Some of our deepest scientific insights have come from the most basic of questions. For our last lesson of the week, we will dig into the question: What is light?

The short answer to the first question (what is light?) is that light is what we experience as a narrow band of waves of specific wavelengths within the visible part of the electromagnetic spectrum. A particle of light is called a **photon**. Visible photons (light) have properties of both a particle and a wave. Photons travel in waves, and waves can be described mathematically by measuring wavelength, amplitude, period, frequency, and speed.

To visualize the parts of a wave, let’s bring in Bill Nye the Science Guy:

Thank’s Bill Nye! Here’s what we learned:

In the vacuum of space, nothing moves faster than light. In fact, we can say the speed of light is the cosmic speed limit. In a vacuum (like space), light travels at 300 million meters per second (3.0 x 10^{8} m/s). Because this number does not change, it is a constant and is assigned the letter *c* (*c* = speed of light). Side note: Thanks to Albert Einstein, you’ve probably heard of the equation *E = mc ^{2}. *In words, the equation says that energy (

*E*) is equal to mass (

*m*) times the speed of light (

*c*) squared. You already knew that

*c*= speed of light!

Ever wondered how long it takes for light from the Sun to reach Earth? Click here to work through the math and find out!

To complete our study of the properties of light, we need to introduce Planck’s constant (*h*):

Next, let’s revisit the parts of a wave and make some connections:

The notes above introduce Planck’s constant, h, which has units of Joule • seconds. Planck’s constant (h) relates a photon’s energy (E) and frequency (f). Frequency is defined as the number of complete waves that pass through a point in one second. The faster a light wave is traveling (greatest speed, measured in meters per second, m/s), the higher the frequency (f, waves/second). Therefore, the faster a light wave is traveling, the higher the energy (E) of the wave. Energy has units of Joule • meters. Waves with the shortest wavelengths (λ) have the highest frequency (f) and therefore have the greatest energy (E).

If you followed all that (and I have no doubt you did!) you are ready for an introduction to Quantum Mechanics. Ars Technica has a fantastic 7-part series of articles focusing on Exploring the Quantum World. The Crash Course videos below are well worth the watch as well. Enjoy!

Return to Week 38 – Light and Color and continue working.

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