Physics 132: Lecture Fundamentals of Physics II Agenda for Today

Transcription

1 Physics 132: Lecture Fundamentals of Physics II Agenda for Today Reflection of light Law of reflection Refraction of light Snell s law Dispersion PHY132 Lecture 17, Pg1

2 Electromagnetic waves A changing electric field creates a magnetic field, which then changes in just the right way to recreate the electric field, and so on This is an electromagnetic wave. PHY132 Lecture 17, Pg2

3 Electromagnetic Waves Like a water wave is a propagating disturbance of water, EM wave is propagating disturbance of electric and magnetic fields Accelerating electric charges will cause changing E and B-fields These disturbances will propagate away from the charges PHY132 Lecture 17, Pg3

4 The Electromagnetic Spectrum Different frequency / wavelength Different names for EM wave PHY132 Lecture 17, Pg4

5 Models of Light The nature of light is elusive Wave Model In some cases light exhibits similar behavior to water or sound waves (Ch 32) Photon Model In the quantum world light consists of photons that have both wave-like and particle-like characteristics (phy 256) Ray Model Light travels in straight line paths (Ch 30 and 31) Good Book: QED: the Strange Theory of Light and Matter by Richard Feynman PHY132 Lecture 17, Pg5

6 Light incident on an object Absorbed Reflects (bounces)** See it Mirrors Refraction (bends) Lenses PHY132 Lecture 17, Pg6

7 Reflection Angle of incident = Angle of reflection q i = q f (Angle is between light beam and normal) q f q i PHY132 Lecture 17, Pg7

8 Reflections We see a point of an object when light rays from it enter our eye. Color is because some light is absorbed Object PHY132 Lecture 17, Pg8

9 Reflections We see a point of an image when light rays appear to come from a single point. Object Image Mirror smooth Student 1: I don't understand what you are doing/mean when you say that you are "backtracking" an image or beam Student 2: According to the video, "virtual images are images where light rays do not actually emanate from/through the image point". Wouldn't PHY132 Lecture 17, Pg9 this make any real object I can see (unless it glows) a virtual image?

10 Clicker Question 1: You are standing in front of a short flat mirror which is placed too high, so you can see above your head, but only down to your knees. To see your shoes, you must move A. closer to the mirror. B. further from the mirror. C. to another mirror. Changing distance doesn t change what you see of yourself PHY132 Lecture 17, Pg10

11 How High Is the Mirror? 11

12 Clicker Question 2: An observer O, facing a mirror, observes a light source S. Where does O perceive the mirror image of S to be located? A. 1 B. 2 C. 3 D. 4 E. The image of S cannot be seen by O when O and S are located as shown. PHY132 Lecture 17, Pg12

13 Index of Refraction Velocity of light in vacuum c = meters/second Velocity in medium Velocity of light in vacuum Index of refraction so PHY132 Lecture 17, Pg13

14 Indices of Refraction 14

15 Clicker Question 3: Suppose the sprinters wish to get from point Q on the beach to point P on the parking lot as quickly as possible. Which path takes the least time? A B C D E Fermat s principle: path taken by a light ray between any two PHY132 Lecture 17, Pg15 points is the path that takes the shortest time.

16 Snell s Law When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends: n 1 sin(q 1 )= n 2 sin(q 2 ) q 1 n 1 q 2 n 2 Student: If angle is measured from horizontal would cos 16be used for refractive index?

17 Plane/Line waves 17

18 Student 1 : why does different material refract light? Snell s Law Student 2: Would you mind going over why the wavelength changes through a medium boundary? Thank you!! Demo PHY132 Lecture 17, Pg18

19 Clicker Question 4: A laser beam passing from medium 1 to medium 2 is refracted as shown. Which is true? A. n 1 < n 2. B. n 1 > n 2. C. There s not enough information to compare n 1 and n 2. PHY132 Lecture 17, Pg19

20 Clicker Question 5: A fish swims below the surface of the water. An observer sees the fish at A. a greater depth than it really is. B. the same depth. C. a smaller depth than it really is. PHY132 Lecture 17, Pg20

21 Clicker Question 5: A fish swims below the surface of the water. An observer sees the fish at A. a greater depth than it really is. B. the same depth. C. a smaller depth than it really is. PHY132 Lecture 17, Pg21

22 Example 22

23 Example 23

24 Total Internal Reflection Recall Snell s Law: n 1 sin(q 1 )= n 2 sin(q 2 ) when n 1 > n 2, q 2 > q 1 When q 1 = sin -1 (n 2 /n 1 ) q 2 = 90 This is critical angle, light incident at a larger angle will be completely reflected q i = q r q 2 n 2 n 1 q c q 1 normal 24

25 Student Questions Does total internal reflection means that the speed in the material in which it travels is so great that the light bends all the way inside? In other words, is this a continuum? Can we always get total internal reflection eventually as long as the emerging light ray bends away from the normal? For total internal refraction, does the index of refraction have to be greater for the origin medium when compared to the index of refraction for the medium the light will travel to? i don't understand the importance of the critical angle. PHY132 Lecture 17, Pg25

26 Fiber Optics At each contact w/ the glass air interface, if the light hits at greater than the critical angle, it undergoes total internal reflection and stays in the fiber. 26

27 Clicker Question 6: A laser beam undergoes two refractions plus total internal reflection at the interface between medium 2 and medium 3. Which is true? A. n 1 < n 3. B. n 1 > n 3. C. There s not enough information to compare n 1 and n 3. PHY132 Lecture 17, Pg27

28 Dispersion The index of refraction n depends on color! n blue = 1.53 n red = 1.52 n blue > n red prism White light Blue light gets deflected more 28

29 Rainbows PHY132 Lecture 17, Pg29