Most plants grown in a hydroponic setting require light, lots of it.
They photosynthesize to produce their own nutrients. They require warmth to keep biochemical reactions going, and going at the right rate. They regulate many processes by the amount of light present, speeding some up and shutting down others.
During photosynthesis plants take in carbon dioxide and water to produce glucose and oxygen. The glucose is used as an internal energy source and much of the oxygen is expelled. But the color and intensity of the light they receive plays a large role in how they do that.
The fact that leaves are generally green and/or yellow shows that those colors are being reflected. The other components of white light (which is a mixture of many wavelengths) are mostly absorbed. But some are taken in more efficiently than others at different growth stages. Wavelengths in the range of the spectrum we perceive as red are used more during growth phases. Blue is absorbed more to produce flowering or fruiting.
Since few hydroponic gardens are simply left out in the sun to take their chances, a lighting scheme is required to assist them in all those activities.
Plants that sit near an open window or are grown outdoors largely self-regulate the wavelengths they absorb, where the whole spectrum is abundant. But those that live indoors can only take up what is available. That makes it important to ensure that they have all they need, both in terms of intensity and wavelength.
Indoor lights are typically less intense (provide less overall energy) and therefore plants may need to be supplemented with natural light. Or, it may be necessary to use special lights to provide the total amount of light energy needed for warmth, photosynthesis and other useful tasks. The mixture will vary from one climate to another.
Indoor lights come in a variety of types, prices and wavelengths, some better for plants than others.
Incandescents provide a lot of heat. They're relatively inefficient at using electricity for that reason. But they produce strongly at the red end and weakly in the blue range.
Many fluorescents, by contrast, tend to produce greenish light. They're filled with mercury and the interior coatings that produce light in the visible range may be better or worse at producing a white-approximating spectrum. But, they are much cheaper to use in the long run.
A relatively inexpensive setup can be achieved with a mixture of incandescents and fluorescents. A 30-watt incandescent with a 100-watt equivalent fluorescent provides a good balance of wavelengths. Fluorescents use much less electricity so the wattages can't be directly compared.
While they cost more, metal halide and sodium lamps make for very good lights for hydroponic gardens. Metal halide lamps produce well in the blue range and sodium lamps provide more reddish light. The bulbs can't be switched between fixtures so it's necessary to buy a housing appropriate to each type.
Some newer LED lamps will produce a very broad spectrum and some can be adjusted to favor one wavelength range. However, they're expensive and many won't produce the same output as any of the other types.
Plants vary in the intensity they require, as well. Ensure that medium light plants receive at least 1,000 foot-candles. Others will need 2,000 foot-candles (about 20 watts per square foot) or more. A good light meter will tell you how much is being given off, or you can sometimes find the rating listed on packaging or advertisements.
Most lamps should be placed about a foot from the plant to provide the proper intensity. The figure will vary considerably, though, depending on whether incandescent, fluorescent or another source is used.
Plants that receive sunshine will need from 8-10 hours per day. If all artificial light is used, the number rises to between 12-14 hours per day. For some plants and lighting schemes 16-18 hours per day may be needed. The specific number depends heavily on the species of plant.