question: Why do large mirrors stop reflecting a single image when cracked?
comment: When a mirror cracks each piece is (almost certainly) at a slightly different angle. Mirrors reflect what in front of them and when you angle the mirror *what* is in front of them changes.
passages: Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.
  
  The strain pattern resulting from tempering can be observed by viewing through an optical polarizer, such as a pair of polarizing sunglasses.
  
  Section::::Uses.
   Christie left an interesting clue in the surname of Marina Gregg for a woman who had a child afflicted with congenital rubella syndrome. This is the same surname of as Sir Normal Gregg, the Australian Ophthalmologist who first described cataracts in congenital rubella syndrome in 1941. Professor David Mackey mentioned this in his 2005 Gregg lecture to the Royal Australian and New Zealand College of Ophthalmologists meeting in Hobart.
  
  Section::::Publication history.
  
  BULLET::::- 1962, Collins Crime Club (London), 12 November 1962, Hardback, 256 pp
  
  BULLET::::- 1963, Dodd Mead and Company (New York), September 1963, Hardback, 246 pp
   Francis Iles (Anthony Berkeley Cox) was somewhat muted in his praise in his review in "The Guardian" of 7 December 1962 when he said, 
   Section::::Developing the character of Marina Gregg.
   Since then, various other refinements have been employed. One approach is to create a so-called non-absorbing mirror (NAM) such that the final 10 µm or so before the light emits from the cleaved facet are rendered non-absorbing at the wavelength of interest. Such lasers are called window lasers.
   The title - shortened from the one used for Christie's book - is part of a line from "The Lady of Shalott" by the English poet Alfred, Lord Tennyson:
  
  Section::::Production notes.:Inspiration theory.
   Section::::Manufacturing.:Tolerances.:Surface defects.
   The title of the novel comes from the poem "The Lady of Shalott" by Alfred, Lord Tennyson. The Lady of Shalott lives in a tower near Camelot, and sees it only reflected in a glass. She will be doomed if she looks directly; when Lancelot appears, she looks directly at him and the looking glass cracks. This poem is referred to by name several times in the novel, with these lines quoted:
  
  At the end, Miss Marple quotes the last three lines in referring to the dead actress:
  
  Section::::Characters.
   Tempered glass must be cut to size or pressed to shape before tempering, and cannot be re-worked once tempered. Polishing the edges or drilling holes in the glass is carried out before the tempering process starts. Because of the balanced stresses in the glass, damage to any portion will eventually result in the glass shattering into thumbnail-sized pieces. The glass is most susceptible to breakage due to damage to the edge of the glass, where the tensile stress is the greatest, but shattering can also occur in the event of a hard impact in the middle of the glass pane or if the impact is concentrated (for example, striking the glass with a hardened point).
   By controlling the thickness and density of metal coatings, it is possible to decrease the reflectivity and increase the transmission of the surface, resulting in a "half-silvered mirror". These are sometimes used as "one-way mirrors".
   Section::::Manufacturing.:Tolerances.:Wedge.
   The process of tempering float glass can cause a NiS to change from its normal state (known as a low-temperature structure) to a different high-temperature, crystalline structure. When the glass is cooled quickly (as part of the process) the NiS particle is unable to change back to its original form. 
  
  Over a certain period of time NiS will slowly convert to the original low temperature structure. This means the NiS increases in size, and the mechanical stresses caused by this cause the tempered pane to shatter, for no apparent reason (hence spontaneous glass breakage).
   Section::::Production notes.:Filming locations.
   Essentially, as a result when light propagates through the cleavage plane and transits to free space from within the semiconductor crystal, a fraction of the light energy is absorbed by the surface states where it is converted to heat by phonon-electron interactions. This heats the cleaved mirror. In addition the mirror may heat simply because the edge of the diode laser—which is electrically pumped—is in less-than-perfect contact with the mount that provides a path for heat removal. The heating of the mirror causes the band gap of the semiconductor to shrink in the warmer areas. The band gap shrinkage brings more electronic band-to-band transitions into alignment with the photon energy causing yet more absorption. This is thermal runaway, a form of positive feedback, and the result can be melting of the facet, known as "catastrophic optical damage", or COD.
   Upon meeting Gregg, Heather shares the story of meeting her in Bermuda and receiving her autograph, during which Mrs. Bantry notices a strange look cross Gregg's face. Mrs. Bantry then takes several other women to see the renovations made to the house only to be interrupted upon discovering Heather has collapsed without explanation. Despite all attempts to revive her, Heather is pronounced dead.
   Cutting "en cabochon" (French: "in the manner of a cabochon") is usually applied to opaque gems, while faceting is usually applied to transparent stones. Hardness is also taken into account as softer gemstones with a hardness lower than 7 on the Mohs hardness scale are easily scratched, mainly by silicon dioxide in dust and grit. This would quickly make translucent gems unattractive—instead they are polished as cabochons, making the scratches less evident.
   In tempered glasses, air jets are used to rapidly cool the top and bottom surfaces of a softened (hot) slab of glass. Since the surface cools quicker, there is more free volume at the surface than in the bulk melt. The core of the slab then pulls the surface inward, resulting in an internal compressive stress at the surface. This substantially increases the tensile strength of the material as tensile stresses exerted on the glass must now resolve the compressive stresses before yielding.
  
