reflection & refraction of light PHY232 Remco Zegers Room W109 – cyclotron building

PHY232 - Remco Zegers reflection & refraction of light 2 quiz (extra credit)  which of the following is not correct in case of an AC LRC circuit? a)in case the inductive (X L ) and capacitive reactances (X C ) are equal in size, the rms current through the circuit is largest. b)The current through and voltage over the resistor are in always in phase c)if the LRC circuit is in resonance, both the capacitive reactance and inductive reactance must be zero d)the rms current at resonance (I.e. when it is largest) is independent of the frequency.

PHY232 - Remco Zegers reflection & refraction of light 3 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 - Remco Zegers reflection & refraction of light 4 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

PHY232 - Remco Zegers reflection & refraction of light 5 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

PHY232 - Remco Zegers reflection & refraction of light 6 Huygens principle II  For a circular wavefront… t=0 vt 1 vt 2

PHY232 - Remco Zegers reflection & refraction of light 7 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

PHY232 - Remco Zegers reflection & refraction of light 8 from now on…  we usually only draw the light rays and not the wave-front

PHY232 - Remco Zegers reflection & refraction of light 9 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

PHY232 - Remco Zegers reflection & refraction of light 10 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…

PHY232 - Remco Zegers reflection & refraction of light 11 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 - Remco Zegers reflection & refraction of light 12 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

PHY232 - Remco Zegers reflection & refraction of light 13 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.

PHY232 - Remco Zegers reflection & refraction of light 14 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 - Remco Zegers reflection & refraction of light 15 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 - Remco Zegers reflection & refraction of light 16 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 - Remco Zegers reflection & refraction of light 17 a harpoon fisher 

PHY232 - Remco Zegers reflection & refraction of light 18 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

PHY232 - Remco Zegers reflection & refraction of light 19 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 - Remco Zegers reflection & refraction of light 20 transport of light! demo

PHY232 - Remco Zegers reflection & refraction of light 21 question  given the above situation and the fact that the critical angle for water->air is , would the harpoonist know where to aim at? I.e. does she see the fish? a)yes b)no  =50 0

PHY232 - Remco Zegers reflection & refraction of light 22 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

PHY232 - Remco Zegers reflection & refraction of light 23 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…

PHY232 - Remco Zegers reflection & refraction of light 24 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…