reflection & refraction of light PHY232 – Spring 2007 Jon Pumplin (Ppt courtesy of Remco Zegers)

PHY232 - pumplin - 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.

PHY232 - pumplin - reflection & refraction of light 3 light rays and wave-fronts  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 rays

PHY232 - pumplin - reflection & refraction of light 4 from now on…  We often only draw the light rays and not the wave-fronts.  But sometimes it is easiest to think in terms of the wave- fronts.

PHY232 - pumplin - reflection & refraction of light 5 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  The mirror must be very flat for this to be true mirror ir normal

PHY232 - pumplin - reflection & refraction of light 6 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 no light present.  This is called a virtual image.

PHY232 - pumplin - reflection & refraction of light 7 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/  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

PHY232 - pumplin - reflection & refraction of light 8 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

PHY232 - pumplin - reflection & refraction of light 9 refraction II  if n new > n old then the light ray bends towards the normal when crossing the boundary.  if n new < n old then the light ray bends away from the normal when crossing the boundary.

PHY232 - pumplin - reflection & refraction of light 10 demo  consider a light ray traveling from air to water at an incident angle of 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  2 =sin -1 (n 1 sin  1 /n 2 ) =sin -1 (1xsin30 0 /1.33)= =  2 =sin -1 (n 1 sin  1 /n 2 ) =sin -1 (1.33xsin30 0 /1.)= =

PHY232 - pumplin - reflection & refraction of light 11 question  a light ray hits a 1 cm glass plate (n=1.5) at an incident angle of The light emerges from the glass plate at an angle  a) <30 0  b) =30 0  c) >30 0  =30 0  =??? at air-glass interface:  2 =sin -1 (n 1 sin  1 /n 2 )=sin -1 (1xsin30 0 /1.5)= at glass-air interface  2 =sin -1 (n 1 sin  1 /n 2 )=sin -1 (1.5xsin /1.)=

PHY232 - pumplin - reflection & refraction of light 12 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 answer c)

PHY232 - pumplin - reflection & refraction of light 13 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  If is larger than 1.00, this equation has no solution,  Total Internal Reflection. (There is always some reflection – reflection gets stronger as the critical angle is approached.) n air sin  2 = n water sin  1 n water sin  1

PHY232 - pumplin - reflection & refraction of light 14 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

PHY232 - pumplin - reflection & refraction of light 15 transport of light! demo

PHY232 - pumplin - reflection & refraction of light 16 dispersion  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

PHY232 - pumplin - reflection & refraction of light 17 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…