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:
“An evident principle ... is the principle of justice to all peoples and nationalities, and their right to live on equal terms of liberty and safety with one another, whether they be strong or weak.”
—Woodrow Wilson (1856–1924)
“The Declaration [of Independence] was not a protest against government, but against the excess of government. It prescribed the proper role of government, to secure the rights of individuals and to effect their safety and happiness. In modern society, no individual can do this alone. So government is not a necessary evil but a necessary good.”
—Gerald R. Ford (b. 1913)
“Perhaps having built a barricade when you’re sixteen provides you with a sort of safety rail. If you’ve once taken part in building one, even inadvertently, doesn’t its usually latent image reappear like a warning signal whenever you’re tempted to join the police, or support any manifestation of Law and Order?”
—Jean Genet (1910–1986)