Types of Crystals

Types of Crystals

Crystals like calcite, quartz, ice and tourmaline having only one optic axis are called uniaxial crystals.

Crystals like mica, topaz, selenite and aragonite having two optic axes are called biaxial crystals

Polarisation

Polarisation

The phenomena of reflection, refraction, interference, diffraction are common to both transverse waves and longitudinal waves. But the transverse nature of light waves is demonstrated only by the phenomenon of polarisation.

Fluoresence

Fluoresence

When an atomic or molecular system is excited into higher energy state by absorption of energy, it returns back to lower energy state in a time less than 10-5 second and the system is found to glow brightly by 

emitting radiation of longer wavelength.

When ultra violet is incident on certain substance, they emit visible light.

It may be noted that fluorescence exists as long as the fluorescing substance remain exposed to incident ultraviolet light and re-emission of light stops as soon as incident light is cut off. 

Fraunhofer lines

Fraunhofer lines

If the solar spectrum is closely examined, it is found that it consists of large number of dark lines. These dark lines in the solar spectrum are called Fraunhofer lines.Solar spectrum is an example of line absorption spectrum.

The central core of the sun is called photosphere which is at a very high temperature of the order of 14 million kelvin. It emits continuous spectrum. The sun's outer layers called chromosphere. this is at a comparatively lower temperature at about 6000 k. It contains various elements in gaseous state.

When light from the central core of the sun passes through sun's atmosphere ,certain wavelengths are absorbed by the elements present in the chromosphere and the spectrum is marked by dark lines.

By comparing the obsorption spectra of various substance with the Fraunhofer lines in the solar spectrum, the elements present in the sun's atmosphere have been identified.

Phosphorescence

Phosphorescence

There are some substance in which the molecules are excited by the obsorption of incident ultra violet light, and they do not return immediately to their original state. The emission of light continues even after the exciting radiation is removed. This type of delayed fluorescence is called phosphorescence.

Band obsorption spectrum

Band obsorption spectrum

If white light is allowed to pass through iodine vapour or dilute solution of blood or chlorophyll or through certain solutions of organic and inorganic compounds, dark bands on continuous bright background are obtained. The band absorption spectra are used for making dyes.

Line absorption spectrum

Line absorption spectrum

When light from the carbon arc is made to pass through sodium vapour and the examined by a spectrometer, a continuous spectrum of carbon arc with two dark lines in the yellow region is obtained.