Light and the Electromagnetic Spectrum

Light and the Electromagnetic Spectrum

1. CP5a

   • We see things when light travels from a source and is reflected by an object into out eyes.
     • The light transfers energy from the source to our eyes
     • Our eyes can detect certain frequiencies of light
       • We refer to these frequencies as visible light
   • Light is a type of electromagnetic wave
   • Different frequencies cause us to see different colours
     • Lower frequencies of visible light appear more red and higher frequencies appear more blue
   • All electromagnetic waves are transverse waves
   • Because electromagnetic waves are transverse this means the vibrations are at right angles to the direction in which the wave is travelling
     • All electromagnetic waves travel at the same speed: (3 x 10 to the power of 8 m/s)
   • Like all waves, electromagnetic waves transfer energy from a source to an observer
   • Electromagnetic waves with frequencies slightly lower than visible light are called infrared
     • All objects emit energy by infrared radiation
     • We can feel the effects of infrared radiation when energy is transferred from the Sun to our skin
   • Electromagnetic waves move at different speeds in different materials
     • Waves that pass at an interface (boundary) between two materials get faster or slower
     • If waves meet an interface (boundary) at an angle the change of speed also makes them change direction
       • The change of direction is called refraction
       • If they meet the interface (boundary) at right angles they do not change direction
   • The first person to discover infrared was William Herschel
     • He put dark coloured filters on his telescope to help him observe the sun safely and noticed that different coloured filters heated the telescope to different extents

2. CP5b

   • Our eyes can detect different colours in visible light and scientists describe the seven colurs in the visible light spectrum in this order:
     • Red, orange, yellow, green, blue, indigo, violet (ROY G BIV)
   • If the frequency of a wave is lower than that of red light, human eyes cannot see it
     • Infrared, microwaves and radio waves have lower frequencies than red light
     • Ultraviolet light has a higher frequency than visible light
       • Even higher frequencies and shorter wavelengths are present in x-rays and gamma rays
   • The full range of electromagnetic waves is called the electromagnetic spectrum
     • The spectrum is continuous so all values of frequency are possible
       • Higher frequency waves have shorter wavelengths and lower frequency waves have longer wavelengths
   • Stars and other space objects can emit energy at all wavelengths.
     • Scientists study this radiation but have to use special telescopes because different materials affect electromagnetic waves depending on the wavelength

3. CP5c

   • Visible light is the part of the electromagnetic spectrum that our eyes detect
     • Light bulbs are designed to emit visible light, while cameras detect it and record images
   • Infrared radiation can be used for communication at short ranges
     • Infrared can be used between computers in the same room or from a TV to its remote control unit
       • Information sent along optical fibres is also sent using infrared radiation
       • A grill or toaster transfers energy to food by infrared radiation
         • The food absorbs the radiation and heats up
   • Thermal images show the amount of infrared radiation given off by different objects
     • Security systems often have sensors that can detect infrared radiation emitted by intruders
       • Some buildings are fitted with systems of infrared beams and detectors and when someone walks through one of the beams will break it and set off an alarm
   • Microwaves are used for communications and satellite transmissions, including mobile phone signals
     • In a microwave oven, microwaves transfer energy to the food, heating it up from the inside
   • Radio waves are used for transmitting radio broadcasts and TV programs as well as other communications
     • Some radio waves are sent via satellites so controllers on the ground communicate with spacecraft suing radio waves
     • Radio waves are produced by oscillations (variations in a current and voltage) in electrical circuits
       • A metal rod can be used as an aerial to receive radio waves
         • Waves travel in straight lines unless they are reflected or refracted

4. CP5d

   • Ultraviolet radiation transfers more energy than visible light
     • It is absorbed by most of the same materials that absorb visible light, including our skin
       • The energy transferred can be used to disinfect water by killing microorganisms in it
     • Some materials absorb ultraviolet light and re-emit it as visible light, this is called florescence
       • Fluorescent materials are often only used in security markings - they are only visible when ultraviolet light shines on them