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:
“I nightly offer up my prayers to the throne of grace for the health and safety of you all, and that we ought all to rely with confidence on the promises of our dear redeemer, and give him our hearts. This is all he requires and all that we can do, and if we sincerely do this, we are sure of salvation through his atonement.”
—Andrew Jackson (1767–1845)
“There is no calamity which a great nation can invite which equals that which follows a supine submission to wrong and injustice and the consequent loss of national self-respect and honor, beneath which are shielded and defended a people’s safety and greatness.”
—Grover Cleveland (1837–1908)
“... what a family is without a steward, a ship without a pilot, a flock without a shepherd, a body without a head, the same, I think, is a kingdom without the health and safety of a good monarch.”
—Elizabeth I (1533–1603)