10. "Why should one never look into the light produced by an arc-welder?"

I believe there are two, related reasons why the light from an arc can damage the eye. Firstly, the combination of the cornea and the lens in the eye produces a focused image on the retina at the back of the eyeball, as shown below:

The retina, which is extremely sensitive to light, consists of two types of receptors, "rods" and "cones", that act rather like antenna. The cones are sensitive to color; the rods are sensitive only to lightness and darkness. So, returning to the figure, the energy of the light that is spread over the opening of the eye - the pupil - is concentrated into a small area of the retina by the focusing effect. So, if the power from the source - an arc, say - is beyond a certain limit then the energy absorbed by the retina at the focused spot will produce sufficient heat to cause irreparable damage. The intensity transmitted through the pupil depends on two factors, the distance of the source from the eye (it "falls off" as r^-2) - so, clearly, there is some advantage in having an arc source a large distance away - and the area of the pupil itself. With an intense incident beam, the eye attempts to prevent damage to the retina by reducing the area of the pupil. However, there is a limiting size below which the pupil cannot contract.

There is a second reason. Not only is there a good deal of power emitted by an arc, but the light produced by the arc itself contains a large amount of uv radiation. The energy of the uv radiation is such that it can cause irreversible cellular damage of the retina. Indeed, the uv could also cause "sunburn" and damage any exposed skin, which is why it's a good idea to wear a full mask and gloves when arc-welding.

So, all in all, it's not a good idea to look at an arc-source!

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