Physics

# Section Summary

PhysicsSection Summary

### 21.1Planck and Quantum Nature of Light

• A blackbody will radiate energy across all wavelengths of the electromagnetic spectrum.
• Radiation of a blackbody will peak at a particular wavelength, dependent on the temperature of the blackbody.
• Analysis of blackbody radiation led to the field of quantum mechanics, which states that radiated energy can only exist in discrete quantum states.

### 21.2Einstein and the Photoelectric Effect

• The photoelectric effect is the process in which EM radiation ejects electrons from a material.
• Einstein proposed photons to be quanta of EM radiation having energy $E=hf, E=hf,$ where f is the frequency of the radiation.
• All EM radiation is composed of photons. As Einstein explained, all characteristics of the photoelectric effect are due to the interaction of individual photons with individual electrons.
• The maximum kinetic energy KEe of ejected electrons (photoelectrons) is given by $K E e =hf−BE, K E e =hf−BE,$ where hf is the photon energy and BE is the binding energy (or work function) of the electron in the particular material.

### 21.3The Dual Nature of Light

• Compton scattering provided evidence that photon-electron interactions abide by the principles of conservation of momentum and conservation of energy.
• The momentum of individual photons, quantified by $p= h λ p= h λ$, can be used to explain observations of comets and may lead to future space technologies.
• Electromagnetic waves and matter have both wave-like and particle-like properties. This phenomenon is defined as particle-wave duality.