Man's physical universe

xanabras

LIGHT IS A FORM OF RADIANT ENERGY 391

lower altitudes, when there are considerable smoke and humidity, the

ultraviolet rays below 3200 angstroms may be absorbed. Inasmuch

as the radiations between 2900 and 3100 angstroms are most effective

biologically, it can readily be understood why ultraviolet lamps are of

distinct value to people who live in cities where smoke vies with clouds

to screen out most of the biologically useful ultraviolet light during the

winter months. There seems to be little doubt that the widespread

popularity of out-of-door life and exposure to sunlight has a sound

physical and physiological basis.

Ultraviolet lamps have been used as a substitute for sunlight in

producing a healthy tan and in producing vitamin D in the human

body, as well as in stimulating various body functions.

Ultraviolet radiations are now widely used to irradiate milk, cereal,

and other products, for such irradiation produces vitamin D in these

foods.

Ordinary window glass does not permit the passage of these healthgiving

ultraviolet rays, but quartz transmits them freely. Certain

types of glass are now manufactured which do permit the passage of

ultraviolet light.

Ultraviolet radiations are of special interest to the chemist because

they affect the outer electrons in atoms.

Recent studies show that plants thrive best under red light and that

ultraviolet light has a definite retarding effect upon plant growth.

The Shorter Wave-length Radiations Likewise Activate Chemical

Changes.

X rays, whose production will be discussed in the next Unit, are

more penetrating than ultraviolet rays and involve the displacement

of inner electrons within atoms.

Short-wave radiations, especially X rays, modify the chromosomes

in living cells. Thus even the sex of flies has been changed by X rays.

A lintless cotton has been grown from X-rayed seed.

Gamma rays, produced by radioactive disintegration, are still more

The

penetrating and displace electrons in the central parts of atoms.

applications of X rays and gamma rays will be studied in the next Unit.

Cosmic rays, studied by Millikan and many others, are the shortest

of all waves and are exceedingly penetrating. A high percentage of

cosmic rays consists of high-speed particles, electrons or protons rather

than photons, which possess enormous energies. While it is true that a

high proportion of cosmic rays are particles rather than waves, the term

"cosmic ray" is still used, just as cathode rays, beta rays, and alpha

rays are used to refer to streams of particles.

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