(a) A proton and an $\alpha$ particle are moving with the same speed. Which one of them has greater value of de Broglie wavelength associated with it? Justify your answer. (b) Lights of wavelengths 430 nm, 450 nm and 660 nm are incident on a metal surface whose threshold wavelength is 600 nm. In which case/cases will photoemission take place and why? Calculate the threshold frequency of the metal surface.