# Laser beam intensity

CMATT
Thread moved from the technical forums -- poster has been reminded of the PF rules
What is the intensity in W/m² of a laser beam used to burn away cancerous tissue that, when 94.0% absorbed, puts 539 J of energy into a circular spot 2.11 mm in diameter in 4.00 s??

I've tried a couple different equations and I keep getting the answer wrong. This is due tonight, I'm super stuck. Any input is greatly appreciated

Gold Member
First imagine the beam is 100% absorbed, find that answer, and multiply that answer by 1/.94 to get the answer for 94% absorbed. To get the 100% absorbed answer, just take that energy, per that area, per that time. You will have to convert mm to m though, and you will need to know the area of a circle.

• CMATT
CMATT
First imagine the beam is 100% absorbed, find that answer, and multiply that answer by 1/.94 to get the answer for 94% absorbed. To get the 100% absorbed answer, just take that energy, per that area, per that time. You will have to convert mm to m though, and you will need to know the area of a circle.

Thank you for your help Ken G! I will try this out right now

CMATT
First imagine the beam is 100% absorbed, find that answer, and multiply that answer by 1/.94 to get the answer for 94% absorbed. To get the 100% absorbed answer, just take that energy, per that area, per that time. You will have to convert mm to m though, and you will need to know the area of a circle.

Yay I got the correct answer, thank you! Would you happen to know how this one below is solved? I did my answer - 1360 W/m^2, and got a number, but I'm not sure if that's correct:

How many times more intense is this than the maximum intensity of direct sunlight (about 1360 W/m²)?

Gold Member
A quick estimate says its about 10,000 times more for the laser. Is that about what you got?