Partial Reflection and Partial Refraction: Refraction is often accompanied by reflection –i.e. when light hits a surface, some of it may pass through and get bent (“refract”) and some of it may bounce back (“reflect”)

Silvered Sunglasses Work on this principle… Some light gets through (so you can see) A lot of light gets reflected (Also happens when looking at a lake - can see fish in water and reflection of trees outside water)

Total Internal Reflection When ALL of the light can “bounce back” into the incident medium (total internal reflection) When does this happen? 1) When the light is passing from a slow to a fast medium AND 2) There is a big enough angle of incidence (bigger than the CRITICAL ANGLE)

When passing from slow medium into fast medium:  As the angle of incidence increases, the refracted angle also increases.  For a big enough incident angle, the refracted angle will reach 90 o (i.e. along the boundary)  For an even bigger incident angle, there will be no refraction – only reflection (total internal reflection!)

CRITICAL ANGLE: the angle of incidence that causes an angle of refraction of 90 0 (see figure above) It is called this because for any angle of incidence bigger than this no refraction would occur (total internal reflection) i.e. if then total internal reflection occurs

Calculating Critical Angle What is the critical angle for light passing from diamond (which has an index of refraction 2.42) into air (which has an index of refraction of 1.00) RECALL: ANGLE OF REFRACTION IS 90 0 for critical angle!! Diamond n d =2.42 Air n a =1.00 The critical angle of diamond is about

For diamond, which of the following angles leads to total internal reflection? a)10 0 b) 20 0 c) 25 0 d) 45 0 e) 78 0 ANSWER: c,d,e because they are all bigger than the critical angle of

Applications of Total Internal Reflection Sparkling Diamond: Because diamond has a low critical angle, a lot of total internal reflection occurs before it exits the diamond. Different incident rays travel along different paths and therefore come out of the diamond at different times and angles, causing a “sparkling effect” Dispersion also occurs, giving the sparkle some colour

Fibre Optics The angle of incidence is higher than the critical angle all along the cable, therefore total internal reflection occurs until it gets to the end of the cable.

Fibre optic cables are used in endoscope (medical cameras used in colonoscopies) Wanna watch the colonoscopy video again? Neither do I…. Lets move on…

Total internal reflection also used in periscopes, binoculars, and retroreflectors

Phenomenon Due to Total Internal Reflection Mirages

Light from the sky passes through hotter and hotter layers of air causing the light to refract Eventually total internal reflection occurs, causing the light to reflect UP to your eye Your brain traces this light back DOWN in a straight line and see an image of the sky on the ground (a MIRAGE that looks like water)

1)Calculate the critical angle for a)Crown glass (n=1.42) into air (n=1.00) a)Water (n=1.33) into air (n=1.00) 2) Read Chapter ) Do pg. 531 #1-3, 5, 7, 8 (answer is 45 0 ) (answer is 49 0 )