Types of Spectra

Spectra Most of you have seen light passed through a prism and separated into a rainbow of colors. Today you will use a different device to get a separation of colors. This device is known as a diffraction grating. The diffraction grating is in the spectroscope that will be used to analyze the colors in various light sources. Your lab instructor will now demonstrate the use of a diffraction grating.

A picture of the spectroscope that you will use in this lab appears below. Also shown is a view of the interior. Slit Wavelength Scale Top Bottom Diffraction Grating

When white light is viewed through a diffraction grating, a continuous spectrum will be produced. It looks like this

Continuous Spectrum It is continuous because all visible frequencies of light are present. Continuous spectra arise from incandescent solids, liquids, and even gases when at high pressure. Incandescent and fluorescent lights produce continuous spectra.

A glowing, low density gas, such as a neon sign, produces a discrete emission spectrum. This type of spectra is seen as having individual lines of different colors. The colors that are emitted by an excited atom are characteristic of that particular kind of atom. They represent a unique set of energy levels (electron orbits) for that kind of atom. These unique sets of energy levels are predicted by quantum mechanics, not classical mechanics. As an example of the kind of spectra that you will see today, a discrete spectrum appears on the next slide.

Discrete Emission Spectrum 700 nm 600 nm 500 nm 400 nm (Nanometer wavelength scale is shown) Continuous Emission Spectrum 700 nm 600 nm 500 nm 400 nm

Fluorescence Some materials that absorb ultraviolet light will emit visible light. To understand this process consider the Bohr model of the atom.

With possible orbits for electrons Here comes a nucleus With possible orbits for electrons e- In this configuration the atom is not excited.

Here comes a UV photon e-

e- Now the atom is said to be excited because the electron is in a higher than normal orbit.

e- The electron can jump back to its original orbit, and in doing so it will emit a photon of the same wavelength as the original excitation UV photon.

e- For some materials the electron might jump to an intermediate orbit first. This would result in the emission a photon of lesser energy than the original UV photon and it would be visible.

e-

e- The atom is still excited and the electron would move to a lower orbit giving off another visible photon.

e- Now the atom is in a non-excited state again.

Fluorescence In conclusion, this simple model shows how UV radiation can stimulate some materials to emit visible light.

Fluorescent Lamps Primary excitation - electron collisions with low pressure Hg vapor, UV given off Secondary excitation - UV photons absorbed by phosphors. Phosphors fluoresce emitting visible light. Remember that the visible light from the excited mercury vapor is also emitted.

You will now receive further explanation on the use of the spectroscope and the procedures of today’s experiment.