
Physics question
How can I calculate the intensity of radiation of the Sun at Earth(at the same distance as the Earth is at). The intenisity of radiation at the Sun is about 64MW/m2. I mean the radiation spreads around so how can I know which portion comes to the Earth?
All help is appreciated.

well saying that the radiation from the sun all travels at the same speed
, this radiation will cover a spherical shape of area.(just like blowing a balloon)
now the distance of the earth to the sun is about $\displaystyle 1.5x10^{11}m$
this means you need to find the surface area of a sphere with a radius of $\displaystyle 1.5x10^{11}m$, then you need to find the area of intersection between the sphere and the earth(assume to be in the middle of the earth)
surface area of the sphere:
$\displaystyle
A_1 = 4\pi r_{ES}^2
$
where $\displaystyle r_{ES} ^2$ is the radius of the earth and has the value of $\displaystyle 1.5x10^{11}$
hence the area of intersection is:
$\displaystyle
A_2 = \pi r _E ^2
$
where $\displaystyle r_E ^2$ is the radius of the earth and has the value of $\displaystyle 6.4x10^6$. The area of intersection is can be assumed as a circle since the distance of the earth from the sun is very much larger tahn the radius of the earth.
hence the fraction of radiation received by earth is
$\displaystyle \frac{A_2}{A_1}$