physics
posted by rick .
The human eye is most sensitive to light having a frequency of about 5.3*10^14 Hz, which is in the yellowgreen region of the electromagnetic spectrum. How many wavelengths of this light can fit across the width of your thumb, a distance of about 2.0 cm?

First you need to calculate the wavelength L of that frequency of light, using the formula
L = (speed of light)/frequency
Use c = 3.00*10^10 cm/s for the speed of light
L = (3.00 cm/s)/(5.3*10^14 s^1)= 5.66*10^5 cm
Divide your thumb's width by that wavelength to get the number of waves. 
Speed of light is 3x10^8 m/s and wavelength is 5.3*10^14, so you can use the equation
v=lambda(frequency) to find the frequency, and use the speed of light for your velocity.
Then just divide the width on your thumb by the wavelength