The radii of two spheres are in the ratio 1 : 2.

Suppose that the radii are r and 2r.

Now, ratio of the surface areas $=\frac{4 \pi r^{2}}{4 \pi(2 r)^{2}}=\frac{r^{2}}{4 r^{2}}=\frac{1}{4}$

= 1:4

∴ The ratio of their surface areas is 1 : 4.