Paper 2 Long Written Qs

Explain how a standing wave is set up on a string (6)

1. Two waves travelling in two different directions (can mention reflection at the far end node)

2. The waves superpose/ interfere

3. Waves in Phase: Constructive Interference, path difference =nλ

4. Waves out of Phase: Destructive Interference, path difference (n+1/2)λ

5. Nodes: Destructive Interference, Min Displacement/ Amplitude

6. Antinodes: Constructive Interference, Max Displacement/ Amplitude

Explain the difference between unpolarised and polarised light (3)

1. Unpolarised – oscillations/vibrations in many directions

2. Polarised – oscillations/vibrations in single direction

3. Oscillations/vibrations are perpendicular to direction of propagation

Explain the photoelectric effect and what it proves in terms of wave behaviour

Waves: EM radiation, e- absorbs energy until it has enough energy to escape. Total energy of e- are increased by increasing intensity.
Prediction: Photoelectrons can be emitted for any frequency of EM ; e- needs time to absorb the energy ; max KE should be greater if intensity is greater ; reaction is not instantaneous

Conclusion: Wave theory does not match the observations made in photoelectric phenomena

Explain the photoelectric effect and what it proves in terms of particle behaviour

Particles: One photon to one e- , Photon interacts with e- and instantly transfer energy in which the e- instantly gets emitted from the metal. The min energy required to remove e- from a metal surface is called the work the work function.
Takeaways: Electron emission happens instantaneously; emission only occurs if incident radiation is above the threshold frequency; differenet metals have different threshold frequencies ; for a given frequency, no. of e- emitted per second is proportional to the intensity of radiation

The Gold Leaf Experiment to demonstrate the Photoelectric Effect

The photo electric effect can be observed on a gold leaf electroscope:

• A plate of metal, usually zinc is attatched to a gold leaf, which is initially negatively charged, causing it to be repelled by a central negatively charged rod

• This causes negative charge/ e- to build up on the metal plate

UV Light (High frequency, low wavelength light; high KE to wave, transferred to e-) is shone onto the metal plate, leading to the emission of photoelectrons

This causes the extra e- on the gold leaf to be removed, so the gold leaf begins to fall back towards the central rod

• This is because they become less negatively charged, and hence repel less.

Describe the Life Cycle of a Star (Low Mass, <1.4xMass of Sun)