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Presented byYingxi WuSiru WenYing Luo

Light

+History

Empedocles (fifth century B.C. Greek philosopher) proposed that light was something that the eye emanated. When objects were within the field of the eye's light flow, they interacted with the light and were transmitted to the mind of the viewer, causing sight.

Plato, another Greek philosopher, proposed that light sources gave off particles.

Robert Hooke, an English physicist, who came up with a different theory about the composition of light. He suggested that light was made up of waves.

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Later, Christian Huygens said that light waves leaving a light source were the result of tiny wavelets, each radiating out from a point on the wave. This theory of the progression of waves is known as Huygens' Principle.

Sir Isaac Newton didn't agree with Huygens' Principal. He thought that if light was a wave, then it should be able to bend around corners. Newton proposed that light was made of particles, sometimes called corpuscular, which radiated out from sources of light. Each color, supposedly, corresponded with a certain type of particle, with white light composed of all of the different kinds of particles.

+Speed of light

The speed of light in a vacuum is defined to be exactly 299,792,458 m/s (approximately 186,282 miles per second)

It is the maximum speed at which all energy, matter, and information in the universe can travel. It is the speed of all massless particles and associated fields– including electromagnetic radiation such as light – in vacuum, and it is predicted by the current theory to be the speed of gravity (that is, gravitational waves).

In the theory of relativity, c interrelates space and time, and appears in the famous equation of mass–energy equivalence E = mc2.

+First measurement attempts

The first quantitative estimate of the speed of light was made in 1676 by Rømer. From the observation that the periods of Jupiter's innermost moon Io appeared to be shorter when the Earth was approaching Jupiter than when receding from it, he concluded that light travels at a finite speed, and estimated that it takes light 22 minutes to cross the diameter of Earth's orbit. Christiaan Huygens combined this estimate with an estimate for the diameter of the Earth's orbit to obtain an estimate of speed of light of 220,000 km/s, 26% lower than the actual value.

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* Additive color: Combine light sources, starting with darkness (black). The additive primary colors are red (R), green (G), and blue (B). Adding R and G light makes yellow (Y). Similarly, G + B = cyan (C) and R + B = magenta (M). Combining all three additive primaries makes white.

* Subtractive color: Illuminate objects that contain dyes or pigments that remove portions of the visible spectrum. The objects may either transmit light (transparencies) or reflect light (paper, for example). The subtractive primaries are C, M and Y. Cyan absorbs red; hence C is sometimes called "minus red" (-R). Similarly, M is -G and Y is -B. The two approaches are illustrated on the right and described in the table below.

+The measurement of light

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Application of light• Brewster’s angle and use

• Reflection

• Refraction

• Convex and Concave Mirrors

+Brewster's angle

Brewster's angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly polarized.

+Use of Brewster’s angle

Polarized sunglasses

Polarized filters in photography

+Polarized sunglasses

Polarized sunglasses use the principle of Brewster's angle to reduce glare from the sun reflecting off horizontal surfaces such as water or road. In a large range of angles around Brewster's angle the reflection of p-polarized light is lower than s-polarized light. Thus, if the sun is low in the sky reflected light is mostly s-polarized.

The polarized sunglasses used by fishermen have polarized filters with the transmission axis oriented vertically to block the light reflected by the water surface. Note that if the fisherman moves his head up or down (nodding) the line joining the temples remains horizontal and the sunglasses continue to block the glare (of course, if he leans his head to one side the filters start to point in the wrong direction). By the way, they are not only used by fishermen but in all other water-sports.  They are also becoming popular with truck drivers and for general outdoor activities.  The last five years have seen a jump in their sales and are the fastest growing segment in eyewear.(http://www.polarization.com/water/water.html)

+ Polarized filters

Light reflected from a non-metallic surface becomes polarized; this effect is maximum at Brewster's angle, about 56° from the vertical (light reflected from metal is not polarized, due to the electromagnetic nature of light). A polarizer rotated to pass only light polarized in the direction perpendicular to the reflected light will absorb much of it.

