What is Light?

What are Electrons?

Light Is light a wave or a stream of

particles?

A wave is characterized by its wavelength, frequency, and velocity.

The speed of light, c, is

3.00 x 108 meters/sec.

Thomas Young

In 1801 Young did an experiment which showed the wave nature of

light.

Young observed light and dark bands on the screen, which was the result of constructive and destructive interference of light waves.

Electromagnetic(EM) radiation

Visible light is one type of EM radiation.

Light as a stream of particles

Albert Einstein (1898 - 1955) did an experiment on the photoelectric effect in 1905 that showed light as a stream of particles (photons).

These particles were massless and each had an energy given by:

E = h

= c /

so,  

E = (constants) /

Note: Planck's constant, h, is

6.626 x 10(-34) joule-sec

and it comes from an important experiment on black body radiation performed in 1900 by

Max Planck (1858 - 1947).

Thus the dual nature of light; it can be modeled as a wave (Young's experiment) or a particle

(Einstein's experiment).

The electron in the hydrogen atom Visible light is composed of all the

colors of the spectrum.

The emission spectrum of the hydrogen atom contains only four colors or lines.

How to explain the four discrete emission wavelengths or energies?

Neils Bohr (1885 - 1962) The Bohr model of the hydrogen atom

(1913)

The further an electron is from the nucleus the more potential energy it has.  So to move from one orbit (which corresponds to a particular radii) the electron must gain/lose energy in the form of a photon.

Energy (n) = Energy (n-1)+ Energy(photon) E(2)=E(1)+E(photon)

Energy (n) = Energy (n+1)- Energy(photon) E(1)=E(2)-E(photon)

Only certain discrete radii or energies are allowed.

We say the allowed energies are quantized.

We can "observe" these gain/loss of energies because energies correspond to wavelengths in the visible spectrum.

Louis de Broglie (1892 - 1987)

If light can be modeled as a particle or as a wave, can an electron be modeled as a wave?

The wavelength of a matter wave (1923) is given by:

Everyday objects are too massive to give observable wavelengths; however, electrons are light enough to give observable wavelengths. Diffraction of electrons was observed in 1927.

The Bohr model could also be explained using standing waves.

Whole numbers (1,2,3,etc.) of de Broglie wavelength give the allowed radii found in the

Bohr model.