Do you enjoy going to the beach?
During the summer, almost everyone enjoys going to the beach. They can swim, have picnics, and work on their tans. But if you get too much sun, you can burn. A particular set of solar wavelengths are especially harmful to the skin. This portion of the soar spectrum is known as UV B, with wavelengths of 280320 nm. Sunscreens are effective in protecting the skin against both the immediate skin damage and the longterm possibility of skin cancer.
Waves
Waves are characterized by their repetitive motion. Imagine a toy boat riding the waves in a wave pool. As the water wave passes under the boat, it moves up and down in a regular and repeated fashion. While the wave travels horizontally, the boat only travels vertically up and down. The Figure below shows two examples of waves.
A wave cycle consists of one complete wave – starting at the zero point, going up to a wave crest, going back down to a wave trough, and back to the zero point again. The wavelength of a wave is the distance between any two corresponding points on adjacent waves. It is easiest to visualize the wavelength of a wave as the distance from one wave crest to the next. In an equation, wavelength is represented by the Greek letter lambda \begin{align*}(\lambda)\end{align*}
Figure B above shows an important relationship between the wavelength and frequency of a wave. The top wave clearly has a shorter wavelength than the second wave. However, if you picture yourself at a stationary point watching these waves pass by, more waves of the first kind would pass by in a given amount of time. Thus the frequency of the first waves is greater than that of the second waves. Wavelength and frequency are therefore inversely related. As the wavelength of a wave increases, its frequency decreases. The equation that relates the two is:
\begin{align*}\ c = \lambda \nu\end{align*}
The variable \begin{align*}c\end{align*}
Sample Problem: Wavelength and Frequency
The color orange within the visible light spectrum has a wavelength of about 620 nm. What is the frequency of orange light?
Step 1: List the known quantities and plan the problem.
Known
 wavelength \begin{align*}(\lambda)\end{align*}
(λ) = 620 nm  speed of light \begin{align*}(c)\end{align*}
(c) = 3.00 × 10^{8} m/s  conversion factor 1 m = 10^{9} nm
Unknown
 Frequency
Convert the wavelength to m, then apply the equation \begin{align*}c=\lambda\nu\end{align*}
\begin{align*}\nu=\frac{c}{\lambda}\end{align*}
Step 2: Calculate
\begin{align*}620 \ \text{nm} \times \left(\frac{1 \ \text{m}}{10^9 \ \text{nm}}\right) &=6.20 \times 10^{7} \ \text{m}\\
\nu = \frac{c}{\lambda}=\frac{3.0 \times 10^8 \ \text{m/s}}{6.20 \times 10^{7} \ \text{m}} &=4.8 \times 10^{14} \ \text{Hz}\end{align*}
Step 3: Think about your result.
The value for the frequency falls within the range for visible light.
Summary
 All waves can be defined in terms of their frequency and intensity.

\begin{align*}c=\lambda\nu\end{align*}
c=λν expresses the relationship between wavelength and frequency.
Practice
Question
Read the material on the link below and answer the questions as they come up:
http://www.absorblearning.com/physics/demo/units/DJFPh064.html
Review
Questions
 Define wavelength.
 Define frequency.
 What is the relationship between wavelength and frequency?