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☞ OPTICS

Optics is all about light and there are several terms that may ring a bell.

Remember “The angle of incidence equals the angle of reflection”? You probably do. But do you remember what it means? Well, the angle of incidence is the angle at which light hits a surface; with specular (mirrorlike) reflection the light is reflected at the same angle. If the surface is rough, you get diffuse reflection, which means that the light bounces off in all directions.

Light may also pass through a medium—such as glass or water—and be refracted (change direction). This is because of the difference in the velocity with which light passes through the two different media (say, air and water), which is measured by the refractive index.

☞ CONDUCTION, CONVECTION, AND RADIATION

There are three ways in which heat is transferred:

Conduction can occur in solids, liquids, or gases and means (more or less) that a cool thing is warmed up by coming into contact with a hot thing. The different levels of conductivity in metals are reflected in their uses in anything from the science lab to kitchenware: Copper, for example, is highly conductive, and therefore it works well for fast cooking (although it may react with certain foods, which is why copper-bottomed pans are often lined with tin); whereas cast iron heats slowly but then cooks evenly.

Convection occurs in liquids and gases and is the basis of the principle that hot air rises. A hot liquid or gas is generally less dense than a cool one; as the hot particles rise, cooler ones rush in underneath to take their place. As the hot particles rise, they cool and come down again, and so on.

Radiation involves the energy that all objects, hot or cold, emit. It is the only one of the three that works in a vacuum and is how the sun’s rays manage to warm the Earth from such a far distance away.

Heat is not the only commodity that is transferred in these ways. There is also electrical conduction, mass convection (of which evaporation is an example), and electromagnetic radiation. So, strictly speaking, you should insert the words “heat” or “therma” in front of conduction, convection, and radiation if that is what you mean.

☞ PHYSICAL LAWS

Physics is based on properties that explain what matter and energy can or can’t do; without these interactions the universe would probably fall apart. From the observation of the interactions, laws were developed. Some of the physical processes and phenomena are revealed in this section. But a few definitions might help first.

Mass is the quantity of matter a body contains. Newton defined it more precisely by bringing in inertia, which is “a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.” All this means is that a thing will sit still until you push it.

Force is calculated by multiplying mass by acceleration and concerns producing motion in a stationary body or changing the direction of a moving one.

Velocity is speed (the dictionary says, “measure of the rate of movement,” but most people call that speed) in a given direction.

Acceleration is the rate of increase in velocity.

Work is the exertion of force overcoming resistance (which might be electrical resistance, or it could be physical resistance, such as friction).

And, regardless of what anyone else may tell you, in this context a body is a thing. The dictionary says, “an object or substance that has three dimensions, a mass, and is distinguishable from surrounding objects.”

☞ THE LAWS OF THERMODYNAMICS

Thermodynamics is the study of heat and its relationship with other forms of energy, and it is important in the study of heat engines such as gas-driven motors and gas turbines.

The other key term here is entropy, which is defined as “a measure of the disorder of a system.” A solid has less entropy than a liquid, since the constituent particles in a solid are in a more ordered state. The flow of energy maintatins