The energy of the incident radiation that causes photoelectrons to be emitted can be calculated using the stopping potential, V, via the equation

$$E = eV,$$

where e is the elementary charge.

The elementary charge, e, is approximately equal to $$1.602 \times 10^{-19}$$ C. So the energy of the incident radiation is

$$E = 1.602 \times 10^{-19} \, \text{C} \times 5 \, \text{V} = 8.01 \times 10^{-19} \, \text{J}.$$

We convert this to electron volts (eV) by using the conversion factor $$1 \, \text{eV} = 1.602 \times 10^{-19} \, \text{J}.$$ So

$$E = \frac{8.01 \times 10^{-19} \, \text{J}}{1.602 \times 10^{-19} \, \text{J/eV}} = 5 \, \text{eV}.$$

This is less than the threshold energy of $$6.2 \, \text{eV},$$ which means that the incident radiation does not have enough energy to overcome the work function of the metal and thus cannot cause photoelectrons to be emitted.

The regions of the electromagnetic spectrum are generally classified by energy as follows:

• Infrared: Less than about $$1.24 \, \text{eV}$$
• Visible: Between about $$1.24 \, \text{eV}$$ and $$3.1 \, \text{eV}$$
• Ultraviolet: Between about $$3.1 \, \text{eV}$$ and $$124 \, \text{eV}$$
• X-rays: Greater than about $$124 \, \text{eV}$$

Therefore, the incident radiation falls in the ultraviolet region.