Young's double slit experiment is performed with monochromatic light of wavelength $6000 \mathring{A}$. If the intensity of light at a point on the screen, where path difference of $2000 \mathring{A}$ is $I_1$ and the intensity of light at a I point on the screen, where the path difference is $1000 \mathring{A}$ is $I_2$, then $I_1: I_2=$