The work functions of Caesium $(Cs)$,Potassium $(K)$ and Sodium $(Na)$ are $2.14\,eV,2.30\,eV$ and $2.75\,eV$ respectively. If incident electromagnetic radiation has an incident energy of $2.20\, eV$, which of these photosensitive surfaces may emit photoelectrons?

One day on a spacecraft corresponds to $2$ days on the earth. The speed of the spacecraft relative to the earth is

In photoelectric effect if the intensity of light is doubled then maximum kinetic energy of photoelectrons will become

Estimating the following two numbers should be interesting. The first number will tell you why radio engineers do not need to worry much about photons! The second number tells you why our eye can never ‘count photons’, even in barely detectable light.

$(a)$ The number of photons emitted per second by a Medium wave transmitter of $10\; kW$ power, emitting radiowaves of wavelength $500\; m$.

$(b)$ The number of photons entering the pupil of our eye per second corresponding to the minimum intensity of white light that we humans can percetve $(-10^{-10}\; W m ^{-2}$). Take the area of the pupil to be about $0.4 \;cm ^{2}$, and the average frequency of white light to be about $6 \times 10^{14}\; Hz$