• Ordinary and Polarized Light
• Polarimetry
• Colorimetry
• Photometry
• Spectrophotometry
• Refractometry
• Luminescence

Ordinary and Polarized light

The waves of ordinary light are arranged in a complex, almost random manner. Unlike ordinary light, polarized light consists of orderly waves that oscillate in only one direction. The process of creating polarized light is called polarization. The device for making polarized light is called a polarizer.

Some crystals change color when exposed to differently polarized light.

Polarimetry

Chemical combinations with asymmetric C atoms are able to turn the plane

of the polarized light for a certain angle. Knowing that angle we can calculate the quantitative concentration of the optically active substance in a solution.

The device for determining the concentration by using this method is called a polarimeter.

Sugars are organic substances which contain asymmetric C atoms, so a polarimeter can be used to find the concentration of sugar in blood, urine, etc.

Many substances give us intensive colored solutions. For example the solution KMnO₄ is coloured violet, K₂Cr₂O₇ is orange, CuSO₄is blue. However, many colorless or vaguely coloured substances together with some reactants, give us intensive coloured solutions. The ions of Fe³⁺in the water solution are vaguely coloured yellow but in the presence of CNS^- , the solution we get is of an intensive red colour. The more concentrated the solution is, the less light will pass through it. If we can measure the intensity of the passing light it is possible to find out the concentration of the dissolved substance. To achieve this, various spectral devices are used, which disperse electromagnetic waves with optical prisms or diffraction gratings.

Colorimetry

Colorimetry is the method we use to determine the concentration of some coloured solution by comparing its colour with the colour of a known concentrated solution. Colorimeter is the apparatus for colorimetrics measurements.

1. Light bulb; 2. Filter; 3. Optical cell; 4. Lightsensitive cell; 5. Meter

Photometry

Photometry is the method we use to measure intensity of light that passes through a solution. The photometer is an instrument used for photometrics measurements. Monochromatic light is required for photometrics measurements.

Spectrophotometry

Spectrophotometry is the measurement of the quantity of light which is absorbed by a substance. For these measurements we use a device called spectrophotometer. All substances have their characteristic absorption spectra in the ultraviolet and infrared regions.

The schematics of the spectrophotometry

1. Light source; 2. monochromator (selects wavelength); 3. Quartz mirror (splits a light beam into a double beam); 4.i 5. quartz cells; 6. Photomultiplier (converts light into electric current); 7. Electric circuitry compares the two currents; 8. Recorder (a pen traces the absorption spectrum).

Refractometry

Refractometry is the method used to determine the concentration of some chemical combination by measuring its refractive index. It is based on the fact that the refraction index of the solutions is directly proportional to their concetration. By using this method we can find the concentration of fat in milk, alcohol in alcoholic drinks, or determine the purity of fat etc. The instrument we use for these measurements is called the refractometer.

1. Eyepiece 2. Scale 3. Double prism with fluid 4. Mirror

Luminescence

Luminescence is the giving off of light without an increase in the temperature. When electrons in the atoms of some substance absorb energy from the source, they tend to get rid of the excess energy. This energy is given off in the form of light. That is how the substance becomes luminescent. When these electrons recombine with other atoms, energy is emitted in the form of light. Different substances have different spectra of luminescent light. Luminescence is used in chemical analysis.

Luminescent substances are mixtures which radiate light by means other than heat. These substances can be divided into two types: fluorescent (the intensity of radiated light decreases rapidly) and phosphorescent (substances which glow after the energy supply is removed). Luminescent substances are used for making posters, luminescent colors etc. By choosing and mixing adequate fluorescent substances, every color in the visible light spectrum can be obtained.

Commonly used phosphorescent substances are Celluloid, ivory and paraffin. Some substances phosphoresce strongly when exposed to nuclear or ultraviolet radiation, or even normal daylight.

Phosphorescent substances are used for some special stage effects: walls and curtains which give off a faint glow.

Phosphorescence can be used in medicine for diagnosing certain diseases.