1. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Unit 18 Reflection & Refraction of Light

2. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Reflection  Observe the reflection of a laser beam due to the mirror.  The Normal is an imaginary line drawn at a 90  angle with the surface of the mirror at the point where the beam strikes the mirror.  The angle between the incident ray and the normal is known as the angle of incidence (  i ).  The angle between the reflected ray and the normal is known as the angle of reflection (  r ). 18-1 Mirror Incident RayReflected Ray Normal

3. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Mirror Types  There are three types of mirrors: plane mirror, concave mirror, and convex mirror.  Each of these three mirror types form images with different properties.  A plane mirror forms an image that is the same size as the object.  A convex mirror forms an image that is smaller than the object.  A concave mirror forms an image that is larger than the object.  What type of mirror is shown in the figure to the right?  Convex Mirror 18-2 ?????Mirror Type????? ObjectReflected Image

4. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Mirror Types  What type of mirror is shown in the figure below?  Plane Mirror. 18-3 ?????Mirror Type????? ObjectReflected Image

5. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Mirror Types  What type of mirror is shown in the figure below?  Concave Mirror. 18-4 ?????Mirror Type????? ObjectReflected Image

6. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Diffuse Scattering.  Imagine that several parallel laser beams are fired at a mirror and a ceiling tile.  The reflected beams from the mirror surface would still be parallel, and  i and  r would be equal.  However, the reflected beams from the tile surface would not be parallel.  The reflected beams from the tile surface exhibit diffuse scattering. 18-5 Mirror Ceiling Tile

7. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Sound Wave Reflection  The picture to the right shows the TV room of a disappointed viewer.  He is very disappointed due to the poor sound quality of his TV.  The rays represent the path the sound travels in the room.  Sound waves, like light waves, reflect off of surfaces.  He is sitting in an acoustic “dead zone.”  Such a zone is typically the result of destructive interference of sound waves.  What could he do in order to improve the sound quality?  If he moves his chair, then he will be able to better hear and enjoy his TV. 18-6

8. © 2001-2005 Shannon W. Helzer. All Rights Reserved.  Simply put, “Refraction” means bends.  When discussing light beams, light bends when it goes from one medium (glass, water, air, etc.) to another.  If it goes from a more dense medium to a less dense medium, then light speeds up and bends away from the Normal.  If it goes from a less dense medium to a more dense medium, then light slows down and bends towards the Normal.  Consider the wheel and axel to the right.  It rolled from pavement (less dense) to gravel (more dense)  The left wheel slowed down causing the axel to turn towards the normal. 18-7 Pavement Gravel Refraction

9. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Refraction  When light undergoes refraction while traveling from a less dense to a more dense medium, the light bends towards the normal.  This is because light travels faster in a less dense medium than in a more dense medium.  Is the fish safe from the laser if we point the laser at the fish?  When laser light travels from air, a less dense medium, and is shined onto the surface of water, a more dense medium, in an fish tank, the light bends towards the normal as shown.  As a result, the light will bend and travel below the fish.  This bending of light at the interface of two mediums is known as refraction. 18-8 Normal

10. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Refraction  Refraction applies to Lenses as well.  Which laser path to the right would the laser beam follow?  When the beam first strikes the lens, it goes from a less dense to a more dense medium.  As a result, the beam will slow down and turn towards the normal.  When the beam strikes the boundary from the lens back to air, it goes from a more dense to a less dense medium.  As a result, the beam speeds up and will turn away from the normal.  It will stay on the same side of the normal.  Therefore, the center beam path would be the correct path followed by the laser beam. 18-9

11. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Refraction  Which of the three paths (A, B, or C) is the one the beam would actually travel?  Remember, lights bends towards the normal when it goes from a less dense to a more dense medium because it slows down in the more dense medium.  Conversely, light bends away from the normal when it goes from a more dense to a less dense medium because it speeds up in the less dense medium.  Which is the correct path? A B C Air Glass Water 18-11

12. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Light Spectrum  Previously, we learned that light travels in waves known as “Transverse” waves.  Light is composed of massless particles known as photons.  The spectrum of electromagnetic radiation spans several wavelengths (frequencies) of the photons.  Of particular interest to us is the visible spectrum.  To remember the visible spectrum, use the name “ROY G BIV.” VisibleInfraredUltra VioletGammaRadio WavesMicrowavesX-rays Red Orange Yellow Green Blue Indigo Violet Frequency Increases Speed Increases 18-12

13. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Dispersion of Light  Dispersion is the separation of light according to its frequencies into its different colors.  A prism is a device often used to demonstrate dispersion of white light.  Light with higher frequency travels slower in a medium; therefore, it will bend more with respect to the normal.  Light with lower frequency travels faster; therefore, it will bend less with respect to the normal.  One of the exit beams is orange and the other is green.  Based on the above information, which exit beam is orange? Top or bottom? Red Orange Yellow Green Blue Indigo Violet Frequency Speed 18-13

14. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Refraction & Fishing  Dr. Physics is watching a fish swimming in a pond.  He sees the fish indicated in the position shown.  Which position, top or bottom, is the actual position of the fish?  Why?  When light beams travel from a more dense to a less dense medium, they speed up and bend away from the normal.  As a result, the actual position of the fish is deeper than the apparent position of the fish. 18-14

15. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Lenses and Magnification – Diverging Lens  When the beam first strikes the lens, it goes from a less dense to a more dense medium.  As a result, the beam will slow down and turn towards the normal.  When the beam strikes the boundary from the lens back to air, it goes from a more dense to a less dense medium.  As a result, the beam speeds up and will turn away from the normal.  Will the resulting image be larger or smaller than the original image?  A diverging lens produces a larger image. 18-15

16. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Lenses and Magnification – Converging Lens  When the beam first strikes the lens, it goes from a less dense to a more dense medium.  As a result, the beam will slow down and turn towards the normal.  When the beam strikes the boundary from the lens back to air, it goes from a more dense to a less dense medium.  As a result, the beam speeds up and will turn away from the normal.  Will the resulting image be larger or smaller than the original image?  A converging lens produces a smaller image. 18-16

17. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Internal Reflection  Suppose we had a laser submerged in water.  If it was pointing straight up, then the beam would pass through the boundary interface without reflecting or refracting.  If we rotate the laser, then some of the beam will reflect and some will refract.  As we continue to rotate the laser, we would eventually observe the refracted beam traveling along the boundary interface.  This phenomena occurs at the critical angle.  As the laser is rotated past the critical angle, all of the beam will be reflected and none will be refracted.  This phenomena is known as total internal reflection. 18-17

18. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Lens Vocabulary  A – Center of Curvature  B – Focal Point  C – Focal Length  D – Principal Axis AABB CC D 18-18

19. © 2001-2005 Shannon W. Helzer. All Rights Reserved. 18-19 Magnification  Magnification is the enlargement or reduction of the size of the image of an object produced by a lens.  If the object is beyond twice the focal length (2f), then the image will be smaller and real (upside down)  If the object is at 2f, the it will be the same size and real.  Between f and 2f, the object will be larger and real.  At f the image will focus at infinity (not be visible).  In between f and the lens, the image will be much larger, virtual (right side up), and will appear on the same side of the lens as the object.  This situation occurs with a magnifying glass.

20. © 2001-2005 Shannon W. Helzer. All Rights Reserved. The Anatomy of an Eye  Label the following eye anatomy (parts) in the figure below.  A – Cornea  B – Lens  C – Iris  D – Blind Spot A B C D 18-20

21. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Nearsighted  A nearsighted person sees objects nearby clearly; however, far away objects appear blurry.  The light rays from far away objects form the image within the eye before the rays reach the retina.  This problem is corrected using a diverging lens.  This diverging lens separates the rays before they reach the eye lens.  The eye lens then focuses these rays on the retina thereby producing a high quality image. 18-21

22. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Farsighted  A farsighted person sees objects far away clearly; however, near objects appear blurry.  The light rays from near objects would form the image beyond the eye in the brain cavity well beyond the retina.  This result, of course, is impossible.  This problem is corrected using a converging lens.  This converging lens brings the rays together before they reach the eye lens.  The eye lens then focuses these rays on the retina thereby producing a high quality image. 18-22

23. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Index of Refraction, n  The index of refraction is a ratio of the speed of light in a vacuum (c) to the speed of light in a medium (v) like glass, oil, water, quartz, diamond,….  The top figure shows a laser aimed into a evacuated cylinder (nothing is inside it).  The speed of light in this cylinder is c = 3 x 10 8 m/s.  The bottom figure shows a laser aimed into a cylinder of water.  The light will travel slower in this cylinder, and its speed (v) can be measured in a lab.  The ratio of these speeds gives the index of refraction (n) of the material through which the light is shined (in this case water). 1

24. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Snell’s Law  Snell’s law allows us to determine the angle of refraction given the angle of incidence of a light ray on a boundary interface.  n 1 is the index of refraction of medium 1, and  1 is the angle of incidence on the boundary between the two interfaces.  n 2 is the index of refraction of medium 2, and  2 is the angle of refraction.  Based upon the picture to the right, which medium us more dense: 1 or 2? Why?  Since the beam bent towards the normal when going from medium 1 to medium 2, medium 2 is more dense than medium 1. n 1 sin  1 = n 2 sin  2 n1n1 n2n2 18-23

25. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Snell’s Law  Given the following information, determine the type of surrounding material and the lens material.   i = 65  R, Use N 1, n S = 1.36, n L = 1.54.  Begin by measuring the incident angle and drawing the incident beam.  Use Snell’s Law to calculate the refracted angle.  Draw the refracted beam to the next material interface.  Draw the normal as shown and measure the new incident angle.  This step not shown.  Use Snell’s Law to calculate the refracted angle, and draw the refracted beam.  The material is determined based upon where your second refracted beam stops.  What are your materials for this problem? Air with diamond lens Air with window glass lens Water with diamond lens Ethanol with Quartz lens N1N1 N2N2 N3N3 18-24 Assignment: 1.  i = 65  R, Use N 1, n S = 1.36, n L = 1.54. 2.  i = 45  R, Use N 2, n S = 1.33, n L = 2.42. 3.  i = 30  L, Use N 3, n S = 1.00, n L = 1.61. 4.  i = 30  L, Use N 3, n S = 1.00, n L = 2.42.

26. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Snell’s Law Air: n 1 = 1.003 Diamond: n 2 = 2.42  1 = 60  Snell’s Law n 1 sin  1 = n 2 sin  2 60 21 9 22

27. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Snell’s Law Water: n 1 = 1.33 Glass: n 2 = 1.52  1 = 40  Snell’s Law n 1 sin  1 = n 2 sin  2 40 34 26 62 18-26

28. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Refraction  Refraction is responsible for the “broken straw” effect you observe when viewing a straw in a drinking glass.  In the next slide, we will use this effect in order to determine the index of refraction of glass. 18-27

29. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Lab – Determination of n i for Glass  Follow the procedure on your lab in order to find the index of refraction for the piece of glass provided by your instructor.  You will do this experiment twice using different angles. P3P3 P2P2 P1P1 N1N1 N2N2 11 22 P4P4 22 11 18-28

30. © 2001-2005 Shannon W. Helzer. All Rights Reserved. This presentation was brought to you by Where we are committed to Excellence In Mathematics And Science Educational Services.

32. A AA 18-1 N3N3 Air with window glass lens N1N1 N2N2 Glass around diamond lens Water around Quartz lens

33. © 2001-2005 Shannon W. Helzer. All Rights Reserved. A AA 18-1 N3N3 Air with window glass lens N1N1 N2N2 Glass around diamond lens Water around Quartz lens

34. © 2001-2005 Shannon W. Helzer. All Rights Reserved. A AA 18-1

35. © 2001-2005 Shannon W. Helzer. All Rights Reserved. A  Keep Me 18-1 Air with diamond lens Air with window glass lens Water with diamond lens N1N1 N2N2 N3N3 Ethanol with Quartz lens

36. © 2001-2005 Shannon W. Helzer. All Rights Reserved.