Laser Safety A 0.75 mW laser emits a narrow beam of light that enters the eye, a

Question

Laser Safety A 0.75 mW laser emits a narrow beam of light that enters the eye, as shown in the figure(Figure 1) .

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

Express your answer using one significant figure.( answer in mJ units)

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?

Express your answer using one significant figure.(answer in kW/cm^2)

C.) Damage to the retina can occur if the average intensity of light exceeds 1.0×10^2W/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

(a) energy E = power * time = 0.75mW * 0.2s = 0.15 mJ

(b) area A = d²/4 = (0.0005cm)²/4 = 1.96e-7 cm²
so avg intensity = power / area = 0.75e-3W / 1.96e-7cm² = 3821 W/cm²

(c) 3821 / 0.01 = 3.8*105

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