1. reflection & refraction of light PHY232 Remco Zegers zegers@nscl.msu.edu Room W109 – cyclotron building http://www.nscl.msu.edu/~zegers/phy232.html http://www.phys.pe.kr/03_wave/03_wave_menu.html

2. PHY232 - Remco Zegers reflection & refraction of light 2 light  c=f  light: wavelike oscillation of E and B fields (just like any electromagnetic radiation). The only things special about it, is that we can see it.

3. PHY232 - Remco Zegers reflection & refraction of light 3 light rays and wave-front  isotropic light source (uniform in all directions):  the wavefronts indicate maxima in the E/B oscillations  the rays indicate the direction of the wave fronts  I=P source /(4  R 2 ) I: intensity (W/m 2 ) P source :power of source. R distance from source wavefront rays

4. PHY232 - Remco Zegers reflection & refraction of light 4 Huygens principle  All points on a wavefront serve as point sources of spherical secondary wavelets. After a time t, the new positions of the wavefront will be the tangent of these secondary wavelets motion of a plane wave: vt 1 vt 2 t=0

5. PHY232 - Remco Zegers reflection & refraction of light 5 Huygens principle II  For a circular wavefront… t=0 vt 1 vt 2

6. PHY232 - Remco Zegers reflection & refraction of light 6 question  consider a square wavefront at t=0. After some time, the wavefront has traveled outward and is now shaped like a  a) circle  b) square  c) ellipse  d) rounded square

7. PHY232 - Remco Zegers reflection & refraction of light 7 from now on…  we usually only draw the light rays and not the wave-front

8. PHY232 - Remco Zegers reflection & refraction of light 8 reflection  consider a light ray that strikes a mirror. It will be reflected.  The angle of incidence (relative to the normal to the plane) equals the the angle of reflection  r=I  of course, the mirror must be very flat for this to be true mirror ir normal

9. PHY232 - Remco Zegers reflection & refraction of light 9 virtual image  if we look at a mirror, we see an image at a location where the object not really is.  the image is formed at a location where there is not light present  this is called a virtual image  see next chapter…

10. PHY232 - Remco Zegers reflection & refraction of light 10 index of refraction and speed of light  In vacuum, the speed of light equals c=3x10 8 m/s.  In any other medium, light propagates more slowly..  in air: v=c/1.0003  in water: v=c/1.33  in glass: v=c/1.5  in general: v=c/n where n: index of refraction  n also depends on the wavelength ( ) n is larger for smaller this effect is called dispersion

11. PHY232 - Remco Zegers reflection & refraction of light 11 refraction  if light passes from one medium with refractive index n 1 to a medium with refractive index n 2 it is bent (refracted):  Snell’s law n 1 sin  1 =n 2 sin  2 or n 1 /n 2 =sin  2 /sin  1

12. PHY232 - Remco Zegers reflection & refraction of light 12 refraction II  if n 1 < n 2 then the light ray bends towards the normal when passing through the medium  if n 1 > n 2 then the light ray bends away from the normal when passing through the material.

13. PHY232 - Remco Zegers reflection & refraction of light 13 demo  consider a light ray traveling from air to water at an incident angle of 30 0. Given n water =1.33, what is the angle of refraction?  Now, consider the light-ray traveling from water to air at the same incident angle. What is the angle of refraction? n 1 /n 2 =sin  2 /sin  1

14. PHY232 - Remco Zegers reflection & refraction of light 14 question  a light ray hits a 1 cm glass plate (n=1.5) at an incident angle of 30 0. The light emerges from the glass plate at an angle  a) <30 0  b) =30 0  c) >30 0  =30 0  =??? http://www.phys.pe.kr/bbs/view.php?id=wave&page=1&sn1=&divpage=1&category=3&sn=off&ss=on&sc=on&select_arrange=headnum&desc=asc&no=11

15. PHY232 - Remco Zegers reflection & refraction of light 15 question: harpoon fishing  You are standing on the shore near a lake and see a fish in the water which you want to spear. Should you aim the spear a) directly at the fish, b) just above it or c) just under it? a b c

16. PHY232 - Remco Zegers reflection & refraction of light 16 a harpoon fisher  http://www.phys.pe.kr/bbs/view.php?id=wave&page=1&sn1=&divpage=1&category=3&sn=off&ss=on&sc=on&select_arrange=headnum&desc=asc&no=2

17. PHY232 - Remco Zegers reflection & refraction of light 17 total internal reflection  consider a ray of light traveling from water to air.  since n water > n air a light ray is bent away from the normal  however, if …this equation has no solution, since sin  cannot be larger than 1  2 =sin -1 (n water sin  1 /n air ) n water sin  1 /n air >1 sin  1 >n air /n water  1 >sin -1 (n air /n water )  1 >48.7 o

18. PHY232 - Remco Zegers reflection & refraction of light 18 internal reflection II  when traveling from a medium 1 to a medium 2 with n 1 >n 2 : if sin  I >n 2 /n 1 refraction is not possible and all light will be reflected;  =sin -1 (n 2 /n 1 ) is called the critical angle

19. PHY232 - Remco Zegers reflection & refraction of light 19 transport of light! demo

20. PHY232 - Remco Zegers reflection & refraction of light 20 question  given the above situation and the fact that the critical angle for water->air is 48.7 0, would the harpoonist know where to aim at? I.e. does she see the fish? a)yes b)no  =50 0

21. PHY232 - Remco Zegers reflection & refraction of light 21 dispersion  we already saw that the index of refraction depends on the wavelength (I.e. color) of the light.  this can be used to separate white light into its component colors. demo

22. PHY232 - Remco Zegers reflection & refraction of light 22 rainbow  depending on the location of the raindrop and the angle a person is looking at it, only light of a certain wavelength (color) is visible.  The angle (location) changes depending on where the drop is, so that one sees different colors at different locations in the sky…

23. PHY232 - Remco Zegers reflection & refraction of light 23 lon-capa  after reviewing this lecture, do questions 1-6 from HW set 8.  to do these problems: be careful with what is the initial and final medium…