Light

When we say the word “light” we usually refer to Visible Light. Visible Light is a section of the spectrum of Ultraviolet Radiation.

We might refer to infrared light or ultraviolet light–these two segments border the section of light that we can see.

A single unit of light is a Photon. When you walk into a room with light in it, you see things because photons are hitting your eyes. Every photon has a wavelength and if the wavelength is between 700 nm (Red) and 380 nm (Violet) then you will see it when it strikes your eye. These two numbers are the extremes, approximately, to what you can see. Wavelengths corresponding to what we consider to be the typical color, are shown below:

  • 650 nm (Red)
  • 600 nm (Orange)
  • 580 nm (Yellow)
  • 550 nm (Green)
  • 450 nm (Blue)
  • 400 nm (Violet)

Wavelength is measured as a distance. Wavelength corresponds to frequency through the following relation:

\lambda \: \nu = c

The speed of light is 299 792 458 m/s, which is a little less than 300,000,000 m/s. You might find the second number easier to read, and to use in a calculation.

Let’s use Dimensional Analysis and convert the wavelength we gave for red light to frequency.

650 nm * \dfrac {1 m} {1 x 10^9 nm} * \dfrac {299,792,458 m} {s} = 461219166.2 MHz

This is 461,219,166,200,000 Hz and we can change that to 461.2 THz (the unit being “terahertz”).

There is more to this topic, and we hope to add said content soon:

The energy of light can be related to its frequency using Planck’s constant.

Appendix A : More Colors

Some people list Cyan at 500. But if we include that then we should probably mention Chartreuse at 565 nm.

And then a Mockingbird student found the following: Vermillion is between Red and Orange; Saffron is between Orange and Yellow; Chartreuse is between Yellow and Green; Turquoise is between Green and Blue; Indigo is between Blue and Violet.

Just as a quick heads up, the name “Cyan” is used much more commonly than “Turquoise”.

Appendix B: Light at Speeds Slower than the Speed of Light

It might (or probably will) seem confusing when you see a calculation that suggests that light moving through a solid moves at speeds slower than the value we call “the speed of light”. Let us try to clear up the confusion.

Imagine you drive a car that always goes at 100 km per hour, and you drive across a 100 km turnpike. If you drive straight through without stopping, the person at the other side looks at your travel time, 1 hr, and calculates that you traveled at a speed of 100 km/hr.

Now, if you stop at 3 rest stops and spend 10 minutes at each rest stop, when you come to the end of the turnpike your travel time will be 1 hour 30 minutes. The person calculates “100 km divided by 1.5 hours equals 67 km/hr”, hence we can explain why the reported speed is less than 100. Light going through a solid can be absorbed by atoms and later re-emitted, thus those absorptions slow it down similar to what rest stops did to you.