What Are Logarithmic Scales?

The pH of a substance equals $-\log[H^+]$, where $[H^+]$ is the concentration of hydrogen ions. $[H^+_m]$ for milk of magnesia is $3.16\times10^{-11}$. $[H^+_o]$ for orange juice is $3.16*10^{-4}$. What is the difference of the pH levels of milk of magnesia and orange juice?