Max Planck discovered that light appears to exist as little packets called quanta. Einstein coined the term photons for these packets. The Energy of a photon is directly proportional to its frequency
E is the energy of a photon. f is the frequency. h is the Planck constant which is 6.63 × 10-34 Js. The equation has both wave elements (λ) and particle elements (E). To determine the value of the Planck constant an LED can be used
LEDs emit light when the p.d. across them reaches a threshold. 
At this p.d. the work done by each electron W = QV Which is about the energy of each emitted photon.W = QV = hfso since Q = eeV = hfThe voltmeter is used to measure the minimum p.d. required to turn on the LED (threshold p.d.). A graph of V against 1/λ can be plotted. 
The gradient is hc / e.
E is the energy of a photon. f is the frequency. h is the Planck constant which is 6.63 × 10-34 Js. The equation has both wave elements (λ) and particle elements (E). To determine the value of the Planck constant an LED can be usedLEDs emit light when the p.d. across them reaches a threshold. At this p.d. the work done by each electron W = QV Which is about the energy of each emitted photon.W = QV = hfso since Q = eeV = hfThe voltmeter is used to measure the minimum p.d. required to turn on the LED (threshold p.d.). A graph of V against 1/λ can be plotted. The gradient is hc / e.