Phys111_ch

PHYSICS 111 LASER AND LIGHT

CHAPTER 1: EARLY IDEAS OF LIGHT

KEY WORDS IN THE CHAPTER

Pharoah in Egypt 1370 B.C. Akhenaton: Sun and its light is divine

Greeks 384-322 B.C. Aristotle: Physical Cosmos

Islamic civilization: Physical and metaphysical light

Alhazen 1000 A.D. theory of vision

European Rennaissance

Nicolaus Copernicus (1473-1543 A.D.) Heliocentric cosmos

Johannes Kepler: Kepler's laws, Elliptical orbits, Orbit radius and time of revolution

Galileo Galilei (1564-1642 A.D.) : Use of telescope, Jupiter has moons, our moon has mountains

Rene Descartes (1596-1650 A.D.): Light travels through a medium

Ether Theory

Outlines of the chapter

1. Ancient civilizations: Chinese, Assyrian, Sumerian, Babylonian, Egyptian, Greek, Roman, Dravidian, Aryan and Mayan.

Sun god as in Egyptian civilization, divinity attached

2. In the book of Genesis: God's first act was creation of light.

In the Koran (600 A.C.): the holy book of over 1 billion Muslims in the world mentions the light (Arabic word "Noor") as a metaphor for knowledge and darkness for ignorance.

3. Attempts by Greeks for rational, non-religious explanations of nature.

Aristotle offered a unified and complete picture of the world. In his explanation, transparent bodies contain substance, also found in the upper shell of the physical cosmos.

4. Flowering of knowledge in Europe: Copernicus, Kepler, and Galilee brought changes in Aristotle's concept of the cosmos. This resulted in modern science as in the topics of Astronomy and mechanics.

5. Rene Descartes tried to explain a unified picture of the natural world. He assumed "ether" pervades all space (?). Light has infinite speed in his theory.

6. The scientific and, mathematically correct explanations of a visual image formed in the eye came from Kepler and Galilee.

7. Descartes is credited for predicting light as a disturbance transmitted through ether and, color associated with periodic motion.

EARLY HISTORY OF LIGHT
Ancient practice of paying homage to the Sun for its power over life and death. The megalithic religions seem to have centred upon the Sun, which
was worshipped throughout the Middle East. In the 14th century BC the radical Egyptian pharaoh Akhenaton made the Sun god Aton the sole god
of the universe in his theology, possibly the first instance of monotheism. In Hinduism, worship of Surya, the Sun god, is an established tradition.

The Sun was a central feature of the religions of Central American peoples such as the Maya. The Sun god was worshipped in Greece as Apollo,
and under other names in Peru, North America, and northern Europe. According to ancient tradition, the Japanese imperial family traces its descent
from the Sun goddess Amaterasu Omikami.

Akhenaton was originally known as Amenophis IV; when he introduced the worship of Aton, superseding the earlier worship of the sun god Ra
(who was later combined with the god of Thebes, Amun-Ra) he renamed himself Akhenaton and built a city in the desert in Aton's honour. After
his death Egypt returned to the old religion and the city was destroyed.
Akhenaton or Ikhnaton

King (pharaoh) of ancient Egypt of the 18th dynasty (c. 1353&endash;1335 BC), who may have ruled jointly for a time with his father Amenhotep III. He
developed the cult of the Sun, Aton, rather than the rival cult of Amen, and removed his capital to Akhenaton. Akhenaton's favourite wife was Nefertiti, and two of their six daughters were married to his successors Smenkhkare and Tutankaton (later known
as Tutankhamen).
Aton: In ancient Egypt, the Sun's disc as an emblem of the single deity whose worship was promoted by Akhenaton in an attempt to replace the many
gods traditionally worshipped.

In Hindu mythology, the sun god, son of the sky god Indra. His daughter, also named Surya, is a female personification of the Sun.

BABYLONIA:

A lens of rock crystal was found in excavations at Nimrud, the royal city of Assyria (Iraq). Natural philosophers who followed Mo Ti in China in the fifth century B.C., studied optics and analyzed the images produced in plane and curved mirrors. Archimedes (287-212 B.C.) of Sicily is known for successfully burning the warships of Rome by gathering sunlight with curved mirrors.

The Greek philosopher Plato believed the fire to be issued from the observer's eye. Some Greeks believed such fire out of the eye combines with sunlight (source) to form a medium, stretching from the visible object to the eye, through which "motions" originating in the visible object are passed onto the eye and ultimately to the soul. Euclid (flourished in 300 B.C.) wrote in his book titled "Optics," that rays emerge from the eye in a cone (called visual cone) with its vertex at the eye and base at the visible object. This is what he called an act of vision and developed a geometrical analysis of the problem of vision.

