In soft-tissue laser surgery, a highly focused beam of light is used to vaporize

Question

In soft-tissue laser surgery, a highly focused beam of light is used to vaporize tissues with a high water content. Typical values for the beam diameter in such applications is 0.4 mm, while the intensity of the beam is 20 W/cm^2 1. How much energy is transmitted by the laser in a time of 0.3 ms? 2. After the laser is used to make small incisions, the beam can be defocused to increase the beam diameter and then be used to cauterize the surrounding tissues to decrease post-operative swelling. For cauterization an intensity of 50 mW/cm^2 is used. Assuming that the power of the laser remains the same as in part 1, what beam diameter should be used for cauterization? Unpolarized light with intensity of 1400 W/m^2 is incident upon a pair of polarizers. The first polarizer is oriented an angle theta_1 = 9 degree with respect to the vertical, while the second polarizer is oriented an angle theta_2 = 87 degree with respect to the vertical. 1. What is the intensity of the light transmitted through the two polarizers? 2. A third polarizer, oriented at an angle theta_3 = 45 degree with respect to the vertical, is placed between the original two polarizers. The same light as before is incident upon the polarizers. What is the intensity of the light transmitted through the three polarizers?

Explanation / Answer

part 1 )

P =I*A

A = 4pi*r^2 ( it radiates spherically )

r = 0.4 mm = 0.4 x 10^-3 m

I = 20W/cm^2 = 20W/10^-4m^2

P = 0.40 W

E = P*dt

dt = 0.3 ms = 0.3 x 10^-3 s

E = 1.2 x 10^-4 J

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