Amplitude Squared Proportional to Intensity?

[Jimmy25]

$$A^{2}$$ $$\propto$$ $$I$$

When is it appropriate to use this formula?

I don't understand how to use a proportionality. Say I have an amplitude A and I make it 2A can I use the equation to determine the intensity will increase by a factor of 4?

I guess I don't know why it is a proportionality as opposed to an equation.

 

[Cryxic]

$$A^{2}$$ $$\propto$$ $$I$$

When is it appropriate to use this formula?

I don't understand how to use a proportionality. Say I have an amplitude A and I make it 2A can I use the equation to determine the intensity will increase by a factor of 4?

I guess I don't know why it is a proportionality as opposed to an equation.

It would be appropriate to use it if you're given an equation for some EM wave. Then the constant in front squared would represent the intensity of the wave.