Explanation: Quartz shows the property of double refraction. Therefore, two pieces of quartz, one right-handed and one left handed are taken and cemented back-to-back in the construction of 60° prism (Cornu mounting), or the energy must be reflected and returned through a single 30° prism, so that it passes through the prism in both directions (Littrow mounting).

Explanation: Most modern instruments now use a diffraction grating as a dispersing element in the monochromator, as prisms in general have a poorer stray light performance and require complex precision cams to give a linear wavelength scale. Replica gratings can even be produced more cheaply than prisms and require only a simple sine bar mechanism for the wavelength scale.

Explanation: A typical reflection grating may have 1200 grooves/mm, which means the grooves are spaced at about 800 nm intervals. The grating may have a width of 20 mm or more, giving a total of at least 24,000 grooves.

Explanation: Holographic gratings used in commercial spectrophotometers are either original master gratings produced directly by an interferometer or are replica gratings. Replica gratings are reproduced from a master holographic grating by moulding its grooves onto a resin surface on a glass or silica substrate.

Explanation: Normally, the absorbance of any material should be less than 0.2 at the wavelength of use. Below 300 nm, quartz or fused silica is utilized. The limit for quartz is 210 nm.

Explanation: Reflections from glass surfaces are reduced by coating these with magnesium fluoride, which is one-quarter wavelength in optical thickness. With this, scattering effects are also greatly reduced.

Explanation: To minimize light losses, lenses are sometimes replaced by front-surfaced mirrors to focus or collimate light beam in absorption instruments. Mirrors are aluminized on their front surfaces. With the use of mirrors, chromatic aberrations and other imperfections of the lenses are minimized.

Explanation: The two beams must retain the spectral properties of the incident beam. Half-silvered mirrors are often used for splitting the beam. However, they absorb some of the light in the thin metallic coating. Beam splitting can also be achieved by using a prismatic mirror or stack of thin horizontal glass plates, silvered on their edges and alternatively oriented to the incident beam.

Explanation: Gases may be contained in cells which are sealed or stoppered to make them air-tight. The sample holder is generally inserted somewhere in the interval between the light source and the detector. For the majority of analyses, a 10 mm path-length rectangular cell is usually satisfactory.

Explanation: Studies of dilute or weakly-absorbing liquid samples, or of samples where trace components must be detected, require a cell with a long path-length. For such applications, a 50 cm path-length with about a 300 ml volume cell is employed.