Page 98 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch08 Elementary Physical Chemistry

Introduction to Quantum Theory 83

cognizant of the confusion and disappointment these failures caused, it is
well to mention some of the disputes scientists engaged in.
In the 17th century there were two theories of light:

(1) The corpuscular theory (proposed by Isaac Newton), which stated that
light consists of “particles” shot out of the luminous body.
(2) The wave theory of light (proposed by Huygens), suggesting that light
consists of waves.

First, the corpuscular theory was most favored as it could account for
the sharp shadows, but as time went on, the appearance of diffraction
patterns when light was passed through two adjacent orifices (Young’s
Experiment) and the prediction of Maxwell’s Theory of Electromagnetic
Radiation, clearly favored the wave theory.
In Young’s experiment (∼1850), the light emanating from the two holes
was observed on a screen. If light were corpuscular, one should observe two
lit points on the second screen. If light were wave-like, the light emanating
from the two holes would set up new waves that would interfere with
each other (constructively and destructively), thus producing interference
patterns. The experiment clearly showed the latter, establishing the wave
theory as the proper one.
Furthermore, Maxwell’s Theory of Electromagnetic Radiation, which
formulated a unified treatment of electricity, magnetism and light (called
electromagnetic radiation), clearly established light to be wave-like. By
the end of the 20th century, the particle theory of light was as dead as
a doornail.

8.2. Failure of Classical Theories
Important experiments which defied classical explanations around 1900
were black-body radiation, photo-electric effect and heat capacity of solids.

8.2.1. Black-Body Radiation
When a body is heated to incandescence, light is emitted. First, dull red
light is emitted. Then, as temperature is increased, bright red light is
emitted. Thereafter yellow light etc. is given off and eventually blue light is
emitted. The distribution of the light colors (really in terms of frequencies or
wavelengths) could be measured by shining the emitted light onto a prism.