Claudius Ptolemy, flourished in 150 A.C. in the city of Alexandria in Egypt, his Optics partially survived. Ptolemy believed radiation to emerge from the eye in a visual cone and interacted with visible object. He, nevertheless conducted experiments and, made first attempt at mathematical analysis of image formation by reflection from plane and curved mirrors and, by refraction at interface of two media. The credit for systematic experimental investigation, gathering of data and formulation of physical concept to describe it, all must belong to Alhazen (964-1040 A.C.) for the first time in the history of Optics.


When the Roman emperor Constantine converted to Christianity AD 313, he changed from being a worshipper of the Sun god Sol Victus, and he
made Sunday, the Sun's day, the legal day of worship for Christians.

Rene Descartes:

Descartes's physical theory of the universe, embodying most of the results contained in his earlier and unpublished Le Monde, is given in his
Principia, 1644, and rests on a metaphysical basis. He commences with a discussion on motion; and then lays down ten laws of nature, of
which the first two are almost identical with the first two laws of motion as given by Newton; the remaining eight laws are inaccurate. He next
proceeds to discuss the nature of matter which he regards as uniform in kind though there are three forms of it. He assumes that the matter of the
universe must be in motion, and that the motion must result in a number of vortices. He states that the sun is the centre of an immense whirlpool
of this matter, in which the planets float and are swept round like straws in a whirlpool of water. Each planet is supposed to be the centre of a
secondary whirlpool by which its satellites are carried: these secondary whirlpools are supposed to produce variations of density in the
surrounding medium which constitute the primary whirlpool, and so cause the planets to move in ellipses and not in circles. All these
assumptions are arbitrary and unsupported by any investigation. It is not difficult to prove that on his hypothesis the sun would be in the centre
of these ellipses, and not at a focus (as Kepler had shewn was the case), and that the weight of a body at every place on the surface of the earth
except the equator would act in a direction which was not vertical; but it will be sufficient here to say that Newton in the second book of his
Principia, 1687, considered the theory in detail, and shewed that its consequences are not only inconsistent with each of Kepler's laws and
with the fundamental laws of mechanics, but are also at variance with the laws of nature assumed by Descartes. Still, in spite of its crudeness
and its inherent defects, the theory of vortices marks a fresh era in astronomy, for it was an attempt to explain the phenomena of the whole
universe by the same mechanical laws which experiment shews to be true on the earth.

CONTRIBUTIONS OF THE ISLAMIC CIVILIZATION: There has been a substantial scientific tradition of Optical Studies in the Islamic civilization. During the period of Islamic civilization (650-1200 A.C.) various Greek approaches were studied seriously, defended, criticized, modified and at times extended by the Muslim scholars. They also opened up new frontiers of knowledge. Later on, these Muslim scholars were able to integrate the separate and many times incompatible Greek optical traditions into a single comprehensive theory of vision.

Following the optical works of Ptolemy, much of the knowledge about Light was added by Ibn al Haytham (965-1040 A.C.), an Arab scientist, known in the West by his Latinized name, ALHAZEN. For example, Euclid and Ptolemy assumed extramission of light from the eye and so did Aristotle, who assumed the extramission theory of vision at least in one occasion while explaining rainbow. Alhazen argued, "The eye is injured by bright objects, so something that starts out from the eye, could not possibly hurt itself." He also pointed out the absurdity of material emanation from the eye to fill up the universe, to make the distant stars visible. The tiny eye could not possibly produce so large an emission (energy) to reach myriad of stars at night and, that too, as many times as we wished to see them. He refuted the extramission theory, and was the first to show that light starts from the illuminated objects and reaches the eye in a visual cone. He formulated and defended the new vision theory of intromission. This was no small achievement without modern theory of radiation. His books were translated from Arabic (named as "Treasures of Optics") at least four different times in Europe. Alhazen should be remembered as the first optical scientist (along with al-Kindi 866 A.C., another scholar of Islamic traditions) who made a successful mathematical formulation of Light and Vision. He for the first time showed defects in image formation by curved mirrors and explained how this could be removed. He predicted the speed of light to be slower in a material medium.

More than 500 years after Alhazen, Johannes Kepler was first to point out the inverted image formation in the retina of the eye, by nearly perpendicular scattered rays entering the eye from the object.