What you’ve always wanted to know about light

“Under the trees light

has dropped from the top of the sky,

light

like a green

latticework of  branches

shining

on every leaf

drifting down like clean

white sand”

 –Ode to Enchanted Light by Pablo Neruda

 Light is everywhere. It shines through our windows, illuminates our bedrooms, and reflects off falling autumn leaves. We see light every day (albeit not as much as would be nice here in New England). But what is it?

 What is light?

Light can be scientifically explained in two ways. On one hand, visible light, along with UV light, X-rays, and other familiar forms of radiation, are electromagnetic waves–that is, waves composed of electric and magnetic fields. But at the same time, quantum mechanics describes light as composed of particles known as photons. These seemingly unrelated explanations are actually fundamentally compatible, and modern scientists accept light as a particle/wave duality (Harris and Freudneric). This article will focus primarily on the wave nature of light, as it is simpler to comprehend, and the wave explanation accounts for light’s most important properties.

light
Light through the trees on the Charles river. Image by Jessi Glueck.

What are these electric and magnetic fields that constitute light?

 Simply put, an electric or a magnetic field is a region in space where a charged particle will experience an electric or a magnetic force, respectively. At each point in space, these fields can be said to have a magnitude and an orientation, which describe the strength and direction in which a particle at that point would experience the force. Electromagnetic waves are simply electric and magnetic fields that oscillate in strength and direction over space, and that propagate over time, much like a wave in the ocean (“Electromagnetic Waves”).

Why is it that these electromagnetic fields form waves?

This wave behavior follows from two key properties of electric and magnetic fields. The first is that a magnetic field that changes in time will produce an electric field. This is actually the principle behind how generators work—a rotating magnet creates a changing amount of magnetic field passing through a wire loop, which generates current. The second crucial property behind electromagnetic waves is simply the converse of the first—changing electric fields produce magnetic fields.  We can mathematically express these crucial properties of the fields (in the form of the famous Maxwell’s equations) and slightly manipulate these equations. If we do this, we arrive at the conclusion that the orientation and  movement of electric and magnetic fields in space  must, in certain cases, satisfy a creatively-named equation called the wave equation. This is the same equation that describes how vibrations manifest and travel along a string, or in the Atlantic ocean. (“Maxwell’s Equations”).

 Why should we care that light is electromagnetic in origin?

While electromagnetic waves behave in many ways like waves that we are used to, they also have some weird quirks. Perhaps the most interesting feature of EM waves is that they are self-propagating, and thus do not require a medium to travel through. In effect, a changing electric field at one point induces a magnetic field at further ahead in space, which in turn induces an electric field, etc. (“Electromagnetic Waves”). This self-propagation allows electromagnetic waves to travel through empty space, in regions where otherwise, electric and magnetic fields could not exist because there is no charge to produce them (Harris and Freudneric). This is the reason why light produced by the sun actually makes it through the vacuum of space and arrives on earth! So when you open your blinds tomorrow and (hopefully) find it’s a sunny day, be sure to acknowledge just how big of an impact two seemingly obscure force fields have on our daily lives.

This piece is part of a series of short, biweekly(ish) columns on topics of general interest, including common-sense explanations of complex scientific phenomena. Stay tuned for more! 

Amir Bitran is a Primary Research Editor for Brevia. He can be reached at amirbitran@college.harvard.edu 

Works Cited

“Electromagnetic Waves.” Hyperphysics. Georgia State University. Web. 16 October 2014. <http://hyperphysics.phy-astr.gsu.edu/hbase/waves/emwavecon.html>.

Harris, William, and Freudneric, Craig. “How Light Works” How Stuff Works. Web. 16 October 2014. < http://science.howstuffworks.com/light.htm>.

 “Maxwell’s Equations.” Hyperphysics. Georgia State University. Web. 16 October 2014.< http://hyperphysics.phy-astr.gsu.edu/hbase/electric/maxeq.html>.

Neruda, Pablo. “Ode to Enchanted Light.” Peaceful Rivers–Wisdom For The Journey. Web. 17 October, 2014. <http://peacefulrivers.homestead.com/PabloNeruda.html#anchor_16108>