The energy of a photon with a wavelength of $\lambda = 4000 \ \mathring{A}$ is equal to ... $eV$.

Write the equation for the effective mass of a photon.

When an $X$-ray photon collides with a proton,its frequency:

$1 \ J$ is equal to how many electron volt?

The fact that photons carry energy was established by

An electric bulb of $100 \ W$ power emits photons of wavelength $410 \ nm$ per second. The number of photons emitted per second is: $(h = 6.63 \times 10^{-34} \ J \cdot s, c = 3 \times 10^8 \ m/s)$