1680s

1. 1680s Dutch physicist Christian Huygens captured this propagation mechanism MATHEMATICALLY Every point on a wavefront behaves as a point source for waves generated in the direction of the wave’s propagation (each point is the center of its own wavelet)

2. A train of waves of fixed frequency is traveling along a thin, light cord toward the juncture to a heavy, thick cord. Both cords are stretched taut and under the same tension. The waves that travel across the heavy cord will have 1) greaterspeed than along the lighter cord.2) the same speed as along the lighter cord. 3) smaller speed than along the lighter cord.

3. A train of waves of fixed frequency is traveling along a thin, light cord toward the juncture to a heavy, thick cord. Both cords are stretched taut and under the same tension. The waves traveling across the heavy cord have 1) largerwavelength than along the lighter cord.2) the same wavelength as along the lighter cord. 3) smaller wavelength than along the lighter cord.

4. A train of waves of fixed frequency is traveling along a thin, light cord toward the juncture to a heavy, thick cord. Both cords are stretched taut and under the same tension. The waves will continue across the heavy cord 1) at lower frequency than along the lighter cord.2) with the same frequency as along the lighter cord. 3) at higher frequency than along the lighter cord.

5. Answers to 3 previous slides 3) smaller speed than along the lighter cord. Speed of mechanical waves, Here the tension remains unchanged, but the density m increases. 3) smaller wavelength than along the lighter cord. Slower speed means the waveforms will bunch up as they pass the juncture. 2) with the same frequency as along the lighter cord.

7. vacuum 2 medium 1 A ray of light travelling in a vacuum encounters an interface with some medium as shown above. In which direction will the ray of light bend ? 1) ray 1 2) ray 2 3) could be either, depending on the medium

8. air air 1 2 Parallel light rays cross interfaces from air into two different media, 1 and 2, as shown in the figures above. In which of the media is the light traveling faster? 1) 1 2) 2 3) same speed in both

9. Consider a light ray which traverses a thick slab • ray bends towards the normal upon entering the glass • ray bends away from the normal when it exits from the glass • exiting light ray is at same angle as original ray, but is shifted over to one side

10. An observer views two closely spaced lines through an angled piece of glass. To the observer, the lines appear: 1) shifted to the right 2) shifted to the left 3) spaced farther apart 4) spaced closer together 5) no change -- exactly as before

11. Answer to 3 previous Concept Question slides: 1) ray 1 As the text describes, light is impeded (slowed) in any medium so travels slower than it does in a vacuum. 1) Medium 1 Light will bend more the greater its change in speed . It slowed down less upon entering the 1st medium. 2) shifted to the left

12. When light goes from a medium with high n into a medium with low n, rays bend away from the normal. Total Internal Reflection c • At a particular incident angle (critical angle qc), the refracted angle becomes exactly 90°. air n2 ( < n1) n1 water • At angles greater than qc there is no refracted ray at all. The incident rays are completely reflected !! • this is total internal reflection

13. What is the condition for total internal reflection? when qi = qc refracted angle is 90° Total Internal Reflection c air n2 ( < n1) n1 water • Remember: this only works when the incident medium has the higher index of refraction.

14. Example: binoculars use 45° prisms to reflect light For glass with n = 1.5 we find that: sin qc = 1.0/1.5 = 0.67  qc = 41.8° so for qi = 45°, the light is totally reflected