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   Section::::Manufacturing.:Tolerances.
   It is this compressive stress that gives the tempered glass increased strength. This is because annealed glass, which has almost no internal stress, usually forms microscopic surface cracks, and in the absence of surface compression, any applied tension to the glass causes tension at the surface, which can drive crack propagation. Once a crack starts propagating, tension is further concentrated at the tip of the crack, causing it to propagate at the speed of sound in the material. Consequently, annealed glass is fragile and breaks into irregular and sharp pieces. 
   While glass is being moved and installed, it is easy for the glaziers to nick or chip the edges of the glass with various tools. It is also possible for fasteners such as nails or screws used to attach glass stops to nick the glass edges if these fasteners are installed at an improper angle. These small nicks or chips may not result in immediate breakage. However, over time, as the glass expands and contracts, stress concentrations can develop around the nick, leading to breakage. In the case of tempered glass the entire unit usually breaks.
  
  Section::::Binding in the frame.
   Section::::Manufacturing.:Tolerances.:Reflectivity.
   Nickel sulfide inclusions ("stones") can be present in the glass. The most common cause of these inclusions is the use of stainless-steel machinery in the glassmaking and handling process. Small shavings of stainless steel containing nickel change structure over time and grow, creating internal stresses in the glass. When these stresses exceed the strength of the glass, breakage results. This type of breakage is almost always found in tempered glass and is indicated by a distinctive "figure eight" pattern, with each "loop" of the figure eight approx. 30mm in diameter.
   Heat-strengthened glass, or tempered glass, is glass that has been heat treated to induce surface compression, but not to the extent of causing it to "dice" on breaking in the manner of tempered glass. On breaking, heat-strengthened glass breaks into sharp pieces that are typically somewhat smaller than those found on breaking annealed glass, and is intermediate in strength between annealed and toughened glasses.
  
  Heat-strengthened glass can take a strong direct hit without shattering, but has a weak edge. By simply tapping the edge of heat-strengthened glass with a solid object, it is possible to shatter the entire sheet. 
   Section::::Developments.
  
  Section::::Developments.:Deformable solid mirror.
  
  Before ferrofluid mirrors were invented, solid deformable mirrors were used to correct atmospheric distortion and keep a steady focus. Deformable solid mirrors use flexible mirrors with complex actuators underneath to correct for atmospheric distortion and keep a steady focus. Shortcomings of traditional deformable solid mirrors include cost, fragility, and the need for continuous power. In addition, images from these mirrors have an undesirable quilt pattern due to the discrete actuators beneath the mirror surface.
  
  Section::::Developments.:Mercury mirror.
   Section::::Conditions that affect the PLC effect.:Material finish.
  
  Polishing the material affects the beginning of the PLC effect and the band velocities that were reported. Apparently a rougher surface provides more nucleation points causing stress raiser which help initiate deformation bands. These bands also propagate twice as fast in the polished specimen
  
  Section::::Conditions that affect the PLC effect.:Non effects.
  
presuppositions: Cracked pieces of a mirror stay at the same angle when broken. A mirror should behave the same way even when cracked.
corrections: Pieces of a cracked mirror are at a different angles which changes what is reflected in each mirror piece. Pieces in a cracked mirror are at slightly different angles, which changes the image that is being reflected.
raw_presuppositions: Cracked pieces of a mirror stay at the same angle when broken. A mirror should behave the same way even when cracked. A mirror should behave the same way even when cracked.
raw_corrections: Pieces of a cracked mirror are at a different angles which changes what is reflected in each mirror piece. Pieces in a cracked mirror are at slightly different angles, which changes the image that is being reflected. Pieces in a cracked mirror are at slightly different angles, which changes the image that is being reflected.