Whiter than White
Introduction
Use a black light to examine the fluorescence from everyday materials. These are great activities to do around Halloween.
Material
To Do and Notice
Turn off the room lights, notice that the "glow-in-the-dark objects continue to glow, they are phosphorescent.
Use the black light in a dark room to search for fluorescent objects. Look at white clothing, white paper, teeth, minerals, liquid tide and quinine water. The glow that emanates from some of these objects is fluorescence.
A simple test to separate phosphorescence from fluorescence is to simply turn off the black light. If an object continues to glow it is phosphorescent, if the glow goes out instantly it is fluorescent.
Liquid detergents such as Tide contain a colorless dye that absorbs ultraviolet and produces a blue fluorescence. In a dark room, shining a blacklight on a detergent will cause a brilliant blue glow.
A ruby will fluoresce red when exposed to ultraviolet light if the ruby has no iron impurities. The red light is the same light produced by a ruby laser.
Quinine water will glow a weird blue. Drinking quinine water illuminated by black light will really entertain an audience. See Eddie Murphy in the nutty professor for example!
Change the temperature of the phosphorescent materials by holding them over hot water or blowing on them with the hot air of a hairdryer. Notice that heating causes them to glow more brightly.
Heat the fluorescent materials. Notice that their temperature does not affect the amount of light they emit.
What’s Going On?
A simple definition of fluorescence is that it is light emitted by an object excited by light, where the emitted light is of a different color, i.e. lower energy, than the exciting light, and where the emitted light stops immediately after the exciting light is turned off. The light will turn off in as little as 10-8 seconds.
In phosphorescence, on the other hand light continues to be emitted after the exciting light is turned off. The light will continue to be emitted for seconds or even hours.
A more advanced definition of phosphorescence is that it is temperature dependent while fluorescence is independent of temperature. Hot phosphorescent objects will glow more brightly for a shorter time than cold ones.
More than you ever wanted to know
Molecules and atoms both have energy levels. Electrons can exist only at certain specific "states" which have specific energies. The lowest energy state is called the ground state, G. In a helium atom the ground state is named 1s and contains two electrons, these electrons have spins, and the spins of the electrons are in opposite directions, one spin up the other spin down. When two electrons have their spins opposite they are said to be in a "singlet state." One electron can be excited into a higher energy state, E. It can then have a spin the same direction as the electron left behind in the ground state, or opposite. If it has a spin in the opposite direction it is in a singlet state, if it is the same it is in a so called "triplet state." It is easier for an electron in a singlet state to decay to a singlet state than it is for an electron in a triplet state to decay to a singlet state. (It is about a hundred million times easier! i.e. 108 times easier.)
When ultraviolet light strikes a molecule it can excite the molecule from its ground state, G, to a higher energy excited state, E. If the electron decays right back down to G it reemits the radiation which is called resonance radiation, the resonance radiation has the same color or energy as the exciting radiation. However, while it is in the excited state a collision with another molecule can take away some energy, a process called internal conversion, leaving the molecule in a middle energy state, M.
When the molecule goes from the M state to the ground state light is emitted which has lower energy than the light that excited the molecule in the first place. Thus a molecule can be excited by high energy ultraviolet light, and emit lower energy blue, green, or even red light. This is fluorescence!
In some materials the transition from the M state to ground is forbidden. For example when the ground state is a singlet state, as most ground states are, and the middle energy state is a triplet state. In these materials the emission of light is delayed. The delayed emission is phosphorescence. (In addition to the rare decays from a triplet to a singlet state, collisions can convert the triplet M state back into a singlet E state followed by a decay to the ground state an light emission.
It is a cultural phenomenon that we in the United states correlate cleanliness with the bluish white appearance of white fabrics. Detergent manufacturers know this and add a colorless dye to the liquid detergents. This dye absorbs ultraviolet and emits blue light. This is a cultural phenomenon and is not true throughout the world. Outside of the United States test detergents to see if they fluoresce.
A typical fluorescent brightener for North
American laundry detergent is:
1,4-bis(styryl) benzene.
References:
The Physics and Chemistry of Color, by Kurt Nassau, Wiley, NY, 1983
|
Scientific Explorations with Paul Doherty |
|
25 June 99 |