A parsec is a measure of distance equal to about 3.26 light-years. This length is equal to the distance an observer must be from earth so that the earth and sun are separated by an angle of one arcsecond. The term parsec is shorthand for “parallax second.” Parallax describes the effect that causes distant objects appear to move less than near objects.

A complete circle is comprised of 360 degrees. Each degree is comprised of 60 arcminutes, and each arcminute is comprised of 60 arcseconds. The angular width of a hand on a fully extended arm is about 10 degrees, and a pinky finger width is about one degree. When observation stellar constellations in the sky, it is handy to know these approximations, as books will often describe stars being separated by a certain number of degrees.

There are a few different types of nebula, but they could all be described as a cloud of gas or dust in outer space. Some clouds are made of very hot gas and emit light in brilliant colors. The most visible emission nebula from Earth is the Orion Nebula (about 1800 light-years away). It can be seen with the naked eye, but it is so dim that photographic equipment is needed to detect the colors. Other nebulae are made of dust and actually obscure the view of stars behind them, or sometimes reflect light from nearby stars. Some of the most spectacular photographs of nebulae in magazines combine emission, absorption and reflection regions into one image.

A polymer is any substance composed of repeating units that are linked to form a larger structure in the way individual metal links are joined to form a long chain. Most chemical polymers are made from hydrocarbon units and produce substances such as plastic, rubber, foam and synthetic fibers, like polyester. The chemical bond between each of the individual units is very strong, but the bond between the chains they make up is very weak. This gives the substance its property of being pliable and springy.

Molecules that are only made from carbon and hydrogen atoms are hydrocarbons. Gasoline is a mixture of different hydrocarbons along with small amounts of detergents to keep engines clean. When a pure hydrocarbon is burned with oxygen, the only products are carbon dioxide and water. Car engines do not burn the fuel completely, and the leftover hydrocarbons that come out the exhaust pipe make up a portion of smog.

The brownish haze that hangs in the air above crowded cities, known as smog, is the result of burning hydrocarbons in an unclean way. There are two main pollutants that are generated by uncleanly burning hydrocarbons: extra unburned hydrocarbons and oxides of nitrogen. These two substances react with each other when ultraviolet light and heat from the sun enter the Earth’s atmosphere. The result is a brown haze, which is thicker on hot, sunny days. The oxides of nitrogen also react with ordinary water in the air, and form nitric acid, or acid rain.

A liquid crystal display is a flat sheet composed of regions that can selectively block or transmit light based on electrical signals. These displays are used in almost all digital watches, laptop screens and some televisions. The LCD sheet is made from a number of layers that each process light in different ways.

The layer closest to the front of the display polarizes the incoming light. The next layer is the liquid crystal itself which rotates the polarization of light, followed by another polarizing layer that is oriented 90 degrees relative to the first, and finally a reflective backing.

The polarizing layers are like a set of Venetian blinds – the light that passes through has a clearly defined direction. The two polarizers in the LCD are set in opposite directions, so without the liquid crystal, the light would pass through the first polarizer, but would not pass through the second polarizer because the orientation does not match. With the addition of the liquid crystal, the light is twisted 90 degrees, so it passes through the second polarizer and is reflected off the mirror.

The light reflects back through the second polarizer and is twisted back by the liquid crystal. Finally the light travels back through the first polarizer, and out from the front of the display. An image is created by applying electricity to the liquid crystal in portions of the display. The liquid crystal portions that are energized lose their ability to twist the incoming light. This causes the light path to stop at the second polarizer because the crystal has not twisted it. The display looks dark in these regions.

Color displays arrange the liquid crystal into thousands of tiny red, green, and blue cells and regulate the electricity going to each cell to control how much the crystal twists the light, and thus how much light passes through.

Plasma TV:
The trendiest televisions use tiny fluorescent lights to make the picture on the flat front panel. Much like LCD TVs, plasma TVs are divided into thousands of tiny red, green and blue cells. Each cell is a fluorescent light, and can be controlled individually by the TV’s circuitry. By supplying more or less power to the fluorescent elements, any color can be mixed. Even though the plasma technology is simpler than LCD technology, it is more difficult to manufacture so many tiny fluorescent lights, and this causes the price to be higher.

Conventional TV:
A standard television is built around a large glass structure called a cathode ray tube. The front of the CRT is coated on the inside with phosphors – compounds that emit light when hit with energy. The cathode ray is a narrow stream of electrons that originate from the back of the CRT. The TV circuitry controls where the beam is aimed, and how much energy it contains. By rapidly sweeping the beam across the screen in sequential rows, an entire picture can be reproduced. Unfortunately, the energy in the cathode ray is only partially converted to light by the phosphors. The remaining energy is converted to X rays that would shoot out the front of the screen if not for the many pounds of lead shielding mixed right into the glass.

Bubble jet printer:
Many inkjet printers use bubble jets to get the ink onto the paper in very precise dots. A bubble jet consists of a long slender tube about 60 microns (0.0024″) square. A typical printhead may have a few hundred jets. The ink reservoir sits above the jet, and keeps the jet constantly filled with ink. There is a small, but powerful heater placed in the jet that is controlled by the printer’s circuitry. When the heater is turned on, a small amount of ink boils and forms a vapor bubble. When the heater is turned off, the bubble collapses, and ink is propelled out the bottom of the jet and onto the paper. This process repeats many thousands of times as the printhead is pulled precisely across the paper.

A transistor is a fundamental electronic component that is the main constituent of microchips, and is used in just about all electronic equipment. A transistor is essentially an electrical switch that can be controlled by an electrical signal. The benefit is that the input electrical signal can be of much smaller magnitude than the output signal. For example, if a microphone were connected to a speaker, and someone yelled into the mic, no sound would come out of the speaker because the mic does not produce a large electrical signal. If a transistor were introduced so that the mic provided the input signal, and a battery provided the switched power, the speaker would sound as loudly as the circuit allows. All commercial audio amplifiers contain more than just one transistor because using more than one will allow for better sound reproduction. Many amplifiers tout their “Power MOSFET” design. A MOSFET is a type of transistor that, when built properly, wastes very little electricity as heat. The cooler running design allows for higher power throughput.

Fuel Cell:
If an electric current is run through water, some of the electrical energy will be used to break water molecules into their constituents – hydrogen and oxygen. A fuel cell performs this process in reverse by combining oxygen and hydrogen and producing an electric current. Some fuel cell cars must store large amounts of hydrogen. The hydrogen is combined with oxygen in the atmosphere to produce the electric energy used to power the car. Other designs are able to extract the hydrogen from more easily stored fuels like gasoline or methanol. This technology is appealing because fuel cells produce no pollution; however, they are still being developed and are not yet ready for mass production.