Electro Magnetic Radiation

The human visible spectrum of light represents a small section of the full range of light which is called the Electro-Magnetic Spectrum (EMS).

The EMS, in-total, can be described as Electro-Magnetic Radiation (EMR).

In the graphic below you can see that visible light represents only a small portion of the full EMS (ranging from roughly 400 nanometers of wavelength to about 750).  

Picture1 – Electromagnetic Spectrum

Picture2 – Electromagnetic Spectrum

Note in the picture above how as lambda gets smaller (λ1 to λ2 to λ3) the frequency (the number of wavelengths counted in one second ie Hertz = cycles per second) gets bigger. 

Picture3 – Electromagnetic Spectrum

EMR Has Important Characteristics

• Electro-Magnetic Radiation, EMR, is radiation energy which travels in the form of electro-magnetic waves.
• EMR is (mathematically)  both a wave and a particle (a photon, a massless unit of energy).
  • Introduction to Light – Khanacademy.org
• Electro-Magnetic Radiation, EMR, has oscillating electric and magnetic fields
  • Professor Dave Explains – What is Light?
• As a wave, EMR has a wavelength (denoted with lambda λ; crest to crest or trough to trough distance) and frequency (denoted as nu ν; number of waves passing a point per unit time).
  • Speed of light c is fixed and equals wavelength x frequency: c = λν
  • Each photon at a certain wavelength will have a unique energy defined as E = hν:  Planck’s Equation
  • Substituting for v,  E = hc/λ ; Planck’s Equation
  • High frequency means lower wavelength and vice versa
  • Higher frequency means higher energy
  • Higher or larger wavelength means lower energy
  • From picture2 above you can see that relative temperature associated with a body that is emitting that radiation increases with increasing frequency ν and decreasing wavelength λ. 

Picture4 – Frequency and Wavelength

• The earth’s average surface temperature is about 14.85 C = 58.7 F = 288 Kelvin. It is an infrared wave radiator. 
• The sun’s average surface temperature is 5597 C = 10,106 F = 5870 K. It is a UV, Visible, and Infrared wave radiator. 
• In climate science a demarcation of about 4 microns wavelength is used to define shortwave radiation (or solar radiation) from longwave radiation (or terrestrial radiation).
  • The sun’s electromagnetic radiation range covers UV 7%, Visible 44%, Infrared 49% with 95% of energy coming from wavelengths between .25 and 2.5 microns.
  • For earth virtually all the radiation is infrared. Over 95% of Earth’s radiation has a wavelength between 2.5 and 25 microns. 
• These waves can travel through a vacuum (it doesn’t need a medium like sound would for example). 
• EMR moves at the speed of light which is super fast (would go around the earth’s equator almost 7.5 times in one second). Nothing can go faster by the way.
  • In a vacuum light travels at
  • 299,792,458 meters/second 
  • 670,616,629 miles/hour
  • 1,079,252,848.8 kilometers/h 
  • 186,282 miles/second approximately

• Sunlight is mostly in the Infrared, Visible, and Ultraviolet parts of the electro-magnetic spectrum

• In the 1600s, Isaac Newton demonstrated that sunlight “contains” the visible spectrum by bending (refracting) light and then “recombining” light through a series of two prisms.
  • By “contains” we mean: possesses the range of  electro-magnetic wavelengths that we see as the rainbow colors.
• Newton probably coined the word Spectrum. It comes from the Latin for ghost , apparition or specter. 
• Visible light has wavelengths between 400 and 750 nanometers.
  • A nanometer or nm is 1 billionth of a meter.  
• In the visible region,
  • Violet light has the shortest wavelength (highest frequency) and
  • Red light has the longest wavelength (lowest frequency). 
• Color doesn’t exist outside our bodies.
  • Electro-Magnetic Waves are sensed and translated by our eyes and brains into colour. 
• The Scot, James Clerk Maxwell (1831 – 1879), and the German, Heinrich Rudolph Hertz (1857 – 1894), did pioneering work in this area.