Refraction

Light Travels in straight lines. We have examined how light can reflect off an object. Today we examine Refraction

Speed of Light (c) Light moves very fast. 300 million m/s (in a vacuum) Light moves more slowly the denser the medium is. Speed of light through air is slower than a vacuum. Speed through glass is slower than air. Refraction is based on the Speed of light.

Refraction Reflection occurs when light bounces. Refraction occurs when light transmits through an object. (or medium) The different densities of the media will make the light bend.

Refraction Below the water, the pencil is bent. As the light rays move from water(dense) to air(not dense) they bend.

Refraction Refraction is most common with water and air, but occurs with any clear substances. (glass, plastic, diamond etc)

Refraction in a Pool Ever lose your key in a pool? Refraction makes it look like the key is somewhere else.

Refraction in a Pool The light bends when it leaves the water and makes the object appear to be “above” the actual position.

Why does light bend? The light travels more slowly in dense media. Some of the light is slowed down and pulls the rest with it.

Index of Refraction Every substance has an index of refraction to tell us how much light will bend when passing through it. (pg 437 Table 11.5) Higher number = greater refraction. We will be using these values for calculations later

How did we come up with all those indices? Index of refraction (n) = Speed of light in a vacuum(c) Speed of light in medium(v) 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.

Predicting Refraction We use a normal with refraction, just like we did with reflection. Remember: Less dense = low refractive index More dense = higher refractive index

Predicting Refraction How does the light bend when going from a less dense to more dense medium? From low index to high index, bend toward normal.

High to Low Refractive Index How does light bend when going from a more dense to less dense media High index to low index, bend away from normal.

Practice Read example problems page 438 Do all 6 calculations page 438

Fibre Optics Light can be bent and controlled with fibre optic cables. (glass tubes as thin as a human hair)

Fibre Optics Core - Thin glass centre of the fibre where the light travels Cladding – glass that reflects light back into core Buffer coating - Plastic coating that protects the fibre moisture

Dispersion White light can be split into component colours. This dispersion is a type of refraction.

Rainbows Light refracts on the way into the raindrop, disperses into colour, reflects off the back, and refracts on the way out yikes. Rainbows are easy...

Work Pg. 916 #1, 2, 3, 5, 7

Snell’s Law A formula that uses the index of refraction to calculate the change in angle of the light ray between two media

Calculations with Snell’s Law n 1 sin  1 = n 2 sin  2 Let the indices of refraction 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.

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 Oi, the Oref also 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 Oi 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 Questions: pg 447 # 11, 12, 13, 16 (Calculator) 17, 18, 19, 21 (No Calculator)