One of the most magnificent astronomical events ever experienced by people
in their life is the total solar eclipse. Of all eclipses, solar eclipses are probably the ones that attract maximum interest from everyone including astronomers, both amateur as well as professional. Thousands of them travel miles and visit different places and locations just to catch a glimpse of the spectacular solar eclipse.
However, watching the sun with a naked eye can prove dangerous and hence, it is essential to resort to adequate precautions. This is because if the retina is exposed to powerful visible light, then there are chances that the exposure may cause injury to its light-sensitive cone cells and rod.
The intense light generates a sequence of chemical reactions inside the cells, which can damage their capacity to react to any visual stimulus. Moreover, in an extreme case, the same exposure can also destroy them entirely. The result, obviously, is a failure of visual functioning that can be either permanent or temporary, depending on how severe the damage is.
In case the person has been observing the sun for a very long time or repeatedly without resorting to proper eye safety measures, then the photochemical retinal injury may be accompanied by thermal damage. Thermal injury refers to the tremendous heating caused by high levels of near-infrared and visible radiation, which literally burns the exposed tissues. This photocoagulation (thermal injury) destroys the cones and the rods, creating a tiny blind area. This is a significant danger to the eye sight because the photic retinal injury occurs without any experience of pain (since the retina does not have any pain receptors), and prominent vision effects do not show up until several hours after the harm has been done.
It is only a total solar eclipse that can be observed with a naked eye because at this time, the moon is completely covering the bright disk of the sun. Viewing an annular or partial eclipse is never risk free unless it is performed with proper techniques and equipment. During a partial phase of solar eclipse when 99 percent of the photosphere (surface of the sun) is obscured, the remaining of its crescent is still sufficient enough to cause retinal burns, even though the illumination level is as small as twilight. People viewing even partial phases of a total eclipse are, therefore, advised to take proper precautionary eye safety measures. Failure to employ proper eye safety methods can result in permanent eye injury or even sever vision loss.
The same methods are used for observing and photographing a partly eclipsed sun and an annular solar eclipse. The most secured and inexpensive technique is the projection method. In this method, a screen is placed behind a small opening (pinhole size) to get the required image of the sun. Whenever there is a solar eclipse, there are many ways to get the images of the sun. You can use a perfboard with many holes, a straw hat (loosely woven), or even your fingers to interlace and produce beautiful images of the eclipse on a wall or a screen. A white card can also be used to get the magnified images of the eclipse with the help of a small telescope or binoculars. A number of people can enjoy the eclipse fraction at a time by using these methods but you need to be careful that your body looks directly at the eclipse or through the device. Projection method is a safer option, since many people can enjoy the spectacular images without looking directly into the sun, while in a pinhole system, very few people can get the benefit and that too, if the screen is placed at least one meter behind that small opening.