Laser Safety A 070 mW laser emits a narrow beam of light tha

Laser Safety A 0.70 mW laser emits a narrow beam of light that enters the eye.

Part A

How much energy is absorbed by the eye in 0.20 s?

ALREADY ANSWERED: E = 0.14mJ

Part B

The eye focuses this beam to a tiny spot on the retina, perhaps 5.0 m in diameter. What is the average intensity of light at this spot in Kw/cm²?

Part C

Damage to the retina can occur if the average intensity of light exceeds 1.0×102W/cm2. By what factor has the intensity of this laser beam exceeded the safe value?

Solution

a) E = power * time = 0.7 mW * 0.20 s

= 0.14 mJ

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b) area A = pi d² / 4 = pi (0.0005 cm)² / 4 = 1.96 * 10^-7 cm²

avg intensity = power / area = 7 * 10^{- 4} W / 1.96 * 10^-7 cm²

= 3571 W/cm²

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c) Factor by which intensity of this laser beam exceeded the safe value

3571 / 0.01 = 357142