Problem 25.29 - MC A typical helium-neon laser found in supermarket checkout sca

Question

Problem 25.29 - MC

A typical helium-neon laser found in supermarket checkout scanners emits 633-nm-wavelength light in a 1.5-mm-diameter beam with a power of 1.5 mW .

Part A

What is the intensity of the beam?

I = 848.8 W/m2

Part B

Damage caused to human eyes by laser light is dependent on the frequency of the laser, as well as the exposure time. For red laser light, a typical value for the amount of energy per square meter that would cause damge is 10J/m2. How long could someone stare directly at this supermarket checkout laser before damage occurs?

Explanation / Answer

A)Intensity is given by

I = P/A

I = 1.5 x 10^-3/(3.14 x 0.00075^2) = 848.8 W/m^2

Hence, I = 848.8 W/m^2

B)E/A = 10 J/m^2

P = E/t ;

I = P/A = E/tA

t = E/IA = 1/I (E/A)

t = 1/848.8 x 10 = 0.012 s

Hence, t = 0.012 s = 12 ms

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