# Homework Help: How to calculate the intensity of light, given area and mass

1. Dec 2, 2017

### Michael Moynihan

1. The problem statement, all variables and given/known data

You want to support a sheet of fireproof paper horizontally, using only a vertical upward beam of light spread uniformly over the sheet. There is no other light on this paper. The sheet measures 20.8 cm by 28.8 cm and has a mass of 1.70 g .
(a) If the paper is black and hence absorbs all the light that hits it, what must be the intensity of the light beam?

2. Relevant equations
I=P/A
A=w*h

3. The attempt at a solution
A=0.0599m^2
I do not know power, so I do not know the intensity. What information does saying that the paper absorbs all the light that hits it actually give me?

2. Dec 2, 2017

### scottdave

Each photon can have momentum, as well as having energy. I'm on my phone, but you can search for momentum photons to get related information.

3. Dec 2, 2017

### haruspex

It affects the momentum conservation equation.

4. Dec 2, 2017

### Michael Moynihan

Thanks guys! Got it!

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