Refraction

Light  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 Optics  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

Refraction  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 Pool  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 Refracts  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 Light ay.jpg  In a vacuum, light travels at almost 300 million m/s or 3.0X10 8 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 vacuum Speed 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.

Practice  Read 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

Dispersion  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 n 1 sin  1 = n 2 sin  2  Let the indices of the two media be n 1 and n 2  Let the angle of incidence be  1  Let the angle of refraction be  2

Practice  Read example problems page  Do all 6 calculations page

Total Internal Reflection  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  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

Mirages

The End