A certain metallic surface is illuminated by monochromatic radiation of wavelength λ. The stopping potential for the photoelectric current for this radiation is 3V0. If the same surface is illuminated with radiation of wavelength 2λ, the stopping potential is V0. The threshold wavelength of this surface for the photoelectric effect is xλ. Find the value of x.

When light of a given wavelength falls on a metallic surface, the stopping potential for photoelectrons is 3.2 V. If a second light having a wavelength twice that of the first light is used, the stopping potential drops to 0.7 V. The wavelength of the first light is ________ m. (h = 6.63 × 10-34 J·s, e = 1.6 × 10-19 C, c = 3 × 108 m/s)

In a photoelectric experiment, ultraviolet light of wavelength 280 nm is used on a lithium cathode having a work function φ = 2.5 eV. If the wavelength of the incident light is switched to 400 nm, find the magnitude of the change in the stopping potential. (Take h = 6.63 × 10-34 J·s, c = 3 × 108 m/s)