Lasers are powerful, important devices, but can also be very hazardous. That is why it is important to understand what lasers can do and why they can be so dangerous when not used with the proper safety precautions. Understanding lasers is an important key to safe usage.
Light from powerful lasers is concentrated to power densities. These power densities are high enough to evaporate tissue, metal or ceramics. Our eyes are at an increased risk to that dangerous light because they are very sensitive. It is even possible to cause irreversible damage to your eyes from only one accidental exposure to a direct or reflected laser beam.
Lasers are much more dangerous than conventional light sources. The main danger of laser light exposure comes from its spatial coherence and its relation to the eye. The wave traits of a laser beam have a fixed relation to space and time, are all the same wavelength, and can travel great distance as nearly parallel beams. This all means that the power that impacts an area like the eye is independent of the distance to the source of radiation.
The risk of losing eyesight from accidental exposure to laser radiation comes from the unique properties in the eye. The eye is sensitive to radiation and usually has natural protective mechanisms, like blinking or turning away, when light appears too bright, but with higher power levels too much energy reaches the eye before it can properly respond. The radiation advances to the retina of the eye but the laser's exposure is only noticed after all the damage is done. The radiation is absorbed at the eye's surface, which leads to overheating and burning of the eye's tissue.
With conventional light sources, the power of the light disperses because the light radiates in different directions. That does not happen with laser light. The light from the laser comes in a much smaller concentration, but much higher density, increasing its power greatly. Because the fovea, which is located on the retina and is the area of the eye responsible for central vision, is only a few micrometers in size, someone can lose their eyesight in one pulse of a laser.
When used properly and with the right protection, lasers are extremely useful tools. It is important to understand the very basics of how lasers work and what makes them so powerful to prevent injury or harm.