We will deal here with another interesting phenomenon connected with the quantum theory. We often deal with this phenomenon in our everyday life. Maybe you have a watch that glows in the dark? This kind of effect is called the phosphorescence. Scientists noticed the phenomenon long before the quantum model of the atom was developed, but just that model let the phenomenon be explained. But let's start with a similar effect called the fluorescence.
As you already know, an atom can absorb a quantum of energy and then achieve an excited state. An electron of such atom changes a lower energy state into a higher one. The excited atom can emit a quantum of energy what is connected with the electron jump-down back to the lower energy state. If, for example, shining ultraviolet light on atoms of some substance, then electrons of those atoms will get to relatively high-energy states. (Ultraviolet light has a bit higher frequencies than visible light, and so its photons transfer more energy than photons of visible light). After a short moment the electrons should get back to the initial state emitting photons having adequate energy. But some electrons don't change their orbits directly into the initial one; they first jump on some intermediate state and just then on the initial one. So in such case they emit two photons, each of the lower energy than one photon of ultraviolet light absorbed before. Those emitted photons represent electromagnetic waves having lower frequencies than the absorbed one, so they could be photons of visible light. Let's sum it all up: If shining ultraviolet light (which we can't see) on atoms of some substances, they begin to emit visible light. The phenomenon is called the fluorescence.
Phosphorescence is a similar effect. The difference is that electrons don't jump to the initial state immediately but stay in the excited state for some time. That time is not precisely defined but with some probability. So if you have a sample consisting of many such atoms (like the dial of a watch) then each will have its own moment of emitting a photon. And so, some atoms will get back to the initial state right after being excited, some other, after a few minutes, and some, even after a few days. If there is no more light then the sample will glow some time more, until all its atoms get back to their ground states. (Of course, after a while, still emitted, the light will reach a point of too little brightness for us to see). So phosphorescence differs from fluorescence only with duration.
Fluorescence is used in fluorescent lamps. Such lamp consists of a hermetic tube filled with atoms of some gas. In the tube there are electrons moving very fast under the influence of applied voltage. Electrons collide with atoms of the gas exciting them. Excited atoms emit photons of ultraviolet light. The tube is covered with some substance that emits visible light under the influence of UV (ultraviolet) light. Why don't you check if you have any fluorescent lamps at home?
We often deal with the phenomena of fluorescence and phosphorescence in our everyday life. But did you know that they are explained on the basis of the quantum theory?
The phenomenon of fluorescence is connected with an electron jump-down on its ground orbit but in more than one jump.
The fluorescence is the glowing of a body when illuminated with electromagnetic waves.
The phosphorescence is the some time long glowing of a body after it has absorbed electromagnetic radiation.
Test no. 6