Light and dual nature of light

 
 

Dual Nature of Light

Light may show properties of a wave or of a particle, called a photon.

Newton (1680) explained light as a particle of energy. In reflection and refraction, light behaved as a particle.

Young, (circa. 1800) showed that light interfered with itself. Therefore, it must be a wave. Reflection and refraction could be explained by light being a wave.

Maxwell (1850) showed that light was a form of high frequency electromagnetic wave.

Einstein (1905) showed that in the photoelectric effect (light causing electrons to be emitted from a metal surface) light must act as a particle.

Planck (1900) developed a model that explained light as a quantization of energy. Energy of a light wave is present in bundles of energy called photons; the energy is said to be quantized into the photons.

Therefore, light must be regarded as having a dual nature; in some cases light acts as a wave; in others it acts like a particle.

 

 
 

In order to see, there must be light. Light reflects off an object and -- if one is looking at the object -- enters the eye.

On the other side of the cornea is more moisture. This clear, watery fluid is the aqueous humor. It circulates throughout the front part of the eye and keeps a constant pressure within the eye.

After light passes through the aqueous humor, it passes through the pupil. This is the central circular opening in the colored part of the eye -- also called the iris. Depending on how much light there is, the iris may contract or dilate, limiting or increasing the amount of light that gets deeper into the eye. The light then goes through the lens. Just like the lens of a camera, the lens of the eye focuses the light. The lens changes shape to focus on light reflecting from near or distant objects.

This focused light now beams through the center of the eye. Again the light is bathed in moisture, this time in a clear jelly known as the vitreous. Surrounding the vitreous is the retina.

Light reaches its final destination within the photo receptors of the retina: the retina is the inner lining of the back of the eye. It's like a movie screen or the film of a camera. The focused light is projected onto its flat, smooth surface. However, unlike a movie screen, the retina has many working parts:

• Blood vessels. Blood vessels within the retina bring nutrients to the retina's nerve cells.
• The macula. This is the bull's-eye at the center of the retina. The dead center of this bull's eye is called the fovea. Because it's at the focal point of the eye, it has more specialized, light sensitive nerve endings, called photoreceptors, than any other part of the retina.
• Photoreceptors. There are two kinds of photoreceptors: rods and cones. These specialized nerve endings convert the light into electro-chemical signals.
• Retinal pigment epithelium. Beneath the photoreceptors is a layer of dark tissue known as the retinal pigment epithelium, or RPE. These important cells absorb excess light so that the photoreceptors can give a clearer signal. They also move nutrients to (and waste from) the photoreceptors to the choroid. Bruch's membrane separates the choroid from the RPE.
• The choroid. This layer lies behind the retina and is made up of many fine blood vessels that supply nutrition to the retina and the retinal pigment epithelium.
• Sclera. Normally light does not get as far as this layer. It is the tough, fibrous, white outside wall of the eye connected to the clear cornea in front. It protects the delicate structures inside the eye.

Signals sent from the photoreceptors travel along nerve fibers to a nerve bundle which exits the back of the eye, called the optic nerve. The optic nerve sends the visual signals to the visual center in the back of the brain where the experience of vision occurs.

Now light, reflected from an object, has entered the eye, been focused, converted into electro-chemical signals, delivered to the barin and interpreted or "seen" as an image.

reference: http://www.webmd.com/eye-health/amazing-human-eye

Light is everywhere in our world. We need it to see: it carries information from the world to our eyes and brains. Seeing colors and shapes is second nature to us, yet light is a perplexing phenomenon when we study it more closely.

Here are some things to think about:

• Our brains and eyes act together to make extraordinary things happen in perception. Movies are sequences of still pictures. Magazine pictures are arrays of dots.

• Light acts like particles—little light bullets—that stream from the source. This explains how shadows work.

