# AP Physics 2 : Photons and Photon Energy

## Example Questions

### Example Question #1 : Photons And Photon Energy

A physicist is trying to determine the identity of a metal by experimentally finding its work function and then comparing the experimental value to a list of known values. If the physicist shines light with a frequency of  on the metal and records an electron speed of , what is the work function of the metal?

Explanation:

To determine the work function of the metal, we can begin by calculating the kinetic energy of the electron from its speed.

Then, convert joules into electron volts:

Next, we'll calculate the energy of the light by using its frequency according to the equation:

Then, we can convert this energy into electron volts:

And finally, we can use the equation for work function:

### Example Question #2 : Photons And Photon Energy

An atomic transition takes place from  to the ground state . What is the wavelength of the photon this transition will emit?

Explanation:

The energy difference between the second excited state  and the ground state  will be equal to the photon energy since energy is conserved.

Since the energy difference is in electron volts, they need to be converted to Joules. Another convenient formula is to notice that since  and  are constants, they can be converted to electron volts and you can use

### Example Question #3 : Photons And Photon Energy

It is possible that electrons can be ejected from a metal plate by light. Which of the following properties of light accounts for this phenomena?

Amplitude

Speed

Electric force

Frequency

Intensity