Brownian Motion

The clusters of atoms, or molecules, that make up the water move rapidly about and collide with the dust particles, setting them into random motion. This is called Brownian Motion, and it provides evidence for the existence of atoms. Nearly 200 years ago, the British botanist, Robert Brown, noticed pollen grains dancing randomly in water. He first wondered if they were alive. Then he found that dust particles in the water had the same motion.

In the adjacent mechanical simulation a large disc represents a dust particle in water and steel balls represent the molecules that make up the water. They are in constant motion due to the heat energy of the water. As the balls hit the disc, they cause it to move about in a random manner.

The Idea of the Atom

As early as 400 B.C.E., a philosopher, Democritus, proposed the concept of the atom which means "indivisible". Since then, the idea of the atom has undergone numerous changes reflecting the thinking of the time. A diagram that illustrates the ideas about the atom including the scientists involved allows visitors to trace its history.

One Particular Wave

What is an Atom? Science's current answer is hard to visualize. We know the Atom as the basic "building block" of matter, yet it combines features of a wave and features of a particle. Like a wave on a pond, it can appear to be spread out. Like a particle of sand, it can appear concentrated in one place. How to visualize something that appears to be both a wave and a particle is not easy. This computer-generated hologram is the work of artist Gerald Marks, and was created as a response to this challenge.

Quantum Atom

Atoms are made of smaller particles: electrons, and the nucleus which is made up of protons and neutrons. In the case of an electron, quantum physics allows us to calculate only where the electron will probably be, this location is described as a "probability cloud". Atoms energized by different colors of light will raise the electron to different levels. The jump from one level to the next is called a quantum change. The electron can be at one level or another, but is never between and when an electron falls to a lower level, energy is given off. Visitors control different wavelengths of light emitted onto a three-dimensional model of a hydrogen probability cloud. The amount of energy an electron receives depends on the light's color. When the electron's energy changes, the atoms shape will change.

Radioactive Atoms

Atoms are made of still smaller particles. Electrons (with negative electrical charges) are pictured as "probability clouds" (this means that we can't say exactly where electrons are at any given time) around the center of the atom. At the center of every atom is the nucleus. 100,000 times smaller than the atom itself! The nucleus is made up of two kinds of particles, called protons (with positive electrical charge) and neutrons (with no electrical charge). For most atoms, the nucleus is stable - it keeps the same number of protons and neutrons forever. For some heavier atoms, however, the nucleus is not stable. It can release energy, and in doing so, change the number of its protons and neutrons. The atom becomes a different chemical element. These unstable atoms are called radioactive. Their released energy is called radiation.

Street Lights and Atoms

At this exhibit visitors look through a spectroscope which contains a large diffraction grating to analyze different types of light. Some light is produced by incandescence with a tungsten bulb - electric current running through the thin tungsten wire produces heat due to resistance. Heat energy causes the atoms that make up the wire to collide. Electrons within the atoms randomly absorb this energy causing them to become "excited" and they "jump" to higher energy levels. At these higher levels the electrons are unstable and fall immediately back to lower levels. When the electrons fall they re-emit the absorbed energy at random wavelengths in the form of heat and light. When observing an incandescent light through the spectrometer all visible wavelengths of light form an unbroken band of light. This is called a continuous spectrum in which colors shade into one another. The sun and other stars produce light in a very similar fashion. The following light sources are on display: Mercury Vapor Lamp, Low Pressure Sodium Gas Lamp, Neon Light, and an Incandescent lamp.

Toward an Even Smaller Realm: Radiation Visualization

The Geiger counter in this exhibit measures the radiation coming from the nearby radioactive rock sample. The rock is a sample of uranium, a naturally occurring radioactive element found throughout the earth. Each click of the counter represents an atom's release of energy. Although the average number of clicks can be predicted, there is no way to know when an individual radioactive atom will decay.

Shadow Wall

Walk into this exhibit and leave your shadow behind. The shadow wall is made up of atoms that absorb and emit light. Light is energy. A blue light striking the wall charges the atoms with energy and makes the wall glow except for that part of the wall blocked by a visitor. This creates "frozen shadows" on the glowing wall. Sooner or later the atoms release that energy in the form of greenish light. When a yellow light strikes the wall, the atoms release their energy quickly, so the wall goes dark. The wall still glows where you block the yellow light causing your "shadow" to remain on the wall.