(b) According to de-Broglie hypothesis, revolving electron in circular orbit shows wave nature. Hence, a circular orbit can be taken to be a stationary energy state only if it contains an integral number of de-Broglie wavelength, i.e. we must have, 2πrn=nλ where, rn= radius of n th orbit and ‌‌n= number of stationary state. For first orbit, n=1⇒2πr=λ So, de-Broglie wavelength of the electron in the first Bohr orbit of hydrogen atom is equal to the circumference of first orbit.