Light!

Where does light come from? Light is the result of electromagnetic radiation from an energized (usually heated) body of matter. Most often thought of as “visible light”, but electromagnetic radiation occurs over a large spectrum of wavelengths.

Electromagnetic radiation occurs at different wavelengths, from long radio waves, to short gamma rays.

Energy and Wavelength The energy of the light wave (radiation) is directly related to its frequency and wavelength.

High Energy Light Waves Increasing frequency (decreasing wavelength) increases energy

Chemistry and Light We have already seen that light emission results from electrons during the excitation from and emission of radiation.

Emission Emission of radiation (light energy) occurs when an electron drops from an excited state Electron absorbs energy from an outside source (flame, solar radiation) Added energy causes it to jump to a higher energy level Electron is very unstable at the higher energy level and quickly drops back down to its “ground state”, releasing energy as it drops.

Bohr’s Back! Each energy level has a “quantized” amount of energy that each electron occupying it holds. So, electrons dropping between the same levels ALWAYS release the same amount of energy

Energy and Wavelengths The energy of emitted radiation (light) is directly related to the wavelength of the light. Emission Animation Dropping to n=1 : UV Light Dropping to n=2 : Visible Light Dropping to n=3 : IR Light

Emission and Chemistry Emission is used all the time to determine the elemental composition of samples because each element has a unique set of wavelengths it emits. The trick is being able to see all of the wavelengths present in a sample of light

Flame Test Gave signature “colors” of emission for different elements Limits Only useful for metals Need a lot of sample to heat to see the color Only see color that results from mixture of all the wavelengths present. (What if 2 are orange?!?)

Spectroscopy – Fancy Flame Test Spectroscopy uses a prism or grating to separate out all of the individual wavelengths present in a light source

Spectroscopy The separated wavelengths proved a unique “signature” for each element.

Applications of Spectroscopy Determining the elements present in unknown samples Mars Rover (soil/rock samples)

Applications of Spectroscopy Determining the elements present in celestial bodies Stars, Gas Clouds

Applications of Spectroscopy Determining the concentration of solutions Measures how much light is absorbed by the sample, showing the amount of dissolved molecules present