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At the end of the 19th century new information was obtained about black-body radiation and the photoelectric effect - AQA - A-Level Physics - Question 2 - 2020 - Paper 7
Question 2
At the end of the 19th century new information was obtained about black-body radiation and the photoelectric effect. This information challenged classical physics th... show full transcript
Worked Solution & Example Answer:At the end of the 19th century new information was obtained about black-body radiation and the photoelectric effect - AQA - A-Level Physics - Question 2 - 2020 - Paper 7
Step 1
State what is meant by black-body radiation.
Answer
Black-body radiation refers to the electromagnetic radiation emitted by a perfect absorber, or 'black body', which is in thermal equilibrium. The spectrum of this radiation has a peak that depends on the temperature of the emitter, with higher temperatures producing more intense radiation and shifting the peak to shorter wavelengths.
Step 2
Describe how the predictions of classical theory compare with Wien and Lummer’s experimental results.
Answer
Classical theory predicted that the intensity of radiation would increase indefinitely as the wavelength decreased, leading to the so-called 'ultraviolet catastrophe'. However, Wien and Lummer's experiments showed that the intensity of black-body radiation peaks at a certain wavelength and then declines at shorter wavelengths, contradicting classical predictions. Figure 2 illustrates this phenomenon, where the curve reaches a maximum and then falls off, indicating that higher energies correspond to shorter wavelengths.
Step 3
Outline the main aspects of his suggestion.
Answer
Max Planck suggested that electromagnetic radiation is emitted in discrete packets called 'quanta'. He introduced the concept that the energy of these quanta is directly related to their frequency by the equation:
where E is the energy, h is Planck's constant, and f is the frequency. This marked a significant departure from classical theories, proposing quantized energy levels for electromagnetic radiation.
Step 4
Discuss Einstein’s explanation of photoelectricity and its significance in terms of electromagnetic radiation.
Answer
Albert Einstein's explanation of the photoelectric effect built upon Planck's ideas. He observed that light can be thought of as consisting of particles (photons), each carrying energy proportional to their frequency. This explained why light below a certain frequency would not cause electrons to be emitted from a metal surface, even if the light was sufficiently intense.
Two key observations in photoelectric experiments include:
- Electrons are emitted from the surface of a metal only when the light frequency exceeds a certain threshold, regardless of light intensity.
- The number of emitted electrons increases with light intensity, but their kinetic energy depends solely on the frequency of the incoming light.
These observations highlighted the limitations of classical wave theories, which could not explain the discrete nature of phenomena observed. Einstein's quantized view of light provided a critical advance in our understanding of electromagnetic radiation, leading to the development of quantum mechanics.