NOTE: Light waves are electromagnetic waves, NOT mechanical waves; however, the principles of energy and inten

Question

NOTE: Light waves are electromagnetic waves, NOT mechanical waves; however, the principles of energy and intensity for electromagnetic waves and mechanical waves are exactly the same. The following problem illustrates this idea. The problem could just as well have been a problem of sound with a given intensity falling upon your eardrum for a specified amount of time. GBA


Light of intensity 3 W/m2 passes through the pupil of one of your eyes and eventually falls on your retina. The radius of the pupil is controlled by the iris. In a human eye, the radius of the pupil can vary from about 1.0 mm to about 3.5 mm. The area of the opening can be calculated from pi (3.1416) times the radius squared. If your pupil is open to a radius of 3.5 mm, how much light energy passes through in one minute? Give your answer in mJ.

?mJ

Answer 1

The area of your iris is
A = pi* r ^2
=3.1459 * (0.0035 m)^2

Light Intensity=LI = 3 W/m^2

Duration= D = 1 min= 60 sec.

Energy = A*LI*D
=3.1459 * (0.0035 m)^2*3 W/m^2*60 sec.
=.00693 J
=6.93 mJ