reflection

1
reflection refraction of light
http//www.phys.pe.kr/03_wave/03_wave_menu.html

• PHY232
• Remco Zegers
• zegers_at_nscl.msu.edu
• Room W109 cyclotron building
• http//www.nscl.msu.edu/zegers/phy232.html

2
light

• cf?
• 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
light rays and wave-front
wavefront
rays

• 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
• IPsource/(4?R2) I intensity (W/m2) Psource
  power of source. R distance from source

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
t0
5
Huygens principle II

• For a circular wavefront

t0
6
question

• consider a square wavefront at t0. After some
  time, the wavefront has traveled outward and is
  now shaped like a
• a) circle
• b) square
• c) ellipse
• d) rounded square

7
from now on

• we usually only draw the light rays and not the
  wave-front

8
reflection
normal

• 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
• rI
• of course, the mirror must be very flat for this
  to be true

i
r
mirror
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
index of refraction and speed of light

• In vacuum, the speed of light equals c3x108 m/s.
• In any other medium, light propagates more
  slowly..
• in air vc/1.0003
• in water vc/1.33
• in glass vc/1.5
• in general vc/n
• where n index of refraction
• n also depends on the wavelength (?)
• n is larger for smaller ?
• this effect is called dispersion

11
refraction

• if light passes from one medium with refractive
  index n1 to a medium with refractive index n2 it
  is bent (refracted)
• Snells law n1sin?1n2sin?2 or
  n1/n2sin?2/sin?1

12
refraction II

• if n1lt n2 then the light ray bends towards the
  normal when passing through the medium
• if n1gt n2 then the light ray bends away from the
  normal when passing through the material.

13
demo

• consider a light ray traveling from air to water
  at an incident angle of 300. Given nwater1.33,
  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?

n1/n2sin?2/sin?1
14
question

• a light ray hits a 1 cm glass plate (n1.5) at an
  incident angle of 300. The light emerges from the
  glass plate at an angle
• a) lt300
• b) 300
• c) gt300

?300
????
http//www.phys.pe.kr/bbs/view.php?idwavepage1
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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?

b
a
c
16
a harpoon fisher

• http//www.phys.pe.kr/bbs/view.php?idwavepage1
  sn1divpage1category3snoffssonsconselec
  t_arrangeheadnumdescascno2

17
total internal reflection

• consider a ray of light traveling from water to
  air.
• since nwater gt nair a light ray is bent away from
  the normal
• however, if
• this equation has no solution,
• since sin? cannot be larger than 1

?2sin-1(nwatersin?1/nair)
nwatersin?1/nairgt1 sin?1gtnair/nwater ?1gtsin-1(nair
/nwater) ?1gt48.7o
18
internal reflection II

• when traveling from a medium 1 to a medium 2 with
  n1gtn2
• if sin?Igtn2/n1 refraction is not possible
  and all light will be reflected ?sin-1(n2/n1)
  is called the critical angle

19
transport of light!
demo
20
question
?500

• given the above situation and the fact that the
  critical angle for water-gtair is 48.70, would
  the harpoonist know where to aim at? I.e. does
  she see the fish?
• yes
• no

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
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
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