Einstein's equation for photoelectric effect is E_("max") = hf - W_(0), where h = Planck's constant= 6.625 xx 10^(-34) J.s, f = frequency of light incident on metal surface, W_(0)= work function of metal and E_("max")= maximum kinetic energy of the emitted photoelectrons. It is evident that if the frequency f is less than a minimum value f_(0) or if the wavelength lamda is greater than a maximum value lamda_(0), the value of E_("max") would be negative, which is impossible. Thus for a particular metal surface f_(0) is the threshold frequency and lamda_(0) is the threshold wavelength for photoelectric emssion to take place. Again if the collector plate is ketp at a negative potential with respect to the emitter plate, the velocity of the photoelectrons would decrease. The minimum potential for which the velocity of the speediest electron becoes zero, is known as the stopping potential, the photoelectric effect stops for a potential lower than this. [velocity of light = 3xx 10^(8) m.s^(-1), mass of an electron m = 9.1 xx 10^(-31) kg, charge of an electron, e = 1.6 xx 10^(-19)C Ultraviolet of wavelength 1800 Å is incident on the metal surface. The maximum velocity of the emitted photoelectron (in m.s^(-1)) is