Question

In our Physics lab we have a 1 milliwatt (0.001W) helium neon laser. Despite the low power, we were cautioned not to even look at reflections of the beam as it could cause permanent eye damage - why is such a low power level hazardous? I have a 3W LED which I can look directly into which is more than 3,000x more powerful.

Answer 1

the diameter of the beams of these HeNe lasers is between 0.5 and 1 millimeter, so the power 1 mW is coming to squared meters or so. The ratio of power and area is Watts per squared meter.

On the other hand, when a 3W LED is watched from the distance 0.1 meters, the power of 3 W is divided to squared meters, so the power per units area is 3/0.13 = 23 Watts per squared meter. In both cases, the eye will refocus the beam so the power per unit area of the retina will actually be much higher in both cases.

In this estimate, the laser has about 40 times higher power per unit area than the LED light bulb. Of course, it's small, so it will only burn one small point in your retina, but that's a bad enough problem. ;-)