Question
(a) What is the intensity in $\textrm{ W/m}^2$ of a laser beam used to burn away cancerous tissue that, when 90.0% absorbed, puts 500 J of energy into a circular spot 2.00 mm in diameter in 4.00 s? (b) Discuss how this intensity compares to the average intensity of sunlight (about $700 \textrm{ W/m}^2$) and the implications that would have if the laser beam entered your eye. Note how your answer depends on the time duration of the exposure.
1. $4.42 \times 10^7 \textrm{ W/m}^2$
2. The laser is more intense than the sun by a factor of 63200. Looking directly at the sun can damage the eye within seconds. The laser would deposit the same damaging amount of energy in the eye in a time that is shorter by a factor of $\dfrac{1}{63200}$, or approximately 0.50ms.
Solution Video