If the de Broglie wavelength of an electron is equal to the 10^–3 times the wavelength of a photon of frequency 6 10¹⁴ Hz, then the speed of electron is equal to : (Speed of light = 3 10⁸ m/s, Planck's constant = 6.63
10^–34 J.s, Mass of electron = 9.1 10^–31 kg)