Spectral lines.

Step 1 of 6

A barcode

A hot solid or a dense gas gives a continuous spectrum, like the lump in the last Lesson. A thin gas behaves differently. In the Scene there is a tube of hydrogen: a high voltage speeds electrons through the gas, and they collide with the atoms. The light passes through a prism, which spreads it out by wavelength on the screen.

Step 2 of 6

The energy of a photon

In the Unit on modern physics we saw that light comes in photons, $E = hf = \dfrac{hc}{\lambda}$$E =$$hf =$$\dfrac{hc}{\lambda}$⁠, and that an atom has fixed energy levels. A collision lifts an electron to a high level, and it comes back down in one jump or several, and each jump gives out one photon with the difference between the levels:

$\lambda = \frac{hc}{E_{\text{upper}} - E_{\text{lower}}}, \qquad hc \approx 1240\ \mathrm{eV \cdot nm}$$\lambda =$$\frac{hc}{E_{\text{upper}} - E_{\text{lower}}}, \qquad hc \approx$$1240\ \mathrm{eV \cdot nm}$⁠

Show the energy jumps shows hydrogen’s ladder of levels and the jump behind each line. Gas in the tube is locked.

Step 3 of 6

Enough energy

A line appears only when the collisions can lift atoms to its upper level. Electron energy is the energy each electron brings to a collision, and for now it is too low. The ladder of Show the energy jumps shows the levels.

Step 4 of 6

Fingerprints

Each element has different levels, so a different barcode. This is spectroscopy: identifying a substance by its light, even from far away. Gas in the tube is now hidden. Look at the barcode.

Step 5 of 6

An element from the Sun

Gas in the tube is locked on helium, and Show the energy jumps shows its levels. The strong yellow line at 587.6 nm was first found in sunlight, before anyone had seen this element on Earth.

Step 6 of 6

Dark lines

In 1814 Joseph von Fraunhofer saw that the rainbow of sunlight is crossed by hundreds of thin dark lines. Forty-five years later Kirchhoff and Bunsen realised that sodium’s dark lines in the Sun are exactly where sodium’s bright lines are in a flame.