If hydrogen atom, an electron jumps from bigger orbit to smaller...

Radius of the orbit, $\mathrm{r}_{\mathrm{n}} \propto \mathrm{n}^{2}$\begin{aligned}& \frac{\mathrm{r}_{\mathrm{nbig}}}{\mathrm{r}_{\mathrm{n} \text{ small }}}=\frac{\mathrm{n}_{\text{big }^{2}}{\mathrm{n}_{\text{small }^{2}}=\frac{4}{1}\\ \Rightarrow & \frac{\mathrm{n}_{\text{big }}}{\mathrm{n}_{\text{small }}}=2 \\ \Rightarrow \text{ and } \frac{\mathrm{n}_{\text{small }}}{\mathrm{n}_{\text{big }}}=\frac{1}{2} \end{aligned}Velocity of electron in $\mathrm{n}^{\text{th }}$ orbit\begin{array}{c} \quad\mathrm{v}_{\mathrm{n}} \propto \frac{1}{\mathrm{n}}\\ \frac{\mathrm{v}_{\mathrm{n} \mathrm{big}}}{\mathrm{v}_{\mathrm{n} \text{ small }}}=\frac{\mathrm{n}_{\text{small }}}{\mathrm{n}_{\text{big }}}=\frac{1}{2}\\ \Rightarrow \mathrm{v}_{\mathrm{n} \text{ small }}=2\left(\mathrm{v}_{\mathrm{n} \mathrm{big}}\right)=2 \mathrm{v} \end{array}$$