Introduction

What this section does

This section only sets the scene for the chapter. It reminds you that light is electromagnetic radiation, that most optical phenomena you have studied are explained by treating it as a wave, and that one class of phenomena — those appearing when radiation interacts with matter — cannot be explained that way at all.

Points worth carrying forward

• Electromagnetic radiation consists of mutually perpendicular oscillating electric and magnetic fields, both perpendicular to the direction in which the wave and the energy travel.
• Reflection, refraction, interference, diffraction and polarization are all explained on the wave picture, across the whole spectrum: X-rays, gamma rays, ultraviolet, visible, infrared and microwaves.
• The black body spectrum, studied in Chapter 3, cannot be explained by the wave nature of radiation. Phenomena of that kind need quantum physics.

Planck's quantization of energy

• Planck first proposed the idea of quantization of energy to explain the black body spectrum. His model says two things: energy is emitted in packets, and at higher frequencies the energy of a packet is larger.
• He assumed atoms behave like tiny oscillators that emit electromagnetic radiation only in discrete packets,

$$E = nh\nu$$

where $\nu$ is the frequency of the oscillator and $n$ is a positive integer.

• Emission occurs only when the oscillator jumps from one quantized energy level to another of lower energy.
• This model became the basis of Einstein's theory of the photoelectric effect, which is the subject of the next section.