Properties of light

Reflection of Light

Reflection is the turning back of the light from the surface it hits. Incoming and reflected lights have same angle with the surface. If the surface reflects most of the light then we call such surfaces as mirrors.

Laws of Reflection

First law of reflection states that; Incident ray, reflected ray and Normal to the surface lie in the same plane.

Angle of incident ray is equal to the angle of reflection ray.

Plane Mirrors and Image Formation in Plane Mirrors

If the reflecting surface of the mirror is flat then we call this type of mirror as plane mirrors. Light always has regular reflection on plane mirrors.

First look at picture and then follow the steps one by one.

In plane mirrors, we use the laws of reflection while drawing the image of the objects. As you see from the picture we send rays from the top and bottom of the object to the mirror and reflect them with the same angle it hits the mirror. The extensions of the reflected rays give us the image of our object. The orientation and height of the image is same as the object. In plane mirrors always virtual image is formed.

Curved Mirrors

We call these types of mirrors also spherical mirrors because they are pieces of a sphere. If the reflecting surface of the mirror is outside of the sphere then we call it convex mirror and if the reflecting surface of it is inside the sphere then we call it concave mirror.

Center of Curvature: As you can understand from the name it is the center of the sphere which the mirror is taken from. It is denoted by C in the diagrams.

Principal Axis: Line coming from the center of the sphere to the mirror is called as principal axis.

Vertex: It is the intersection point of the mirror and principal axis. We show it with the letter V in ray diagrams.

Focal Point: For concave mirrors and thin lenses rays coming parallel to the principal axis reflects from the optical device and pass from this point. For convex mirrors and thick lenses rays coming from this point or appear to coming from this point reflect parallel to the principal axis from the optical device. Another explanation for this term is that, it is the point where the image of the object at infinity is formed. It is denoted with the letter F or sometimes f in ray diagrams.

Radius of Curvature: It is the distance between center of the sphere and vertex. We show it with R in ray diagrams.

Mirror Equations of Curved Mirrors

Refraction

When light passes from one medium to another medium velocity of it changes and so, its direction changes. We call this change in the direction of light refraction.

Refractive Index

It is the ratio of the speed of light in vacuum to the speed of the light in given medium. Refractive index of medium A is given below;

The Laws of Refraction

1. Incident ray, reflected ray, refracted ray and the normal of the system lie in the same plane.
2. Incident ray, coming from one medium to the boundary of another medium, is refracted with a rule derived from a physicist Willebrord Snellius. He found that there is a constant relation between the angle of incident ray and angle of refracted ray. This constant is the refractive index of second medium relative to the first medium. He gives the final form of this equation like;

Critical Angle and Total Reflection

According to the angle of incident ray, at one point it does not refract but goes parallel to the boundary of the mediums. We called this angle as critical angle. If the angle of incident ray is larger than the critical angle then it does not refract but it does total reflection.

 

 Reference:

 http://www.physicstutorials.org/

The Eye is like a Camera

Blue sky

The blue color of the sky is caused by the scattering of sunlight off the molecules of the atmosphere. This scattering, called Rayleigh scattering, is more effective at short wavelengths (the blue end of the visible spectrum). Therefore the light scattered down to the earth at a large angle with respect to the direction of the sun's light is predominantly in the blue end of the spectrum.

Note that the blue of the sky is more saturated when you look further from the sun. The almost white scattering near the sun can be attributed to Mie scattering, which is not very wavelength dependent.

 
 

Pasted from <http://hyperphysics.phy-astr.gsu.edu/HBASE/atmos/blusky.html>

 
 

Red sunset

Sunsets are reddened because for sun positions which are very low or just below the horizon, the light passing at grazing incidence upon the earth must pass through a greater thickness of air than when it is overhead. Just before the sun disappears from view, its actual position is about a diameter below the horizon, the light having been bent by refraction to reach our eyes. Since short wavelengths are more efficiently scattered by Rayleigh scattering, more of them are scattered out of the beam of sunlight before it reaches you. Aerosols and particulate matter contribute to the scattering of blue out of the beam, so brilliant reds are seen when there are many airborne particles, as after volcanic eruptions.

 
 

The equivalent phenomenon can be seen at sunrise

 
 

Pasted from <http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html>

 
 

How a Rainbow is Formed

10 Great Motivational Quotes

Light

• Our primary source of light is the sun.
  
• Light travels in straight lines at a speed of 186,000miles per second.
  
• *Light waves travel faster than sound waves!
  
• Light energy from the sun travels through space , reaches earth, and some of it turns to heat energy and warms the earth's air.
  
• Light from the sun also travels to the cells of green plants (producers) and is stored as energy.
  
• When light reaches an object, it is absorbed, reflected, or passes through it.
  

 

SOURCES OF LIGHT

• SUN=warms air, water, and land.
  
• Fire=provides heat, light, and cooking fuel.
  
• Lightning
  
• Firefly
  
• Flashlight
  
• Light bulb
  
• Laser beams
  
• Optical
  
  telephone
  
  fibers
  
  *Traffic lights
  
  

 

EXAMPLE OF TRANSPARENT OBJECTS:

• The windows on a school bus,
  
• A clear empty glass,
  
• A clear window pane,
  
• The lenses of some eyeglasses,
  
• Clear plastic wrap,
  
• The glass on a clock,
  
• A hand lens,
  
• Colored glass…
  
• ALL of these are transparent. Yes, we can see through them because light passes through each of them.
  

 

EXAMPLE OF TRANSLUCENT OBJECTS:

• Thin tissue paper,
  
• Waxed paper,
  
• Tinted car windows,
  
• Frosted glass,
  
• Clouds,
  
• All of these materials are translucent and allow some light to pass but the light cannot be clearly seen through.
  

 

EXAMPLE OF OPAQUE OBJECTS:

• Heavy weight paper,
  
• Cardboard
  
• Aluminum foil,
  
• Mirror, bricks, buildings,
  
• Your eyelids and hands,
  
• Solid wood door,
  
• All of these objects are opaque because light cannot pass through them at all.
  
• They cast a dark shadow.
  

 

WHAT IS REALY LIGHT? . .Electromagnetic wave radiation

• Light waves are three dimensional.
  
• Light waves vibrate in all planes around a center line.
  
• The waves have high points called "crests."
  
• Waves also have low points called "troughs."
  
• *The distance from one crest to the next crest is called a "wavelength."
  
• *The number of waves passing a given point in one second is called the "frequency."
  

 

 

ELECTROMAGNETIC WAVE

• Electromagnetic radiation can be described as a stream of photons. Each photon traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy.
  
• The only difference amongst radio waves, visible light, and gamma-rays is the amount of energy of the photons. Radio waves have photons with low energies. Microwaves have a little more energy than radio waves. Gamma-rays and cosmic rays have highest energy waves and are the deadliest.
  

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>