Theories About Light

Good Morning

Seniors!!!

Let your light shine. Shine within you so that it can shine on someone else. Let your light shine.

Theories regarding the nature of light

Light has been characterized by six major theories over the past 3,000 years. The six theories are known as:

1. The tactile theory2. The emission theory3. The corpuscular theory4. The wave theory5. The electromagnetic theory6. The quantum theory

The tactile theory was based on the ability to touch. The theory stated that the eye sent out invisible probes to "feel" objects. The emissions theory, however, was the opposite of the tactile theory. The emissions theory stated that bright objects sent out beams or particles that would ricochet off objects and enter the eye. The emission theory became generally accepted over the tactile theory by the eleventh century.

What is a theory?

A hypothesis is an educated guess, based on

observation. Usually, a hypothesis can be

supported or refuted through experimentation or more observation. A

hypothesis can be disproven, but not proven

to be true.

Example: If you see no difference in the cleaning

ability of various laundry detergents, you might hypothesize that cleaning effectiveness is not affected by which detergent you use. You can see this hypothesis can be disproven if a stain is removed by one detergent and not another.

On the other hand, you cannot prove the hypothesis. Even if you never see a difference in the cleanliness of your clothes after trying a thousand detergents, there might be one you haven't tried that could be different.

A scientific theory summarizes a hypothesis or group of hypotheses

that have been supported with repeated testing.

A theory is valid as long as there is no evidence to dispute it. Therefore, theories can be disproven. Basically, if evidence accumulates to support a hypothesis, then the hypothesis can become accepted as a good explanation of a phenomenon.

One definition of a theory is to say it's an accepted hypothesis.

Example: It is known that on June 30, 1908 in

Tunguska, Siberia, there was an explosion equivalent to the detonation of about 15 million tons of TNT. Many hypotheses have been proposed for what caused the explosion. It is theorized that the explosion was caused by a natural extraterrestrial phenomenon, and was not caused by man.

A law generalizes a body of observations. At the time it is

made, no exceptions have been found to a law. Scientific laws explain things, but they do not describe them. One way to tell a law and a theory apart is to

ask if the description gives you a means to explain 'why'.

Example: Consider Newton's Law of Gravity.

Newton could use this law to predict the behavior of a dropped

object, but he couldn't explain why it happened.

http://chemistry.about.com/od/chemistry101/a/lawtheory.htm

What is light?

Light or visible light is electromagnetic radiation

that is visible to the human eye, and is responsible for the

sense of sight.

Visible light has wavelength in a range from about 380 nanometers to

about 740 nm, with a frequency range of about 405 THz to 790 THz. In

physics, the term light sometimes refers to electromagnetic radiation of

any wavelength, whether visible or not.

What are the theories regarding the nature of light?

René Descartes (1596–1650)

held that light was a

mechanical property of the luminous body.

In 1637 he published a theory of the refraction of light that assumed, incorrectly, that light travelled faster in a denser medium than in a less dense medium. Descartes arrived at this conclusion by analogy with the behavior of sound waves.

Although Descartes was incorrect about the relative speeds, he was correct in assuming that light behaved like a wave and in concluding that refraction could be explained by the speed of light in different media.

Descartes is not the first to use the mechanical analogies but because he clearly asserts that light is only a mechanical property of the luminous body and the transmitting medium, Descartes' theory of light is regarded as the start of modern physical optics.

Pierre Gassendi (1592–1655), an

atomist, proposed a particle theory of light which was

published posthumously in the

1660s.

Isaac Newton studied Gassendi's work at an early

age, and preferred his view to Descartes' theory of the plenum. He stated in his

Hypothesis of Light of 1675 that light was composed of

corpuscles (particles of matter) which were emitted in all directions from a source.

One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.

Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater.

His reputation helped the particle theory of light to

hold sway during the 18th century.

