Other visible light hazards : Ordinary, visible light can present a hazard under certain conditions. You can burn paper by focussing the sun's rays using a small magnifying lens. The same effect can occur with the eye's lens on the back of the eyeball (the retina), and this usually happens when we are viewing an eclipse. The result can be total blindness or at least partial blindness. Similar dangers can occur when using daylight to illuminate a microscope slide.

The source of the danger is the sudden and intense illumination of a human eye by the sun. The accidents occur because it is difficult to ensure that a filter is going to absorb sufficient light to make it safe to view a sudden appearance of the sun, such as an eclipse, directly. The absorbent powers of a filter won't be instrument. discovered until it is too late, and the time-honoured method of using smoked glass is much unreliable. You can imagine how much worse the damage would be to the human eye if an eclipse was viewed through any optical instrument, e.g. binoculars, reflecting devices on microscopes, etc.

The only safe way to view an eclipse is to look at an image of the sun projected onto a screen. This means that a laboratory or classroom would have to be turned into a projection room, i.e. blacked out with a screen and an optical system need only be a 1 cm hole in a blind to a limit the required light and a plain mirror to direct the light onto the screen, Figure.

Figure: Safe way to view an eclipse