Laser Safety A 0.95 mW laser emits a narrow beam of light that enters the eye, a

Question

Laser Safety A 0.95 mW laser emits a narrow beam of light that enters the eye, as shown in the figure (Figure1) How much energy is absorbed by the eye in 0.20 s? Express your answer using one significant figure. 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. Damage to the retina can occur if the average intensity of light exceeds 1.0 Times 10^-2 W/cm&2. By what factor has the intensity of this laser beam exceeded the safe value? Laser Safety A 0.95 mW laser emits a narrow beam of light that enters the eye, as shown in the figure (Figure1) 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. Damage to the retina can occur if the average intensity of light exceeds 1.0 Times 10^-2 W/cm^2. By what factor has the intensity of this laser beam exceeded the safe value? Express your answer using two significant figures.

Explanation / Answer

Part A

calculate energy

Expression power

P = W/t

W = P t

W = 0.95 10-3 0.20

W = 0.19 10-3 J

The work is equal to the variation of kinetic energy.

As we ask in mJ result, we reduce the units

W = 0.19 10-3 J (103 mJ/J)

W = E = 0.19 mJ

Part B)

The intensity or fluence is given by the expression

I = P/ A

A = R2= (D/2)2

D = 5 m = 5 10-6m 102(cm/1 m) = 5 10-4cm

A = (5 10-4/2)2

A =19.63 10-8 cm²

I = 0.95/(19.63 10-8)

I = 4.84 106W/cm²

reduce W to KW

I = 4.84 103KW/cm²

Part C)

I = 4.84 106W/cm²

Io = 1.0 10-2 W/cm²

Factor= I/Io

Factor= 4.84 106 /1.0 10-2

Factor = 4.84 108

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