# Confused about intensity of a laser beam

For normal natural light coming from the sun, the electromagnetic waves emanate from the sun with a spherical wavefront, then we can calculate the intensity of these waves reaching us on Earth by $$I = \frac {P}{4 \pi r^2}$$

We used the surface area $$4 \pi r^2$$ because we know the wavefront is spherical, but in the case of lasers, how do we approach this using the idea of wavefronts?

I know it's easy to imagine that a laser beam with a negligible divergence has a specific diameter and we can calculate the intensity by dividing by $$\pi r^2$$ because the beam has a circular cross section, but how are we sure that's right? I searched for the type of the laser wavefront and I found that it is called the Gaussian wavefront, is it responsible for this kind of intensity and is it even relevant to what I'm talking about or am I just overthinking?

• Search for the "waist" of a laser beam. – Cinaed Simson Apr 15 at 20:39
• I would add to @CinaedSimson, search Gaussian beam, Gaussian optics, laser modes.Basically there exist very reliable approximations to what the laser light is, as an analytical expression, when it propagates in free space. – Cryo Apr 15 at 23:08