Involving an electrical effect produced by the action of light or other radiation.

The *photoelectric effect* occurs when light (or similar radiation such as X- rays) falls on a material such as a metal plate and causes it to emit electrons. The discovery of the photoelectric effect led to important new theories about matter (and to a Nobel Prize for Albert Einstein). *Photoelectric cells,* or *photocells*, are used in burglar-alarm light detectors and garage-door openers (both employ a beam of light that is broken when something moves across it), and also to play soundtracks on movie film (where a light beam shines through the soundtrack encoded on the film and is “read” by the photocells).

The *-voltaic* part of *photovoltaic* comes from the name of Alessandro Volta, inventor of the electric battery. Thus, unlike photoelectric cells, which use electricity for certain small tasks, photovoltaic (or PV) cells actually produce electricity. Solar cells, the standard type of photovoltaic cells (often called simply *photocells*), operate without chemicals and with no moving parts to create energy directly from sunlight. Much research is now being done on creating an alternative technology—solar film, which could be stuck onto almost any surface, or possibly even sprayed on.

A tiny particle or bundle of radiant energy.

It was Albert Einstein who first theorized that the energy in a light beam exists in small bits or particles, and scientists today know that light sometimes behaves like a wave (somewhat like sound or water) and sometimes like a stream of particles. The energies of photons range from high-energy gamma rays and X-rays down to low-energy infrared and radio waves, though all travel at the same speed. The amazing power of lasers is the result of a concentration of photons that have been made to travel together in order to hit their target at the same time.

The process by which green plants use light to produce organic matter from carbon dioxide and water.

The Greek roots of *photosynthesis* combine to produce the basic meaning “to put together with the help of light.” Photosynthesis is what first produced oxygen in the atmosphere billions of years ago, and it's still what keeps it there. Sunlight splits the water molecules (made of hydrogen and oxygen) held in a plant's leaves and releases the oxygen in them into the air. The leftover hydrogen combines with carbon dioxide to produce carbohydrates, which the plant uses as food—as do any animals or humans who might eat the plant.

(1) Very clear and easy to understand. (2) Able to think clearly.

Mental *lucidity* is easy to take for granted when we're young, though alcohol, drugs, and psychological instability can confuse the mind at any age. We all hope to live to 100 with our mental abilities intact, which is entirely possible; avoiding the condition called dementia (which includes the well-known Alzheimer's disease) often involves a combination of decent genes, physical and mental activity, and a good diet. Writing *lucidly*, on the other hand, can take a lot of work at any age; you've probably had the experience of trying to read a set of instructions and wondering if the writer even grew up speaking English.

To clarify by explaining; explain.

The basic meaning of *elucidate* is “to shed light on.” So when you elucidate, you make transparent or clear something that had been murky or confusing. *Elucidation* of a complex new health-care policy may be a challenge. Elucidation of the terms of use for a credit card may be the last thing its provider wants to do. The physicist Carl Sagan had a gift for elucidating astronomical science to a large audience, his *lucid* explanations making clear how stars are born and die and how the universe may have begun.

(1) Hard and difficult study. (2) The product of such study.

*Lucubration* came to mean “hard study” because it originally meant study done by lamplight, and in a world without electric lights, such study was likely to be the kind of hard work that would only a dedicated student like Abe Lincoln would make a habit of. The word has a literary feel to it, and it's often used with a touch of sarcasm.

Partly transparent; allowing light to pass through without permitting objects beyond to be seen clearly.

With its prefix *trans-*, meaning “through,” *translucent* describes material that light shines through without making anything on the other side clearly visible, unlike a *transparent* material. Frosted glass, often used in bathroom windows, is translucent, as is stained glass. Red wine in a crystal goblet, when held before a candle in a dark corner of a quiet restaurant, usually proves to be translucent as well.