The Energy in a Light Wave 
Why is it people will pay money to see reflections of light at a laser light show, or even spend time in a tanning booth yet they get paranoid when out on a hot sunny day because they fear skin cancer?   The answer lies in what they've been told. Visible light is harmless, tanning booth light is safe if used in moderation but intense direct sunlight can be harmful. Why is that?

In 1900,  Max Planck (1858-1947), a German physicist, coined the term "photons".  It seems that light can be thought of in two ways. Either as waves with troughs and peaks or as bundles of energy.  Anyone standing on a beach can attest to this.  Have you even been knocked over by a wave?  Max Planck proposed that electromagnetic radiation is emitted in tiny packets of energy which he called "photons".  Each photon pulses with a frequency, v, and travels at the speed of light.

Planck also proposed that photons with very high frequencies carried more energy than ones with lower frequencies.  What he actually said was "the energy of a radiation is proportional to its frequency."  Albert Einstein latter confirmed this and coined a new term, the quantum of energy.

                                      Energy of a photon(E) = hv

where   E = energy       v = frequency     h = Planck's constant = 6.63 X 10-34 J.s


Lets take a look at a particular event in everyday life,  photosynthesis in plants.  Photosynthesis is started when a plant absorbs light. It is the frequency of light that is important. Therefore different kinds of light will have different effects on the efficiency of how plants do photosynthesis. We know that plants grow best in blue light (it just happens to have one of the higher frequencies) and not as well in red light. Plants do not grow well in green light which is a middle frequency because plant pigments do not absorb green light. Plants in fact reflect green light, which is why plants appear green to us humans!

To sum up: the higher the frequency the more energy a photon has.  Since ultraviolet light has a higher frequency than infrared light, ultraviolet light photons each carry more energy than do infrared photons.  Infrared photons warm you, ultraviolet photons burn you, cosmic rays are so tiny and so energetic they pass through you before any damage can be done.
            Go to The Energy in a Light Wave Worksheet