Pre-1900 Science

a.k.a. “Classical Physics”

Wavelength ()

Amplitude

Frequency ()

Speed

Distance between identical points on consecutive waves

Distance between origin and crest (or trough)

Number of waves that pass a point per unit time

= wavelength x frequency

Wavelength and frequency are inversely related.

Speed = ln

1873 – James Clerk Maxwell

• Visible light is composed of waves

• “Electromagnetic waves”

“Electromagnetic radiation”

Visible light is just one small portion of the entire electromagnetic spectrum.

Electromagnetic waves . . .

. . . always move at the same speed, the speed of light.

c = 3.00 x 108 m/s

speed = = 3.00 x 108 m/s

Classical Physics:

Matter is made of particles.

Energy is made of waves.

Particles (and matter) have mass, exist in a specific space.

Waves (and energy) are massless and delocalized.

Classical Physics:

Energy varies continuously. In other words, there’s no limit (big or small) on the amount of energy a system could gain or lose.

Like walking up a ramp. You can take as big or as small a step up/down as you want.

So as of 1900 . . .

• Energy is continuous (like a ramp).

• Matter is composed of particles. Energy is composed of waves.

• Physics is “done”.

But, scientists can’t seem to explain . . .

1. Blackbody radiation

2. The photoelectric effect

3. Atomic emission spectra

4. Periodic trends in properties of the elements

Blackbody Radiation

• release of energy by the vibrating atoms of a solid

• At room temp, black bodies radiate infrared light (heat). As temp increases, moves to red, orange, yellow, white, blue, and eventually ultraviolet.

Blackbody Radiation

• scientists were having difficulty developing mathematical equations to correctly describe the spectrum of radiation from a black body observed in experiments

The Photoelectric Effect

When light is shone onto a metal surface, electrons are ejected from the surface.

The Photoelectric Effect

The Photoelectric Effect

Classical physics says light is a wave.

Electron emission should be based on amplitude (intensity) of the wave of light.

- lag time with low amplitude light

- higher amplitude light = faster moving e-

The Photoelectric EffectFor any given metal, the frequency of light used must be above a certain minimum value called the threshold frequency.

Classical physics says the current should gradually decrease as frequency decreases.

Below the threshold frequency, no current flows.

The Photoelectric Effect

If light is a wave, amplitude (intensity) should affect the electron emission.

Instead, energy of the ejected electrons is related to the frequency of light shining on the metal surface.

Scientists had observed continuous and line emission spectra . . . .

Atomic Emission Spectra

When light is emitted by atoms of a gas through which electricity is passing, it produces a spectrum consisting of a few isolated parallel lines.

Hydrogen Line Emission Spectrum

Scientists had observed atomic emission spectra . . . .

. . . but couldn’t explain them.

From all the trouble explaining these events . . .

Blackbody radiation

. . . quantum mechanics will arise.

Atomic emission spectra

The photoelectric effect

Periodic

Trends