Chapter 1 – Introducing quantum mechanics

Light as particles

Two experiments forced physicists to rethink light…

The Ultraviolet Catastrophe

Hot objects radiate energy with a spectrum the depends on the temperature – hot objects glow red and the color moves to white as it gets even hotter.

Classical physics said a hot object should radiate energy at each wavelength proportional to 1/wavelength.

A hot object should radiate an infinite amount of ultraviolet! This did not match observation but physicists at the time did not understand why.

In 1900, Max Planck explained the observations by saying heat generates light in chunks of energy proportional to 1/wavelength (He didn’t know why!)

It takes a lot of energy to create short wavelength chunks so there are less of them

Photoelectric effect

The photoelectric effect is when light hitting a material releases electrons.

Most people would predict that the electrons knocked out of the metal with a brighter light should come out with more energy. However, a brighter light releases more electrons with the same energy. The energy of the electrons released depends only on the color of the light, not its brightness.

In 1905 Albert Einstein explained this by going further than Max Planck. He said the light is a stream of photons with each photon’s energy proportional to 1/wavelength.

That means photons can’t have any energy – the energy depends only on wavelength:

• Blue photons have high energy and cause high energy electrons to be released.
• Red photons have lower energy and cause lower energy electrons to be released.