Visible white light that humans can perceive is only a small part of the Electromagnetic Spectrum, as described by James Clerk Maxwell, which includes ultraviolet (UV), infrared (IR), x-rays, and radio waves, to name a few. When visible white light is broken down into its components by the process of dispersion, it reveals red, orange, yellow, green, blue, indigo, and violet light. This dispersion is due to the difference in frequency and wavelength between the red and violet components: violet has a shorter wavelength than red and a lower frequency. Therefore, violet is refracted by a greater degree than its preceding components, indigo, all the way to red. The wavelengths of this visible spectrum range from 400 nanometers (violet) to 700 nanometers (red).
There is an inverse relationship between wavelength and frequency; this means, the shorter the wavelength, the greater the frequency, and the greater the energy it possesses. In addition, when light travels through a medium which is more dense than air (such as glass) - it slows down, and then speeds up again when re-entering air - the principle behind refraction. It is worth noting that the ratio between the speed of light in a vacuum (c), 299,792,458 meters per second, and its speed in the medium being used (v), determines the medium's refractive index (n):
Refractive Index (n) = c/v
UV rays, which have a shorter wavelength than the visible spectrum, have a greater frequency and, accordingly, possess greater energy. Additionally, UV rays can be broken down into:
UV-A: Wavelength: 315-399 nanometers
UV-B: Wavelength: 280-314 nanometers
UV-C: Wavelength: 100-279 nanometers
UV-C rays, ultimately the most harmful of the three, are thankfully absorbed by the ozone layer. However, UV-A and some of the UV-B rays can reach the earth and are the cause of skin cancer, cataracts, and many other health issues.
These are useful pieces of information to share with patients, in order to better educate them as to the importance of using premium quality sunglasses. Having the ability to discuss technical details such as these also helps to add to a patient's perceived value of paying a little more to receive their eye care from a trained and knowledgable optical professional, as opposed to purchasing their glasses online.