When radiation is emitted from a source, it spreads out
     • Until reaches some matter
       • Air
       • Glass
       • Walls
   • Reflected
     • Bounce back
     • EG. Light reflected off of a mirror
   • Absorbed
     • The photons transfer their energy to the matter
     • EG. sunbather absorbs UV rays from the sun
     • Objects that absorb radiation are called detectors
       • Our eyes are light detectors
     • Radiation can be absorbed by objects a long way from source
       • A parked car warms up in the sun
         • Infrared radiation from sun travels 150,000,000km through space to Earth
           • Some of it is absorbed or reflected by atmosphere
           • Some reaches that car and warms it up
             • It absorbs the thermal (heat) radiation
   • What happens depends on...
     • What substance is like
     • Type of radiation
   • 2 of 3 things can happen at once (transmit, absorb, reflect)
     • EG. sunlight shines on glass
       • A lot of light is transmitted
         • You can see inside your house during the day
         • Some is reflected
           • You can see yourself in shop windows

3. Intensity decreases as distance from source increases

   • When radiation is absorbed, photons transfer their energy
     • The energy 'deposited' by the photons depends on 2 things
       • Amount of photons there are
       • Energy of each photon
       • Total energy = no. of photons X energy of each photon
   • The intensity (or strength)
     • Means how much energy arrives at each square metre of surface per second
     • Units = W/m2 (squared)
       • watts per square metre
   • Intensity of beam of radiation decreases because...
     • The beam spreads out
       • Less photons per square metre
     • The beam gets partially absorbed as it travels
     • EG.
       • Infrared radiation travels from Sun through space and atmosphere
         • It spreads out
           • Some is absorbed or reflected by the atmosphere
             • So radiation is less intense on Earth
               • Than Mercury as it is closer to the sun and has NO ATMOSPHERE
               • Why Pluto is so cold
                 • Furthest away from sun