Laser physics

  • #1
Hi,I need help with this question

Assume you have a laser emitting a wavelength of 800 nm with power 20 W. You irradiate a goblet filled with water whose layer is 0.25 cm thick.
The absorption coefficient of water at this wavelength is 0.65 cm-1.
a) What is the remaining power behind the goblet?
b) How large should the layer thickness of the water be, if you desire only 1% of the incident power behind the goblet? (equals 99% absorption in the water)
Note:You can neglect the reflection losses and all absorption losses in the walls of the goblet.

I am not really sure how to start.
 

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