Quantum Physics Flashcards
(8 cards)
What is a Photon?
A photon is a quantum(packet) of electromagnetic energy.
What is the conversion from eV to Joules?
Multiply by Elementary Charge.
What is the conversion from Joules to eV?
Divide by Elementary Charge.
Explain the experiment to determine a value for Planck Constant using LEDs.
LEDs convert electrical energy into light energy. They emit visible photons when the p.d. is above a critical value which is the threshold p.d. .
At this p.d. the LED lights up and starts emitting photons with a certain wavelength. At this p.d the work done is about the same as the energy of the emitted photon. This is given by W=VQ. We can use a voltmeter to measure the minimum p.d. when the LED lights up. If we also know the wavelength of the photons we can determine the Planck Constant.
At the threshold p.d. the energy transferred by a single electron is approximately the same as the energy of the single photon it emits.
Therefore eV=hc over wavelength or eV=hf
We could use this to calculate h or to be more accurate we should use a variety of different wavelength LEDs.
Then use that data to plot a graph of V against 1 over wavelength.
Planck Constant can be determined by gradient of graph which is hc/e.
Therefore just multiply gradient by e/c and you have obtained the experimental value for Planck Constant.
What is the Photoelectric Effect?
The emission of photoelectrons from a metal surface when EM radiation above a threshold frequency is incident on the metal.
What are photoelectrons?
Electrons emitted from the surface of a metal by the photoelectric effect.
What is the Gold Leaf experiment?
Charging the Electroscope:
If you negatively charge the metal cap, electrons flow into the stem and leaf.
The gold leaf and stem become negatively charged and repel each other, causing the leaf to rise.
Testing the Photoelectric Effect:
Shine ultraviolet (UV) light onto the metal cap (often zinc).
If the frequency of UV light is above the threshold, photoelectrons are emitted from the metal surface.
This causes the metal to lose negative charge, so the leaf falls.
What does the Gold Leaf Experiment show?
Also why it supports the Photon Model?
What Does It Show?
The fall of the leaf indicates that electrons are leaving the metal — a clear sign of the photoelectric effect.
If light of low frequency (e.g. visible light) is used:
Even with high intensity, no electrons are emitted, and the leaf does not fall.
Why It Supports the Photon Model
The leaf only falls if photon energy (E=hf) is greater than the work function.
Emission happens instantly — no delay, which contradicts wave theory predictions.
Intensity alone doesn’t cause emission — only frequency matters, again supporting the quantised nature of light.
