Electromagnetic radiation has particulate, as well as wave, nature. A photon is the quantum pf energy of this EM radiation (quantised describes something that takes discrete values). 

Energy of a photon is given by the equation = ℎ and therefore E=frac{hc}{lambda }.h is Planck’s constant, which has a value of

6.63 × 10−34Js. 

The electronvolt (eV) is a unit of energy; it is the energy transferred to (or from) an electron when it moves through a potential difference of 1V. 1ev is equivalent to 1.6 × 10−19J. 

We can do experiments involving LEDs to determine a value for the Planck constant, using the fact that LEDs only emit visible photons when the p.d. across them is above the critical value. 

V = frac{W}{Q}therefore W =VQ

W = eV for the case of one electron

At the threshold p.d. the energy transferred by an electron to the LED is approximately equal to the energy supplied to the photons emitted. Therefore:

eV = frac{hc}{lambda }

You can plot a graph of V against , and the gradient is equal to  frac{1}{lambda }frac{hc}{e} Dividing by c and multiplying by e yields h. .