Light

The only thing we can really see is light. But, what is light ?

During the day the primary source of light is the sun and secondary sources are flames, white-hot filaments in lamps and glowing gasses in glass tubes.

Some materials, such as air, water or window glass, allow light to pass through. Others, such frosted glass, light passes partially and other things like wood or steel, block it.

• Early Concepts of light:

Some ancient Greek philosophers thought that light were tiny particles that traveled from our eyes to the objects we look.

For Newton and beyond also it was thought thatlight consisted in particles but coming from the objects to our eyes.Huygens argued that light was a wave.

Scientist now agree that light has a dual nature, part particle and part wave.

• The Speed of Light Until the 17th century it was not known if light travels instantaneously or with

finite speed.Galileo and others tried to measure the time a light beam takes to travel to a distant mirror. All they succeeded in doing was measuring their own reaction time.

The first demonstration that light travels at a finite speedwas supplied by the Danish astronomer Olaus Roemer in1675 measuring the periods of Jupiter’s inner moon: Io.Light coming from Jupiter’s moon Io takes a longer timeto reach Earth at position D than at position A.The extra distance that the light travels divide by the extratime it takes gives the speed of light.

Huygens knew that distance and the time and was able to calculate that the speed of light is 300,000 km/s.

The most famous experiment measuring the speed of light was performed by the American physicest Albert Michelson in 1880

• Electromagnetic Waves

Light is energy that is emitted by accelerating electric charges, electrons in atoms. This energy travels in a electromagnetic wave.Light is only a small portion of the electromagnetic spectrum which includes: radio waves, microwaves and X-rays.

• The lowest frequency of light that our eyes can see is Red.• The highest visible frequency and nearly twice than red is Violet.• Electromagnetic waves with frequency lower than visible light are Infrared• Electromagnetic waves with frequency higher than visible light are Ultraviolet

• Light and Transparent MaterialsLight passes through materials whose atoms absorb the energyand immediately remit it as light.Depending the “natural frequency” of the material some vibrationwill pass through and some vibrations will remain in the material

Also the speed of light is affected by the material. Vaccum: c / Water: 0.75c / Glass: 0.67cWhen light emerge from these materials into the air, it travels atits original speed, c.

Glass blocks both, infrared and ultraviolet, but it is transparent to all the frequencies of visible light.

• Opaque materialsMost materials absorb light without reemission and thus allow no light through them; they are opaque. Wood, stone and people are opaque to visible light.Metals are also opaque although they are shiny.Light doesn’t pass through material when the energy is changed to random kinetic energy of the atoms.

• Shadows

When light shines on an object, some of the rays may be stopped while others pass on in a straight-line path. A shadow is formed where light rays cannot reach.

Shadows can be produced from a small light source nearby or by a large source further away.A total shadow is called an umbra, and a partial shadow a peumbra. A penumbra

appears where some of the light is blocked, but where other light fills in. Or a penumbra

occurs where light from a broad source is only parcialy blocked. A dramatic example

of this occurs when the moon passes between Earth and the sun- during a solar Ecliplse

.

• PolarizationLight travels in waves. The fact that the waves are transverse – and not longitudinal – is demonstrated by the phenomenon of polarization.

If you shake the end of a horizontal rope, a transverse wave travels along the rope. The vibrations are back and forth in one direction and the wave is said to be polarized.

Shake it up & down Vertically polarized waveShake it side to side Horizontally polarized wave

A common light source such an incandescent or fluorescent lamp, a candle flame or the sun, emits light that is not polarized.

When these lights shines on a polarizing filter, such as Polaroid sunglasses, the result is the emergence of polarized light. Since some of the waves are filtered, the ones that are not aligned with the polarized filter, the intensity of light decreases.

Light will pass through a pair of polarizing filters whentheir polarization axes are aligned, but not when they are crossed at right angles.

• Polarized Light and 3-D Viewing Vision in three dimensions depends on the fact that both eyes give

impressions simultaneously, each eye viewing a scene from a slightly different angle. The combination of views in the eye-brain system gives depth.

A pair of movie frames, taken a short distance apart, can be seen in 3-D when the left eye sees only the left view and the right eye sees only the right view.

This effect is accomplished by projecting the pair of frames through polarization filter onto a screen. The polarization axes are at right angles.

To see in 3-D the viewer wears polarizing eyeglasses with the lens axes also at right angles. In this way, each eye sees a separate picture. The brains interprets the two pictures as a single picture with a feeling of depth.