Electromagnetic Radiation

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

The EMS, in-total, can be described as Electromagnetic 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).  

• A mnemonic for memorizing the colors from longest wavelength to shortest is ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet)
• A mnemonic from short to long wavelength color is VIBGYOR

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 (Vertical View)

EMR Has Important Characteristics

Electromagnetic Radiation, EMR, is radiation energy which travels in the form of electromagnetic waves.

EMR is (mathematically)  both a wave and a particle (a photon, a massless unit of energy).

• Introduction to Light – Khanacademy.org

Electromagnetic 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

Read my posts: Global Warming Primer  and Sunlight on Earth – Black Nights , Blue Days

• 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. 

• Sunlight is mostly in the Infrared, Visible, and Ultraviolet parts of the electromagnetic spectrum 

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 
• or 670,616,629 miles/hour
• or 1,079,252,848.8 kilometers/h 
• or 186,282 miles/second approximately

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  electromagnetic  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).

Picture: VIBGYOR or ROYGBIV – The Visible Spectrum

Color doesn’t exist outside our bodies

• Electromagnetic Waves are sensed and translated by our eyes and brains into colour. 
• See my post A Color Primer (Including How to Create an Effective Colour Palette) for more on this.
• Also refer to my posts on vision which you can access via this post:  Understand Your Eyes

The Scot, James Clerk Maxwell (1831 – 1879), and the German, Heinrich Rudolph Hertz (1857 – 1894), did pioneering work in this area. 

• Faraday’s 1831 Electromagnetic Experiments
• See my posts on electricity also