Laser Safety A 0.65 mW laser emits a narrow beam of light that enters the eye, a

Question

Laser Safety A 0.65 mW laser emits a narrow beam of light that enters the eye, as shown in the figure(Figure 1) .

Part A

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

Express your answer using one significant figure. mJ

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?

Express your answer using two significant figures. KW/cm^2

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?

Express your answer using two significant figures.

Explanation / Answer

According to the question;

We know that,

power of the laser beam, P=0.65 mW

A)

time, t=0.2 sec

enegry absorbed, E=P*t

From using formula

E=0.65*10^-3*0.2

E=0.13 mJ

Then

B)

diameter of the retina, d=5um

radius of the ratina, r=2.5 um

intensity I=P/A

From using formula

I=0.65*10^-3/(pi*(2.5*10^-6)^2)

I=331.04*10^5 w/m^2

C)

maximum avg intensity

I'=1*10^-2 w/cm^2 =100 W/m2

I/I'=331.04*10^5/(100)

I/I' =3.31*10^5

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