Why does human skin darken when exposed to sunlight for a long period of time?

Which of the following has the minimum frequency?

$A$ monochromatic source of light operating at $15 \text{ kW}$ emits $2.5 \times 10^{22} \text{ photons/s}$. The region of an electromagnetic spectrum to which the emitted electromagnetic radiation belongs to . . . . . . . (Take $h = 6.6 \times 10^{-34} \text{ J}\cdot\text{s}$ and $c = 3 \times 10^8 \text{ m/s}$).

In the electromagnetic spectrum,the visible light region lies between which of the following?

Which of the following is not an electromagnetic wave?

The range of wavelength for ultraviolet is from . . . . . . .