Refraction

Lighthttp://arrowheadradio.com/images/fiber-optics.jpg

Tends to travel in straight lines

If you need to bend light or shine it into difficult-to-reach places, optical fibres can be used

Fibre Opticshttp://static.howstuffworks.com/gif/fiber-optic-fiber-edit.jpg

Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair.

Core - Thin glass center of the fiber where the light travels

Cladding - Outer optical material surrounding the core that reflects the light back into the core

Buffer coating - Plastic coating that protects the fiber from damage and moisture

Refractionhttp://thescienceroom.pbworks.com/f/brokpen.jpg

Light also bends when it passes from one medium into another such as from water to air

The pencil appears to bend because light rays change direction as the move between mediums

Refraction in a Poolhttp://www.wildpackets.com/images/compendium/FDRflct.gif

You may have stood by a pool and seen an object on the bottom that you want to pick up

When you dived in to get the object, it wasn’t where it appeared to be

Why? This happens because the light rays change

direction at the surface of the water as they pass between the water and the air

When we view the refracted light rays we assume they have traveled in a straight line

If you trace the light rays back in a straight line you will find that they don’t lead to the object

How Light Refracts If light strikes a new

medium at and angle it will bend

Think of a light beam as the leading edge of a wave, the part that strikes the surface first slows down

How Light Refractshttp://optics.mbhs.edu/snells-law/image1.JPG

As light enters the plastic block it slows down and refracts

As light leaves the block and enters the air it speeds up and refracts

Light does not refract inside the block, only when changing mediums

Speed of Lighthttp://2.bp.blogspot.com/_2ii88_lzmW0/TSSZcbuLLOI/AAAAAAAAAMM/x_iuQyjc5Ak/s1600/

speed_of_light_highway.jpg

In a vacuum, light travels at almost 300 million m/s or 3.0X108 m/s

Light travels almost that fast in air

The more dense the medium, the slower light can travel through it.

The Index of Refraction

The amount by which a transparent medium decreases the speed of light is indicated by a number called the index of refraction

The larger the refractive index, the more the medium decreases the speed of light

The refractive index of the speed of light in a vacuum (and approx. air) is 1.00

Calculating the Index of Refraction

Index of refraction =

Speed of light in a vacuumSpeed of light in medium

Or

n = c/v

The refractive index of a medium, n, is calculated by comparing the speed of light in the medium, v, with the speed of light in a vacuum, c.

PracticeRead example problems page 438

Do all 6 calculations page 438

Angles of Refraction

Angles of refracted light rays are usually measured from the normal, drawn at 90º to the surface where the light ray crosses between two media

Low to High Refractive Index

When light travels from air, with a low refractive index to water, with a high refractive index, it bends toward the normal

Low to High Refractive Index

When light travels from water, with a higher refractive index (denser medium) to air, with a lower refractive index (less dense medium), it bends away from the normal

Dispersionhttp://www.educationalelectronicsusa.com/l/images/light-XVa.gif

The splitting of a ray into its component colours is known as dispersion of light and the band of colours is known as a spectrum

Rainbows The most common type

of dispersion is in the formation of a rainbow

When sunlight passes through a rain drop, some of the light is refracted, once on entering and again on exiting the drop

Both refractions cause separation of the white light

Snell’s Law A formula that

uses the index of refraction to calculate the new angle that a ray will take as a beam of light strikes the interface between two media

Calculations with Snell’s Law

n1 sin 1= n2 sin 2

Let the indices of the two media be n1 and n2

Let the angle of incidence be 1

Let the angle of refraction be 2

PracticeRead example problems page 441 - 442

Do all 6 calculations page 441 - 442

Total Internal Reflectionhttp://www.hsc.csu.edu.au/senior_science/core/info_systems/9_4_6/Laserpath.gif

Sometimes light does not pass from one medium to another, but stays within the medium

This happens in the case of fibre optics

Total Internal Reflection http://labspace.open.ac.uk/file.php/6211/t305_002i.jpg

Light reflects completely off the inside wall of a denser medium (higher index of refraction) rather than passing through the wall into a less dense medium (lower index of refraction)

RECALL!

When light passes from a denser material into a less dense material, the light refracts away from the normal

The Critical Angle

As the angle of incidence increases, the angle of refraction increases

At the critical angle, the refracted ray of light follows a path exactly along the surface of the water

Past The Critical Angle If the angle of incidence is increased past

the critical angle, the light ray is reflected not refracted

Mirages

Both total internal reflection and refraction play a role in forming a mirage

Mirages Light rays pass through layers of air with

progressively lower indices of refraction, eventually the light is totally internally reflected

Mirageshttp://www.islandnet.com/~see/weather/graphics/photos/infmirg2.gif

The End