Given the wavelength of the yellow light (700 nm. in this case) we can find the frequency
by dividing the speed of light c by the wavelength w, that is: f = c/w and we know that
c is equal to 2.998 * 10**8 meters per second.
So the frequency f = (2.998 * 10**8) / (7.0 * 10**-7) = 4.283 * 10**14 cycles per sec.
(or Hz.) Since the threshold frequency of Cs is 9.39 * 10**14 Hz, the red light doesn't
have a high enough frequency (or energy) to cause electron emission.
Hope this answers your question.