+Use in photography

Blue sky

Black and white photography

Reflections

Neutral density substitute

(http://dslrtips.burnerphotography.com/5-reasons-why-every-landscape-photographer-needs-a-polarizing-filter)

+Blue sky

Polarizing filters are used, primarily to enhance the color of the sky to make it appear a deeper, darker shade of blue. With a circular polarizer attached to the front of the lens, the photographer can rotate the front element to block out stray light and increase the effect, making the sky darker.

(http://dslrtips.burnerphotography.com/5-reasons-why-every-landscape-photographer-needs-a-polarizing-filter)

+Neutral density substitute

Neutral density filters are designed to cut down the light in these situations; however, polarizing filters can have a similar effect.

A polarizer helped to achieve a shutter speed of 1/6 of a second—slow enough to blur the water.

(http://dslrtips.burnerphotography.com/5-reasons-why-every-landscape-photographer-needs-a-polarizing-filter)

+Black and white photography

One of Ansel Adams most famous images. He used a colored filter to darken the sky

(http://dslrtips.burnerphotography.com/5-reasons-why-every-landscape-photographer-needs-a-polarizing-filter)

+Reflections

From a dictionary standpoint, a reflection is “the production of an image by or as if by a mirror”.

However a physics perspective would state that a reflection is the change in direction of a wave-front at an interface between two different media so that the wave-front returns into the medium from which it originated.

http://en.wikipedia.org/wiki/Reflection_%28physics%29

+Reflections Cont.

Law of Reflection Main article: Specular reflection

If the reflecting surface is very smooth, the reflection of light that occurs is called specular or regular reflection. The laws of reflection are as follows:

The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane.

The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.

http://en.wikipedia.org/wiki/Reflection_%28physics%29

+Reflections Cont.

Due to the discovery of reflections, man created the mirror which lead on to many successful inventions that we still use today.

A mirror provides the most common model for specular light reflection, and typically consists of a glass sheet with a metallic coating where the reflection actually occurs. Reflection is enhanced in metals by suppression of wave propagation beyond their skin depths. Reflection also occurs at the surface of transparent media, such as water or glass.

http://en.wikipedia.org/wiki/Reflection_%28physics%29

+Refraction

Refraction is the bending of a wave when it enters a medium where it's speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. The amount of bending depends on the indices of refraction of the two media and is described quantitatively by Snell's Law.

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/refr.html

+Refraction Cont.

Refraction is responsible for image formation by lenses and eyes.

+Curved mirror

A curved mirror is a mirror with a curved reflective surface, which may be either convex (bulging outward) or concave (bulging inward).

With mirrors, the curved surface is referred to as either concave or convex depending upon whether the center of curvature occurs on the side of the reflecting surface or the opposite side.

+Convex mirror

A convex mirror, fish eye mirror or diverging mirror, is a curved mirror in which the reflective surface bulges toward the light source.

Such mirrors always form a virtual image, since the focus (F) and the centre of curvature (2F) are both imaginary points "inside" the mirror, which cannot be reached.

+Convex mirror

convex mirrors have wider fields of view than other reflective surfaces, such as plane mirrors or concave mirrors, they are commonly used in automobile side mirrors. Having a fish eye on your automobile will allow you to see more of your rear.

A convex mirror is also a good security device. Store owners, for instance, install a number of them inside their stores and orient them in such a way that a single security personnel can see large portions of the store even while monitoring from a single location.

+Concave mirror

A concave mirror, or converging mirror, has a reflecting surface that bulges inward (away from the incident light). Concave mirrors reflect light inward to one focal point.

Unlike convex mirrors, concave mirrors show different image types depending on the distance between the object and the mirror.

+Concave mirror

concave reflectors are used in car headlights. The bulb of the head light is placed at the focal point. The reflected light is parallel and gives more visibility to the driver at night.

concave mirrors are used in solar powered gadgets. The parallel rays of the sun meet at the focal point F. The solar energy concentrated at F is used for further applications such as heating, etc.

A concave mirror is used for making a shaving mirror or a make-up mirror.