Reflection & Refraction Learning Targets: L3: Reflection L4: Refraction – Definition and Ray Dig L5: Critical Angle & Total Internal Reflection L6: Snell’s Law & Index of Refraction

Normal? Everyday definition... Physics definition... a line drawn perpendicular to a surface. Normal Line 90 o

Angle of Incidence & Angle of Reflection Angle of Incidence – the angle between the normal and the light ray incident on the surface. Angle of Reflection – the angle between the normal and the light ray reflected off the surface. Normal Line Incident ray ii Reflected ray RR

Law of Reflection The angle of incidence is equal to the angle of reflection.  i =  R

Plane mirror – flat mirror (smooth on the microscopic level) Object – the source of the light rays (can be luminous or illuminated sources) Image – the point where the light seems to have come from; where the light focuses - it is a “likeness” of the object Virtual image – an image formed from diverging light rays

Specular and Diffuse Reflections Specular Reflection Reflection from a smooth surface; all of the light rays reflect parallel to each other Diffuse Reflection Reflection from a rough surface; light rays reflect in all directions

Refraction What is refraction? The change in direction of a wave at a boundary where the wave passes from one medium into another due to the change in speed of the wave. 123

How can refraction be explained? When a wave is incident on a boundary between media, some of the energy is reflected and some is transmitted, thus the wave changes speed.

Three parts of the Law of Refraction (A.) When a wave strikes a boundary at an angle from a less dense medium to a more dense medium the wave is refracted toward the normal. Normallight ray Boundary between AIR GLASS “if” the light did not bend Less dense More dense

(B.) When a wave strikes a boundary at an angle from a more dense medium to a less dense medium the wave is refracted away from the normal. Normallight ray Boundary between GLASS AIR “if” the light did not bend More dense Less dense

(C.) When a wave strikes a boundary straight on the wave moves straight through the boundary. The wave is not refracted. Normal light ray Boundary between Medium 1 Medium 2

Remember: Refraction is... the bending of a light wave due to a change in the speed caused by a change in medium. Index of Refraction is... the ratio of the speed of light in a vacuum to the speed of light in that medium. - symbol is n- it has no units n = c v

Index of Refraction It depends on the substance. Examples: Vacuum n = 3.00x10 8 m/s= x10 8 m/s Air n = We will use 1.00 Diamond n = 2.42 More dense the material the more the light slows down and the larger the index of refraction.

Snell’s Law Now we can calculate exactly how much the light bends (the angle). n i sin  i = n r sin  r Normallight ray Boundary between AIR GLASS Less dense More dense ii rr n air = n i n glass = n r

Square of glass on the left-hand side of paper. n i = n air = 1.00  i = 60.0 o  r = 34.5 o n r = ? Glass is more dense than air Thus the light slows down when it goes from air into glass Thus the light bends or refracts. n i sin  i = n r sin  r 1.00 sin 60.0 o = n r sin 34.5 o sin34.5 o sin34.5 o n r = 1.53

Square of water on the right-hand side of paper. What happens as the light enters the water? n i = n air = 1.00  i = 60.0 o  r = 40.6 o n r = ? Water is more dense than air Thus the light slows down when it goes from air into water Thus the light bends or refracts. n i sin  i = n r sin  r 1.00 sin 60.0 o = n r sin 40.6 o sin40.6 o sin40.6 o n r = 1.33

Homework: Read Chapter 17.1 Do Reading Notes Worksheet (Transparency 17-1 Worksheet front + back) Do Problems #1-5 on page 364

Total Internal Reflection We already learned... n i > n r n i sin  i = n r sin  r more to less  away  i >  r Normal “if” the light did not bend Water n = 1.33 Air n = 1.00 ii rr

Now...As  i increases...  r increases... eventually  r = 90 o Normal “if” the light did not bend Water n = 1.33 Air n = 1.00 ii rr

Critical Angle When  r = 90 o Then... sin 90 o = 1 sin  i = n r sin  r n i sin  c = n r n i We call this the critical angle  c 1

Now...As  i increases...  i >  c  r increases...  r > 90 o This changes from refraction to reflection.  i =  R Total Internal Reflection Normal Water n = 1.33 Air n = 1.00 ii rr RR cc

Other Cool Light Things Mirage - an optical phenomenon, especially in the desert or at sea, by which the image of some object appears displaced above, below, or to one side of its true position as a result of spatial variations of the index of refraction of air.

Inferior Mirages

Superior Mirages

Survivor Skills – You learned it here

Sound Refraction

Rainbows