The electrons are somewhere in the atom (we have not yet specified where), but it takes energy to remove them, and the energy is used to overcome the attraction of the negative electron and the positive nucleus. Recall that, in the photoelectric effect, each photon ejects an electron from an atom on the surface of the metal. Enough energy for what, you might ask, and the answer is that the lower energy photons simply do not have enough energy to overcome the attraction between an electron and the nucleus. Einstein explained this observation by assuming that only photons with “enough” energy could eject an electron from an atom if photons with less energy hit the atom, no matter how many, no electrons are ejected58. The surprising result is that the same total amount of energy can produce very different effects.

An analogy is with a vending machine that can only recognize quarters, you could put many nickels or dimes into the machine all day and nothing will come out. Once the wavelength is short enough to eject electrons, increasing the intensity of the light increases the number of electrons emitted. But shine low intensity (few photons per second), high energy, short wavelength light, such as ultraviolet light or X-rays, on the plate and electrons are ejected. This is why we don’t mind being surrounded by radio waves essentially all the time yet we closely guard our exposure to gamma-rays, X-rays and UV light their much higher energies cause all kinds of problems with our chemistry, as we will see later.īecause of the relationship between energy and wavelength, you can shine light of long wavelength (low energy - such as infrared light) but high intensity (many photons per second) on a metal plate and no electrons will be released.

So radiation with a very short wavelength, such as X-rays (λ= ~10 -10 m) and ultraviolet light (between 10 -7 to 10 -8 m) have much more energy per particle than long wavelength radiation like radio and microwaves (λ= ~10 3 m). Since wavelength and frequency are inversely related (as one goes up the other goes down), so energy is directly related to frequency by the relationship E = hν or inversely related to the wavelength E= hc/λ, where h is Planck’s constant. The higher the frequency ν (cycles per second, or Hertz) the shorter the wavelength λ (length per cycle), and the greater the energy per photon. The intensity of the light is related to the number of photons that pass by us per second, while the energy per photon is dependent upon its frequency (or wavelength, since wavelength and frequency of light are related by the formula λν = c, where c, the speed of light in a vacuum, is a constant and equal to ~3.0 x 10 8 m/s). Let us assume that light comes in the form of particles, known as photons, that also have a wavelength and frequency (we know: this doesn’t make sense, but bear with us for now). It turns out that there is a threshold wavelength of light, which is characteristic for the metal used, below which no electrons are ejected. It was known that there is a relationship between the wavelength of the light used, the type of metal the plate is made of, and whether or not electrons were ejected. The photoelectric effect occurs when light shines on a (usually) metal plate and electrons are ejected, creating a current.

In 1905 Albert Einstein used the idea of quanta to explain the photoelectric effect, which was first described (and patented) by Nikola Tesla (1856 – 1943). Chemistry, life, the universe and everything Chapter 2.2: Taking quanta seriously

Intensity equation light