Camille Walker

09/24/03 

Intro to Technical Theater

Hills

 

Agustin Jean Fresnel

 

Agustin Fresnel was born on May 10, 1788 in Broglie France.  At age twelve Fresnel began his studies at the École Centrale in Caen.  His interest in mathematics led him to a career in engineering. In 1804 he entered the École Polytechnique in Paris and after two years there he entered the École des Ponts et Chaussées.  After completing the course there in three years, he was qualified as a civil engineer.[i]

 

His first job was building roads, but in his spare time he did scientific work.  He became fascinated with light and in the middle of 1814 he began experimenting.  At that time, Newton’s corpuscular theory, which held that light is composed of tiny particles (corpuscles) emitted by luminous bodies, was favored over the wave theory, which held that light traveled through space in transverse waves.[ii]

 

Fresnel began by experimenting with diffraction and made a breakthrough when he attached a piece of black paper to one edge of a diffracter and observed that then the bright bands within the shadow vanished. From this he correctly deduced that these bright bands were produced by light coming through both edges of the diffracter but since bright bands outside the shadow remained he deduced that they must result from light reflected from only one edge of the diffracter. [iii]

 

He was able to calculate formulas that gave the position of the bright and dark lines based on where the vibrations were in phase and where they were out of phase. He published his first paper in October 1815 on his wave theory of light and made a first attempt to explain the phenomenon of diffraction. He then used his same mathematical formulas that worked for his diffraction experiments to give theoretical results on interference patterns obtained by reflecting a light source with two mirrors. He verified the theoretical results by experiment.  His mathematical deductions, later called “Fresnel’s integrals”, allowed him to calculate the intensity of light at every point behind the diffractor.[iv]

 

Important experiments were done on the diffraction and interference of light by Fresnel that could only be interpreted in terms of the wave theory. The polarization of light was still another phenomenon that could only be explained by the wave theory. Thus, in the 19th Century the wave theory became the dominant theory of the nature of light.[v]

 

Fresnel was employed by the Lighthouse Commission and as part of his effort he developed the use of compound lenses instead of mirrors for lighthouses.  In 1822 he invented a system of lenses and prisms that magnified and focused as much as 85% of the light from a single source.  Fresnel lenses are comprised of many smaller prisms, individually crafted and suspended in a complex arrangement. They concentrate the light into parallel, horizontal beams that may be directed into a narrow, cylindrical beam or a wide, flat beam visible far out to sea. The concentration is accomplished by passing the light through glass prisms in which the beam is bent by refraction or reflection.[vi] 

 

Early lens designs resembled a giant glass beehive, with a light at the center.  The lens had concentric rings of glass prisms above and below a center drum section to bend the light into a narrow beam. Later designs incorporated a bull's eye design into the center of the lens shaped like a magnifying glass, so the concentrated beam was even more powerful. Tests showed that while an open flame lost nearly 97% of it’s light, and a flame with reflectors behind it still lost 83% of it’s light, the Fresnel lens was able to capture all but 17% of it’s light.  This efficiency meant the Fresnel lens could easily throw its light 20 or more miles to the horizon.

 

Fresnel lenses are divided into 7 classes (called "orders"). The distance from the flame to the lens determines the order. The "First Order Fresnel lens" is the largest lens widely used, and was installed in many of the largest "sea coast lights". Two larger sizes were built in limited quantity and they are there for a couple of special installations. Smaller Fresnel lenses, such as the sixth-order lens, were installed in smaller lighthouses, such as breakwater lighthouses.[vii]

 

In theatrical lighting, the Fresnel lens is a type of step lens with the glass cut away from the convex face of the lens instead of its plano side.  The reduction of the thickness of the lens allows more light transmission and lessens the chance of heat fracture.[viii]

 

In 1823 Fresnel was elected to the Académie des Science. He was also elected to the Royal Society of London and he received its Rumford Medal in 1827.  He died of tuberculosis in 1827 at the age of 39.