The Tools the Physicist Uses

C. H. Kunsman.

Physics, a basic and a far-reaching branch of science, gets right at the heart of many things that affect our life, comfort, and happiness. Agriculture, national defense, transportation, communication, and the control of climate are concerned with the applications of physics in particular fields.

Physics deals with matter and energy. It comprises the use and study of mechanics, heat, light, sound, electricity, magnetism, and atomic physics.

The physicist's tools are simple ones like the measuring rules, weighing balances, clocks or watches, thermometers, speedometers, and the electrical instruments. He also uses more complicated tools like the vacuum tubes, electronic amplifiers, recording meters, spectrophotometers, electrostatic generators, Cyclotrons, and the atomic pile.

Physics solves many of the problems that pertain to the processing and utilization of farm goods. A market exists for many an agricultural product simply because of its special physical property. Fibers have values based almost entirely on their physical characteristics appearance, durability, and wrinkle-proofness. Plastics are wanted for their strength, durability, color, and ease of shaping. Processed food, fresh fruit, vegetables, poultry products, and frozen or canned products are bought by housewives according to color, crispness, texture, or moisture content.

Physicists and what they do are less commonly known than chemists or engineers, because so many of them become engineers and technologists when they apply the basic principles of physics to a particular field.

A GREAT DEAL of science (particularly physics, chemistry, engineering, and biology) deals with seeing, sorting, comparing, and measuring the optical properties and dimensions. The microscope makes visible to the eye details not otherwise resolvable. Such devices range from a single lens or reading glass that magnifies type only two times to the electron microscope, developed in the past decade, which has a useful magnification up to 100,000 times. The magnification of this instrument is equivalent to making a period on this page appear to be 300 feet in diameter. In between these developments are the ordinary optical microscope, with a useful magnification of 100 to 500 times, and the ultraviolet microscope, with a useful magnification of 2,000 times.

Because the nature of the light used in microscopy is basic in obtaining definition or useful magnification, many devices have been used to control or modify the illumination. The one most recently made available for general use is known as the phase-contrast microscope, which employs minor changes in the phase relation of the wave fronts, where the crests of some of the light waves coincide with the troughs of others, so that highly transparent objects-whose structural details vary only slightly can be seen and photographed. Phase microscopes have been employed in the study of bacteria, molds, yeasts, suspensions in body fluids, muscular tissue, feldspar, dusts, and natural and synthetic fibers. The magnification varies from 300 to 1,000 times.

Because electrons in motion have properties similar to those of light, they are now used as an aid to sight in the electron microscope. The electron has a unit negative charge of electricity. Its weight is one eighteen-hundredth of the weight of our smallest atom, hydrogen. It is a part of all atoms. It is also at the heart of a radio or television set. In the electron microscope, those negative-charged particles are deflected and controlled by electric fields, in much the same way as glass lenses bend and control the light beam in the ordinary microscope. The present limit of vision or resolution of the electron microscope is a solid particle that consists of 100 or more atoms. This powerful pair of eyeglasses therefore helps us to see many of the larger molecules and to get a photographic record of them.

Scientists use the electron microscope for determining the size and shape of very small particles, or for observing very thin sections of substances, such as soils, starches, proteins, dyes, pigments, cellulose, natural and synthetic textile fibers, viruses, and other biological materials. The results are of maximum value when they are used with other information on molecular structure obtained by X-ray, spectroscopic, or light-scattering methods. To the extent that the information is available on these small units of agricultural materials largely depend their uses as foods, feeds, or industrial materials. The information is as important in agricultural research as information on the structural properties of brick and mortar is in the construction of buildings.

The physicist uses the spectroscope for study of the structure of atoms and molecules through the light they emit when in a high state of excitation, as in a flame, electric arc, or spark. Emission spectroscopy gives him a sensitive, accurate, and rapid method of determining, in amounts as small as a few parts per million, essential and desirable . constituents, or harmful and objectionable ones. Lead, tin, iron, and copper from spray residues, processing equipment, and containers used to store foods are examples of substances that may be harmful; calcium and magnesium are considered essential minor, or trace, elements for the growth of plants.

