The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan By Mark Jordan © ACTION REFR PART_1 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 of Education and Science under the National Development Plan

3 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.

4 Light is stated to travel at, approx, 3.00 x 10 8 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

5 Glass Air i r Ray of light travelling from less dense medium (e.g. air) to more dense medium (e.g. glass) changes direction or bends – called Refraction. A normal (90 o ) to point where the light enters dense medium (glass) shows ray bending into the normal. Snell, a Dutch mathematician, found that :- Incident ray Refracted ray Angle of refraction Angle of incidence N o r m a l Refraction

6 Refraction Glass Air 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 r r i We can verify Snell’s Law with an Experiment Angle of refraction Angle of incidence N o r m a l Refraction is the bending of a wave at the boundary when it is going from one medium to another

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

8 i/ o r / o 35 o 23 o 40 o 26 o 45 o 29 o 50 o 32 o 55 o 34 o 60 o 36 o 65 o 38 o Find the sine of angles of incidence and refraction and record Draw graph of sin i (y- axis) against sin r (x-axis) sin isin r

9 Choose coordinates on line Draw graph of sin i (y-axis) against sin r (x- axis) Refractive index (n) = = 1.48 S l o p e = k = n (0.68, 1.0) (0.14, 0.2) Straight line graph through the origin proves Snell’s Law i.e. Sin i  Sin r

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

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

12 i i i r = 90° r r c  light ray travelling from more dense to less dense medium  refraction occurs  refracted ray bends away from the normal. 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 Glass Air  Angle of incidence becomes bigger than the critical angle then..  Total internal reflection occurs

13 r = 90° c Glass Air It is possible to calculate the refractive index using the critical angle 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 Refraction

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Critical angle of glass in prism 41.9 o (approx)  Light from air to glass at 90 o  Light attempting to go from glass to air but angle of incidence greater than critical angle.  No Refraction  Total Internal Reflection  Light from glass to air at 90 o  No Refraction

Critical angle of glass in prism 41.9 o (approx)  Light from air to glass at 90 o  No Refraction  Light attempting to go from glass to air but angle of incidence greater than critical angle.  Total Internal Reflection  Light from glass to air at 90 o  No Refraction

17 Hot air Warm air Low density High densityCool air Hot ground Light rays from sky Mirage ---- Total Internal Reflection of light from sky

18 R e a l d e p t h A p p a r e n t d e p t h n = Real depth Apparent depth Glass of water Refraction

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

20 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? 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. Click the links below to view a video (must have Quicktime installed) and for more information/images.

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

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

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