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# Electromagnetic Spectrum Activity - Visible Light

## Resources

• NASA EM Spectrum Information Pages Visible
• Discovery Education EM Spectrum
• List of SI (metric) prefixes . e.g. nano, milli, mega, tera.

Humans are most familiar with the visible light band of the electromagnetic spectrum because we don't need tools to sense visible light. Newton anchored the scientific understanding of light with his book Opticks, published in 1704.

With shorter wavelengths it is more common to refer to the wave's wavelength rather than its frequency. Visible light covers the range from 390 to 750 nm (nm = nanometers = 1x10-9). The frequency would be expressed in Thz. A high frequency wave has a short wavelength. The relationship is defined as:

• Variables:   λ=wavelength (m)     v = speed of light (3 x 108 m/s)     f = frequency (Hz). Note THz = 1 x 1012
• Formula:   λ = v ÷ f
• Example:   λ = 3 x 108 m/s ÷ 600 THz = 500 nm

Visible light has more energy than infrared but less than ultraviolet light. Visible light is non-ionizing radiation. Ionizing radiation has enough energy to tear electrons away from their atoms. The atoms are now 'ions'. These ions can cause unwanted chemical reactions in a material or organism.

The ability to detect a portion of the EM spectrum depends on the antenna. Your car radio needs an antenna about 18" long for the long wavelengths of radio. For the much shorter wavelengths of visible light, living organisms use molecules! When light strikes special molecules in your eyes the light energy causes the molecules to change shape. A nerve cell detects that shape change and informs the brain. There are different molecules for different wavelengths of light. Humans have three different molecules. If your body can not make one of more these molecules then you are 'color blind' to that wavelength.

Light is emitted by objects due to the movement of electrons and protons in the object. This is called thermal radiation. The incandescent light bulb emits only 10% of its total energy in the visible light range. In what band of EM spectrum does the light bulb emit most of its energy?

Astronomers use the idea of thermal radiation to measure the temperature of stars. If you know the EM wavelength a star emits, you know its temperature! Light pollution is a problem for astronomers. Did you know that Island County has an ordinance prohibiting light pollution ?

Light also pushes on objects it hits. The Japanese space agency in 2010 launched a space craft with a solar sail that will travel to Venus!

## EM Spectrum Data Table

Name Wavelength Range Wavelength
(comparative size)
Frequency Range Energy
ionizing/ med / lo
Sources Uses
Visible Light

## Waves Glossary

Electromagnetic waves
Radiation consisting of waves of energy sent through space and matter associated with electric and magnetic fields resulting from the acceleration of an electric charge (light, x-rays, microwaves, gamma rays, etc.). 3.9
Frequency
The number of back-and-forth cycles per second, in a wave or wave-like process. 3.5
Period
The time for one wavelength to pass a point. The time for a wave to travel the distance of one wavelength. P= 1/frequency. 3.5
Wave
A rhythmic disturbance that transfers energy from place to place. 3.5
Wavelength
The distance measured from crest to crest of one complete wave or cycle. 3.5