The wavelength of violet light is given as 4.2x10^-7 m and the speed of light is 3x10^8 m/sec
The equation relating between the frequency and the wavelength is as follows:
speed of light = wavelength x frequency
Substitute with the givens in the above equation to get the frequency of violet light as follows:
frequency = (3 x 10^8) / (4.2 x 10^-7) = 7.14285 x 10^14 Hz
The threshold frequency of cesium is 9.39x10^14 Hz.
Since the frequency of violet light is less than the threshold frequency of cesium, therefore, the exposure to violet light won't cause electrons to be emitted from cesium.
