An electron is moving in Bohr's fourth orbit. Its de-Broglie wavelength is λ. Calculate the circumference of the fourth orbit.

According to Bohr's concept, an electron always move in the orbit with angular momentum (mvr) equal to nh2π.∴  mvr=nh2π  ∵λ=hmvand m = 4∴   Circumference =2πr=4·hmv=4λ