N IO T C REF R A 1

N IO T C REF R A 1 _ T R A P By Mark Jordan © The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan

OUTLINE OF THE DAY To Be Completed by Class Teacher The Professional Development Service for Teachers is funded by the Department The Professional Development Service for Teachers is funded by the of Education and Science under the National Development Plan Department of Education and Skills under the National Development Plan

A bowl has a coin placed on the bottom just out of view. When water is poured into the bowl suddenly the coin can be seen – the water seems to ‘lift’ the coin. Why do you think this happens? The pencil on the left seems broken where it enters the water. Why? These are both examples of light bending or Refraction. Can you explain how this is caused? Hint next Slide. 3

• Light is stated to travel at , approx, 3. 00 x 108 m/s. Does light always travel at that speed, or can it travel faster or slower? • How does the line of a marching band make a turn without getting out of step? How could this be related to the bending of light through glass 4

Normal Refraction Angle of incidence i en cid In r y A normal (90 o) to point where the light enters dense medium (glass) shows ray bending into the normal. Re fra cte dr ay Air Glass a tr Ray of light travelling from less dense medium (e. g. air) to more dense medium (e. g. glass) changes direction or bends – called Refraction. Snell, a Dutch mathematician, found that : - Angle of refraction 5

Normal Refraction Air Glass r Angle of refraction Light ray travelling from a more dense medium (glass) to a less dense medium (air) bends away from the normal - Snell’s Law again applies i. e. sin i α sin r Refraction is the bending of a wave at the boundary when it is going from one medium to another i We can verify Snell’s Law with an Experiment Angle of incidence 6

Using a ray box and a block of glass record the values for the angle of incidence & angle of refraction as shown. 7

Find the sine of angles of incidence and refraction and record i/ o r/o sin i sin r 35 o 23 o 0. 57 0. 39 40 o 26 o 0. 64 0. 44 45 o 29 o 0. 7 0. 49 50 o 32 o 0. 76 0. 53 55 o 34 o 0. 81 0. 56 60 o 36 o 0. 86 0. 59 65 o 38 o 0. 90 0. 62 Draw graph of sin i (y- axis) against sin r (x-axis) 8

Draw graph of sin i (y-axis) against sin r (x- axis) (0. 68, 1. 0) Straight line graph through the origin proves Snell’s Law i. e. Sin i Sin r n = k = e p o l S (0. 14, 0. 2) Choose coordinates on line Refractive index (n) = 9 = 1. 48

Waves going from air to glass at angle other than 90 o Ø velocity decreases Ø frequency remains constant Ø Wavelength decreases (from c = f λ) In slowing and changing wavelength, the angle at which they proceed MUST change i. e. they REFRACT 10

Waves going from air to glass at 90 o Ø velocity decreases Ø frequency remains constant Ø Wavelength decreases (from c = f λ) If the waves arrive perpendicular to the boundary they do not REFRACT 11

r Air Glass i Ø light ray travelling from more dense to less dense medium Ø refraction occurs Ø refracted ray bends away from the normal. r= 90° Air Glass ic As the angle of incidence gets bigger, angle of refraction gets bigger & eventually becomes 90 o. This angle of incidence is called the critical angle 12 i r ØAngle of incidence becomes bigger than the critical angle then. . ØTotal internal reflection occurs

Refraction r= 90° Air Glass It is possible to calculate the refractive index using the critical angle c As the angle of incidence gets bigger, the angle of refraction gets bigger & eventually becomes 90 o. This angle of incidence is called the critical angle 13

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Critical angle of glass in prism 41. 9 o (approx) Ø Light from air to glass at 90 o ØNo Refraction 450 450 Ø Light attempting to go from glass to air but angle of incidence greater than critical angle. ØTotal Internal Reflection Ø Light from glass to air at 90 o ØNo Refraction 15

Critical angle of glass in prism 41. 9 o (approx) Ø Light from air to glass at 90 o ØNo Refraction Ø Light attempting to go from glass to air but angle of incidence greater than critical angle. 450 ØTotal Internal Reflection 450 Ø Light from glass to air at 90 o ØNo Refraction 16

Mirage Li gh ---- Total Internal Reflection of light from sky tr ay sf ro m Cool air sk y High density Warm air Hot air Low density Hot ground 17

Refraction Apparent depth Real depth n = Real depth Apparent depth Glass of water 18

Refraction Cork Pin To find refractive index of a liquid (water) by measuring Real depth over Apparent depth of an object in the liquid. Apparent depth Mirror Real depth Water Image Pin 19

An optical fiber is a thin, flexible, transparent fiber of glass that acts as a "light pipe”. See image. Using your knowledge of TIR can you explain the following. • How can a beam of laser light travel through a clear optical fibre without any significant amount of light escaping when the beam strikes the side of the fibre? • Do you think the laser light could stay in a fiber that is bent? Why or…. why not? Click the links below to view a video (must have Quicktime installed) and for more information/images. http: //www. teachersdomain. org/asset/lsps 07_vid_laserfall/ http: //en. wikipedia. org/wiki/File: Fiber_optic_illuminated. jpg 20

Optic Fibre Glass cladding of low refractive index 21 Glass core of high refractive index

Glass of high refractive index Glass of low refractive index N N Normal 22 N

The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan