Safety
A transparent sphere of any material with refractive index greater than 1 brings parallel rays of light to a rough focus (that is, a focus with significant coma). For typical values of the refractive index, this focus usually lies very close to the surface of the sphere, on the side diametrically opposite that where the rays entered. If the refractive index is greater than 2, the focus lies inside the sphere, so the brightest accessible point is on its surface directly opposite the source of light. However few materials have that property. For most materials, the focus actually lies slightly outside the sphere. The closer the refractive index is to 2, the closer the focus is to the surface of the sphere and hence the brighter the spot formed on an object touching the sphere.
This effect is used in the Campbell–Stokes recorder to record the brightness of sunlight by burning the surface of a piece of cardboard placed near the sphere. The same effect can occur with a crystal ball that is brought into full sunlight; it may either burn a hand that is holding it, or even start a fire if it is resting on or very close to dark coloured flammable material. Consequently, such a ball should always be covered before being taken outdoors on a sunny day, and should not be left uncovered near a window.
Read more about this topic: Crystal Ball
Famous quotes containing the word safety:
“Once women begin to question the inevitability of their subordination and to reject the conventions formerly associated with it, they can no longer retreat to the safety of those conventions. The woman who rejects the stereotype of feminine weakness and dependence can no longer find much comfort in the cliché that all men are beasts. She has no choice except to believe, on the contrary, that men are human beings, and she finds it hard to forgive them when they act like animals.”
—Christopher Lasch (b. 1932)
“He had a gentleman-like frankness in his behaviour, and as a great point of honour as a minister can have, especially a minister at the head of the treasury, where numberless sturdy and insatiable beggars of condition apply, who cannot all be gratified, nor all with safety be refused.”
—Philip Dormer Stanhope, 4th Earl Chesterfield (1694–1773)
“Can we not teach children, even as we protect them from victimization, that for them to become victimizers constitutes the greatest peril of all, specifically the sacrifice—physical or psychological—of the well-being of other people? And that destroying the life or safety of other people, through teasing, bullying, hitting or otherwise, “putting them down,” is as destructive to themselves as to their victims.”
—Lewis P. Lipsitt (20th century)