Laser Safety A 0.85 mW laser emits a narrow beam of light that enters the eye, a

Question

Laser Safety A 0.85 mW laser emits a narrow beam of light that enters the eye, as shown in the figure .

d=5 mu(m)

Part A
How much energy is absorbed by the eye in 0.20 {s}?
Express your answer using one significant figure.
E = mJ


Part B
The eye focuses this beam to a tiny spot on the retina, perhaps 5.0 mu {m} in diameter. What is the average intensity of light at this spot?
Express your answer using two significant figures.
I = kW/cm^2


Part C
Damage to the retina can occur if the average intensity of light exceeds 1.0 x 10^{ - 2}, {W/cm}^2. By what factor has the intensity of this laser beam exceeded the safe value?

Explanation / Answer

Part A)

The laser power is .85 X 10-3 W

P = Energy/time

Energy = (P)(t) = (.85 X 10-3)(.2) = 1.7 X 10-4 W which is .17mW

Part B)

Intensity = Power/Area         Area = r2 = ()(2.5 X 10-6)2 = 1.96 X 10-11 m2

I = (.85 X 10-3)/(1.96 X 10-11)

I = 4.33 X 107 W/m2 = 4.33 X 103 W/cm2 = 4.33 kW/cm2

Part C)

This value is over the max safe level by...

4.33 X 103/1 X 10-2 = 4.33 X 105 times

So this exceeds the safe level by a factor of 4.33 X 105

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