Newton’s corpuscular theory of light is based on the following points1. Light consists of very tiny particles known as “corpuscular”.2. These corpuscles on emission from the source of light travel in straight line with high velocity3. When these particles enter the eyes, they produce image of the object or sensation of vision.4. Corpuscles of different colours have different sizes.

Pierre-Simon Laplace

The particle theory of light led Laplace to argue that a body could be so massive that light could not escape from it. In other words it would become what is now called a black hole.

Laplace withdrew his suggestion when the

wave theory of light was firmly established.

In 1672 he discovered the

phenomenon of diffraction (the bending of light

rays around corners); to

explain it, he offered the

wave theory of light.

In 1678, Dutch physicist, Christiaan Huygens, believed that light was made up of waves vibrating up and down perpendicular to the direction of the light travels, and therefore formulated a way of visualizing wave propagation. This became known as 'Huygens' Principle'.

According to huygen’s wave theory:1. Each point in a source of light sends out waves in all directions in hypothetical medium called "ETHER".2. Light is a form of energy 3. Light travels in the form of waves.4. A medium is necessary for the propagation of waves & the whole space is filled with an imaginary medium called Ether5. Light waves have very short wave length

Augustin Fresnel (1788-1827) elaborated on Huygens' Principle by stating that the amplitude of the wave at any given point equals the superposition of the amplitudes of all the secondary wavelets at that point (with the understanding that the wavelets have the same frequency as the original wave).

Thomas Young (13 June 1773 – 10 May 1829) was an English polymath. He is famous for having partly deciphered Egyptian hieroglyphics He was admired by, among others, Herschel and Einstein.

In Young's own judgment, of his many achievements the most important was to establish the wave theory of light.

To do so, he had to overcome the century-old view, expressed in the venerable Isaac Newton's "Optics", that light is a particle. Nevertheless, in the early 19th century Young put forth a number of theoretical reasons supporting the wave theory of light, and he developed two enduring demonstrations to support this viewpoint

With the ripple tank he demonstrated the idea of interference in the context of water waves. With the Young's interference experiment, or double-slit experiment, he demonstrated interference in the context of light as a wave.

James Clerck Maxwell

(13 June 1831 – 5 November

1879)

At the end of the nineteenth century, James Clerk Maxwell combined electricity, magnetism and light into one theory. He called his theory the electromagnetic theory.

According to Maxwell, light was an electromagnetic wave. Because light was an electromagnetic wave, light carried the same properties as the other electromagnetic waves. Maxwell was able to predict the speed of light by using electrical and magnetic constants. His calculations proved to be very close to the accepted value.

Max Karl Ernst Ludwig Planck, (April 23, 1858 – October 4, 1947) was a German physicist who discovered quantum physics, initiating a

revolution in natural science and philosophy.

He is regarded as the founder of

quantum theory, for which he received the

Nobel Prize in Physics in 1918

According to quantum theory “Energy radiated or absorbed can not have any fractional value. This energy must be an integral multiple of a fixed quantity of energy. This quantity is called “QUANTUM”OREnergy released or absorbed is always in the form of packets of energy or bundles of energy. These packets of energy are known as QUANTA or PHOTONS

Using Max Planck’s quantum Theory he formulated the photon theory of light and explains the photoelectric effect.

In physics, a photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation, and the force carrier for the electromagnetic force.

Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, exhibiting properties of both waves and particles.

For example, a single photon may be refracted by a lens or exhibit wave interference with itself, but also act as a particle giving a definite result when its position is measured.

Einstein's hypothesis that light itself is quantized; the quanta of light are photons.

Louis-Victor-Pierre-Raymond, de Broglie

In a thesis published in 1922, when he was only thirty years old, he

suggested that light could behave either as a wave or as a steam of

He argued that if light which was normally a form of wave motion

could take on a corpuscular (particle) form, then small particles such as electrons could also have

wave-like characteristics associated with them. However, he had to wait five years for the evidence.

That is all for today!!!

Questions???

Seatwork!!!

Identify the scientist and his contribution/s in the field of optics.

1.

2.

3.

4.

5.

Answers!!!