The spectroscopic method also has the advantage that spectra of a number of elements may be recorded on a photographic plate simultaneously and held as an impersonal record for study and analysis later. The spectrograph has been used in agricultural research to determine such trace elements as magnesium, calcium, manganese, copper, iron, boron, cobalt, lead, zinc, tin, and aluminum in soils, fertilizer materials, agricultural products (such as fruits, vegetables, and poultry), and industrial products and byproducts (such as pectin, fats and oils, microbiological culture media, and the fermentation residues).

In spectrophotometry the physicist has a means for studying some of the fundamental physical characteristics of materials. The devices he employs measure the ability of the material to reflect or transmit the electromagnetic waves. As the experimental techniques necessarily vary with the spectral regions, it is natural to divide the subject into visible, ultraviolet, X-ray, infrared, electrical, radio, and microwave spectrophotometry. Those physical methods are important because they give the necessary characterization and identification, without chemical or physical change in the materials under study. They are often the only methods available for obtaining those results.

LIGHT is the part of the spectrum that is ordinarily visible to the eye. A material appears colored because it selectively reflects or transmits light. Color is a significant factor in the acceptability of foods and most items of the home. The more nearly the original color of fruits, vegetables, and poultry products can be kept in processing and storage, the more acceptable they will be to the consumer.

In researches on agricultural products, color is treated only from a physical standpoint. The physicist can state with certainty that two beams of light are identical, or he can determine how two beams of light differ with no reference whatsoever to the eye. A recording spectrophotometer is used in the investigations because it permits a record of the percentage of light reflected from the sample (or transmitted through a clear extract) to be automatically recorded on a chart. The physicist thus can measure quickly and accurately differences in color in the fresh produce or the changes in color due to processing and storage.

Typical curves are given in the accompanying chart for light reflected from an orange, a winesap apple, yellow sweet corn, and fresh green peas, all products of average market maturity. Differences in color are related to differences in percentage of reflectance in the visible range of 400 to 700 milli-microns (4,000 to 7,000 angstroms, the units which are used to measure the wave length of light and which correspond to the colors indicated in the figures). In the chart on page 29, reflectance curves are given for lemons in different states of maturity. You will notice that the most marked differences occur at about 680 millimicrons, corresponding to one of the chlorophyll absorption bands. As the amount of chlorophyll, the predominant green pigment in vegetation, decreases, the lemon becomes progressively yellower. Two important plant pigments, carotene (provitamin A) and chlorophyll, are identified and characterized by their absorption spectra. The deterioration or loss of carotene and the transformation of chlorophyll to pheophytin, a brown pigment, in agricultural products are also followed with the spectrophotometer.

Crystalline quartz is the material used for the optical system in the ultraviolet spectrograph and transmits to about 1,850 A. (angstroms). The useful range covered by such an instrument extends from 2,000 to about 8,000 A.; or from the ultraviolet through the entire visible region of the spectrum. When diffracted, or bent, light rays from the spectrograph are allowed to pass through a quartz cell containing the clear liquid to be characterized, one or more absorption bands usually result.

Because most of the measurements are made on substances in solution, solids as well as liquids can be investigated. Constants and calculations for the spectrophotometric determination of fatty acids, important constituents of fats and oils and having direct bearing on spoilage and rancidity, have been established.

A new spectrophotometric method has been developed at the Western Regional Research Laboratory for analyzing hop extracts for humulon and lupulon, two essential constituents that have antibiotic properties that is, they prevent the growth of certain bacteria. Although the two substances are colorless, their ultraviolet absorption spectra are different enough to permit the determination of the amounts of humulon and lupulon in a hop extract in about 15 minutes. We do that by measuring the optical density of the extract at two different wave lengths in the ultraviolet region of the spectrum. No satisfactory chemical method existed previously for determining lupulon in the presence of humulon.

THE PRINCIPLES used for measurements in the visible and ultraviolet regions of the spectrum are applied also in infrared spectrophotometry. The only differences are imposed by materials and energy sources and receivers best adapted to infrared radiation. The prisms are usually rock salt, lithium fluoride, or potassium bromide, which are more transparent than either glass or quartz to the longer infrared radiations and also have characteristic dispersions higher for certain bands that may be under study.

Substances of agricultural origin that have been characterized by application of infrared spectrophotometry are cellulose, cotton and wool, vitamin C and related compounds, amino acids and amino acid complexes, penicillin, plant and animal tissues, some plant pigments, vegetable oils, long-chain compounds such as gutta-percha and rubber in plants, extracted natural rubbers, and synthetic rubbers.