Page 16 - Applied Photovoltaics

Chapter

1

THE

CHARACTERISTICS

OF SUNLIGHT

1.1 PARTICLE-WAVE DUALITY
Our understanding of the nature of light has changed back and forth over the past few
centuries between two apparently conflicting viewpoints. A highly readable account
of the evolution of quantum theory is given in Gribben (1984). In the late 1600s,
Newton’s mechanistic view of light as being made up of small particles prevailed. By
the early 1800s, experiments by both Young and Fresnel had shown interference
effects in light beams, indicating that light was made up of waves. By the 1860s,
Maxwell’s theories of electromagnetic radiation were accepted, and light was
understood to be part of a wide spectrum of electromagnetic waves with different
wavelengths. In 1905, Einstein explained the photoelectric effect by proposing that
light is made up of discrete particles or quanta of energy. This complementary nature
of light is now well accepted. It is referred to as the particle-wave duality, and is
summarised by the equation
E hf hc Ȝ / (1.1)

where light, of frequency f or wavelength O, comes in ‘packets’ or photons, of energy
–34
E, h is Planck’s constant (6.626 × 10 Js) and c is the velocity of light (3.00 × 10 8
m/s) (NIST, 2002).
In defining the characteristics of photovoltaic or ‘solar’ cells, light is sometimes
treated as waves, other times as particles or photons.