If the eye receives an average intensity greater than 1x10^2 W/m^2, damage to th
ID: 1299545 • Letter: I
Question
If the eye receives an average intensity greater than 1x10^2 W/m^2, damage to the retina can occur. THis quantity is called the damage threshold of the retina. A)What is the largest average power (in mW) that a laser beam 1.5 mm in diameter can have an still be considered safe to view head-on? B) What are the maximum values of the electric and magnetic fields for the beam in part A. C) How much energy would the beam in part A deliver per second to the retina? D) Express the damage threshold in W/cm^2
Explanation / Answer
a) power = intensity * area = 100 * pi*0.00075^2 = 0.0001767 Watts = 0.1767 mW
b) E = sqrt(240pi * intensity) = sqrt (240pi * 0.0001767) = 0.365 V/m
c) B = E/c = 0.365 / 3x10^8 = 1.22 x 10^-9 Tesla
d) energy = power * time = 0.0001767 W * 1 sec = 0.0001767 Joules
e) 100 W/m^2 = 0.0100 W/cm^2