• Light also acts like waves—ripples in space—instead of bullets. This explains how rainbows work. In fact, light is both. This "wave-particle duality" is one of the most confusing—and wonderful—principles of physics.

read more about light:
http://www.learner.org/teacherslab/science/light/
http://en.wikipedia.org/wiki/Light

WAVES
wave is a disturbance that propagates (travels) through space and time, usually by transference of energy. A mechanical wave is a wave that propagates through a medium due to restoring forces produced upon its deformation. For example, sound waves propagate via air molecules bumping into their neighbors.

vibration can be defined as a back-and-forth motion around a reference value. However, a vibration is not necessarily a wave.
http://en.wikipedia.org/wiki/Wave

Nature of waves

 
 

WAVES

 
 

http://physics.info/waves/

 
 

Brief history of wave and particle viewpoints

Aristotle was one of the first to publicly hypothesize as to the nature of light, proposing that it was a disturbance in the element air (hence it was a wave-like phenomenon). On the other hand, Democritus – the original atomist – argued that all things in the universe, including light, were composed of indivisible sub-components (light being some form of solar atom).[3] At the beginning of the 11th century, the Arabic scientist Alhazen wrote the first comprehensive treatise on optics; describing refraction, reflection, and the operation of a pinhole lens via rays of light traveling from the point of emission to the eye. He asserted that these rays were composed of particles of light. In 1630, René Descartes popularized and accredited in the West the opposing wave description in his
treatise on light, showing that the behavior of light could be re-created by modeling wave-like disturbances in his universal medium (plenum). Beginning in 1670 and progressing over three decades, Isaac Newton developed and championed his
corpuscular hypothesis, arguing that the perfectly straight lines of reflection demonstrated light's particle nature; only particles could travel in such straight lines. He explained refraction by positing that particles of light accelerated laterally upon entering a denser medium. Around the same time, Newton's contemporaries – Robert Hooke, Christian Huygens, and Augustin-Jean Fresnel – mathematically refined the wave viewpoint, showing that if light traveled at different speeds in different media (such as water and air), refraction could be easily explained as the medium-dependent propagation of light waves. The resulting Huygens–Fresnel principle was extremely successful at reproducing light's behavior and, subsequently supported by Thomas Young's discovery of double-slit interference, effectively disbanded the particle light camp.[4]

 
 

Thomas Young's sketch of two-slit diffraction of waves, 1803.

The final blow against corpuscular theory came when James Clerk Maxwell discovered that he could combine four simple equations, which had been previously discovered, along with a slight modification to describe self propagating waves of oscillating electric and magnetic fields. When the propagation speed of these electromagnetic waves was calculated, the speed of light fell out. It quickly became apparent that visible light, ultraviolet light, and infrared light (phenomenon thought previously to be unrelated) were all electromagnetic waves of differing frequency. The wave theory had prevailed – or at least it seemed.

While the 19th century had seen the success of the wave theory at describing light, it had also witnessed the rise of the atomic theory at describing matter. In 1789, Antoine Lavoisier securely differentiated chemistry from alchemy by introducing rigor and precision into his laboratory techniques; allowing him to deduce the conservation of mass and categorize many new chemical elements and compounds. However, the nature of these essential chemical elements remained unknown. In 1799, Joseph Louis Proust advanced chemistry towards the atom by showing that elements combined in definite proportions. This led John Dalton to resurrect Democritus' atom in 1803, when he proposed that elements were invisible sub components; which explained why the varying oxides of metals (e.g. stannous oxide and cassiterite, SnO and SnO2 respectively) possess a 1:2 ratio of oxygen to one another. But Dalton and other chemists of the time had not considered that some elements occur in monatomic form (like Helium) and others in diatomic form (like Hydrogen), or that water was H2O, not the simpler and more intuitive HO – thus the atomic weights presented at the time were varied and often incorrect. Additionally, the formation of HO by two parts of hydrogen gas and one part of oxygen gas would require an atom of oxygen to split in half (or two half-atoms of hydrogen to come together). This problem was solved by Amedeo Avogadro, who studied the reacting volumes of gases as they formed liquids and solids. By postulating that equal volumes of elemental gas contain an equal number of atoms, he was able to show that H2O was formed from two parts H2 and one part O2. By discovering diatomic gases, Avogadro completed the basic atomic theory, allowing the correct molecular formulas of most known compounds – as well as the correct weights of atoms – to be deduced and categorized in a consistent manner. The final stroke in classical atomic theory came when Dimitri Mendeleev saw an order in recurring chemical properties, and created a table presenting the elements in unprecedented order and symmetry. But there were holes in Mendeleev's table, with no element to fill them in. His critics initially cited this as a fatal flaw, but were silenced when new elements were discovered that perfectly fit into these holes. The success of the periodic table effectively converted any remaining opposition to atomic theory; even though no single atom had ever been observed in the laboratory, chemistry was now an atomic science.

 
 

Pasted from <http://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality>