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:
“The safety of the republic being the supreme law, and Texas having offered us the key to the safety of our country from all foreign intrigues and diplomacy, I say accept the key ... and bolt the door at once.”
—Andrew Jackson (1767–1845)
“A lover is never a completely self-reliant person viewing the world through his own eyes, but a hostage to a certain delusion. He becomes a perjurer, all his thoughts and emotions being directed with reference, not to an accurate and just appraisal of the real world but rather to the safety and exaltation of his loved one, and the madness with which he pursues her, transmogrifying his attention, blinds him like a victim.”
—Alexander Theroux (b. 1940)
“To emancipate [the slaves] entirely throughout the Union cannot, I conceive, be thought of, consistently with the safety of the country.”
—Frances Trollope (1780–1863)