Background

The sun is a fusion reactor in the sky that produces almost all of the energy available on Earth. Unlike nuclear power plants that make energy by splitting atoms, the sun fuses or combines smaller atoms together to create energy. In fact, the sun makes enough energy each minute to meet all of the people of the world's total energy demands for an entire year. The sun has generated energy for about 4 billion years and is expected to produce energy for another 4 billion years.

Law of Reflection

Most of the things you see do not emit light of their own. You can see them because the objects reflect part of the light that falls upon them. When light hits the surface of a material, it can be re-emitted or absorbed into the material and turned into heat. For most materials, both of these processes happen to some extent. Light re-emitted from a material back into the medium from which it came is called reflection.

The Law of Reflection states that the angle that a light ray strikes a surface is equal to the angle at which the light ray leaves the surface. For smooth surfaces, like a mirror, this relationship is easy to see and understand.

For uneven surfaces, like a sheet of paper, many light rays hit the paper at different angles due to the roughness of the paper's surface. The individual light rays still follow the Law of Reflection, they just appear to scatter because the rays hit and reflect at so many different angles.

What is the focal point?

The focal point is the point at which beams of parallel light reflected by a mirror or refracted by a lens appear to come together. An image may appear at the focal point of a mirror or lens.

Material Reflectivity

The reflectivity of a material describes how much light is returned from the surface of the material. The chemical nature of the material and the smoothness of the surface impact the amount of reflected light and the angle at which the light is reflected from a surface. A shiny, polished piece of silver metal, like a mirror, has a high material reflectivity, while a piece of charcoal would have a low reflectivity.

Kinetic Energy

Kinetic energy is the energy associated with a moving object. Kinetic energy is a scalar quantity that can never have a negative value. The kinetic energy of an object depends on two factors, the mass of the object and its speed. Speed impacts kinetic energy more than mass. For instance, if two objects are rolling along the floor with the same speed, but object A has a mass of 2 kg and object B has a mass of 4 kg; object B would have twice the kinetic energy as object A. However, if two other objects C and D, both with the same mass, were rolling across the floor and object C had twice the speed as object D, object C would have four times more kinetic energy than object D.

Potential Energy

Potential energy is energy that is stored in an object whether due to its position or the object's shape. It's called potential energy because the energy has the ability or potential to do work at a later time. A stretched spring or a block moved to a greater height would be examples of potential energy. Just think how much work the block could do on your head if it was dropped from a height of only five or six feet. The falling block would have a large "potential" for disaster.

What is heat?

Heat is the total kinetic energy of all the particles in a given system. Heat is thermal energy that always spontaneously flows from a region of higher temperature to one of lower temperature. Remember "total of all" for heat. A good way to think of the concept of the "total of all" is to imagine that you wanted to measure the height of students in your class. One way to do it would be to have each student stand on the head of another student and so on stacking the students on top of each other. Ouch! Not only would you most likely have to cut a hole in the roof, but you'd need a very long measuring device to get the students' heights. This would give you the "total height of all" students. Heat is the "total kinetic energy of all" particles.

Remember: Heat always flows from a higher temperature to a lower temperature and never the reverse!

What is temperature ?

Temperature is the average kinetic energy (motion) of the particles. Average is the key word here. In any given system some particles have a little energy, some have a medium amount, and some have lots of energy. A good way to help visualize the concept of "average of all in a system" is to think of something that you might commonly want to average. Let's say that you wanted to know the height of the students in your class. One way to find this would be to measure each person's height and to find the average height. You'd do this by adding up all of the student's individual heights and dividing by the total number of students measured. This would give you the "average of all" students. Temperature is the "average kinetic energy of all" of the particles.

What is specific heat?

Specific heat is the amount of heat energy needed to raise the temperature of one gram of a substance by one degree Celsius. A substance with a high specific heat takes more energy to change its temperature than a substance with a low specific heat. Water has a high specific, while aluminum metal has a low one. That means if the same amounts of water and aluminum were exposed to the same amount of heat, aluminum would increase its temperature more quickly than the water.

Table of specific heats
Substance	Specific heat (calories/gramCelsius)
air	0.23
sandstone rocks	0.20
water	1.00

What is thermal conductivity?

Thermal conductivity is a measure of how well a material transfers heat. You can think of thermal conductivity as how well a material can give you back the heat that was stored in it at an earlier time. Thermal conductivity varies depending on the density of free electrons in a material. Since metals have more free electrons than nonmetals, metals have thermal conductivity values that are much larger than the nonmetals. So a silver spoon transfers stored heat about 500 times better than a piece of concrete.

Table of thermal conductivity
Material	Relative thermal conductivity
air	0.11
concrete	12
glass	4
iron	320
sandstone	3
water	4.2

What is thermal mass?

Thermal mass is a measure of how well a material stores and transfers heat. The thermal mass efficiency depends on two factors: specific heat and thermal conductivity. A material with a high thermal mass efficiency would have higher values in specific heat and thermal conductivity. The best light collector design would use materials that had a high specific heat value and a high thermal conductivity value.