Given de- Broglie's explanation of quantisation of angular momentum as proposed by Bohr.

Explanation

For an electron moving in `n^(th)` drcular orbit of radius `r_(n)`, the total distance is
`=2pir_(n)`
Circumference of a stationary Bohr orbit of radius `r_(n)` is equal to integral multiple of wavelength of matter waves
`2pir_(n)=nlamda" "......(i)`
The de-broglie wavelength of the electron moving in the `n^(th)` orbit.
`lamda=(h)/(mv)" "......(2)`
From equation (1) and (2),
`2pir_(n)=(nh)/(mv)`
i.e., `mvr_(n)=(nh)/(2pi)`
But, angular momentum of the electron is
`L=mvr_(n)`
Hence `L=(nh)/(2pi)`