The idea of the quantum nature of light has emerged in an attempt to explain:

When a photon enters glass from air,which one of the following quantities does $\underline{\text{NOT}}$ change?

$A$ source $S_1$ is producing $10^{15}$ photons per second of wavelength $5000 \; \mathring{A}$. Another source $S_2$ is producing $1.02 \times 10^{15}$ photons per second of wavelength $5100 \; \mathring{A}$. Then,$(\text{power of } S_2) / (\text{power of } S_1)$ is equal to

$A$ radio transmitter radiates $1 \text{ kW}$ power at a wavelength of $198.6 \text{ m}$. How many photons does it emit per second?

The rest energy of an electron is

The wavelength of a photon with energy $35 \text{ keV}$ is . . . . . . .
$(h = 6.625 \times 10^{-34} \text{ J s}, c = 3 \times 10^{8} \text{ m/s}, 1 \text{ eV} = 1.6 \times 10^{-19} \text{ J})$