Docstoc

Light

Document Sample
Light Powered By Docstoc
					          Property 5: Refraction
experiment ?
particle (photon)?
wave (E&M) ?
          Property 5: Refraction
• experiment: objects in water seem closer
  than they really are when viewed from air

                                         eye
  air

  water
          apparent
          location

                     real object
                Property 5: Refraction
 • particle (photon) ?
                  incident ray

          air

surface
          water



                                 refracted ray
                Property 5: Refraction
 • particle (photon) ?
                  incident ray                      vxair = vxwater
                       vxair
          air                vyair                  vyair < vywater

surface
                                                   therefore
          water
                                     vxwater       vi < vr

                                         vywater
                                         refracted ray
                  Property 5: Refraction
                            normal line

   • wave (E&M) ?
                incident wave
          air


surface                                           surfac
                                                  e
      water

                                 refracted wave
                            normal line
                   Property 5: Refraction
                                         crest of following wave
     • wave (E&M) ?
                                              crest of wave
                  incident wave
          air                                           crest of preceding wave
                                  air           air
surface                      x
                    water
          water

                                      refracted wave
                                 normal line
       Property 5: Refraction
• particle (photon) theory: vwater > vair
• wave (E&M) theory:        vwater < vair
• experiment ?
       Property 5: Refraction
• particle (photon) theory: vwater > vair
• wave (E&M) theory:        vwater < vair
• experiment:               vwater < vair
  wave theory works!
   particle theory fails!
          Properties 1, 2 & 5

Speed, Color and Refraction
• Speed of light changes in different materials
• Speed is related to frequency and
  wavelength: v = f
• If speed changes, does wavelength change,
  frequency change, or BOTH?
       Properties 1, 2 & 5
• Speed, Color and Refraction
• Speed of light changes in different materials
• Speed is related to frequency and
  wavelength: v = f
• What changes with speed?
   – Frequency remains constant regardless
     of speed
   – Wavelength changes with speed
    Refraction and Thin Lenses
Can use refraction to try to control rays of
 light to go where we want them to go.

Let’s see if we can FOCUS light.
           Refraction and Thin Lenses
   What kind of shape do we need to focus light
    from a point source to a point?
                                 lens with some shape for front & back




point
source
of light                                                screen
                                  s’ = image distance
           s = object distance
    Refraction and Thin Lenses
Let’s try a simple (easy to make) shape:
  SPHERICAL.

Play with the lens that is handed out
  Does it act like a magnifying glass?
    Refraction and Thin Lenses
Let’s try a simple (easy to make) shape:
  SPHERICAL.

Play with the lens that is handed out
  Does it act like a magnifying glass?
  Does it focus light from the night light?
    Refraction and Thin Lenses
Let’s try a simple (easy to make) shape:
  SPHERICAL
Play with the lens that is handed out
  Does it act like a magnifying glass?
  Does it focus light from the night light?
  Does the image distance depend on the shape
   of the lens? (trade with your neighbor to get a
   different shaped lens)
    Refraction and Thin Lenses
Spherical shape is specified by a radius.
The smaller the sphere (smaller the radius),
the more curved is the surface!
                    R
       R                         R1
                          R2
 Refraction and the Lens-users Eq.
                                         f>0
1   1   1                                s > 0 AND s > f
  =   +
f   s   s'
                                         s’ > 0 AND s’ > f




               f                     f

           s
                                s’
 Example: f = 10 cm; s = 20 cm; s’ = 20 cm: 1/20 + 1/20 = 1/10
     Refraction and the Lens-users Eq.
   1   1   1                                as s gets bigger,
     =   +
   f   s   s'                               s’ gets smaller




                  f                     f

              s
                                   s’
Example: f = 10 cm; s = 40 cm; s’ = 13.3 cm: 1/40 + 1/13.3 = 1/10
      Refraction and the Lens-users Eq.
   1   1   1                                 as s approaches infinity
     =   +
   f   s   s'                                s’ approaches f




                    f                    f

                s
                                   s’
Example: f = 10 cm; s = 100 cm; s’ = 11.1 cm: 1/100 + 1/11.1 = 1/10
 Refraction and the Lens-users Eq.
                                         f>0
1   1   1                                s > 0 AND s > f
  =   +
f   s   s'
                                         s’ > 0 AND s’ > f




               f                     f

           s
                                s’
 Example: f = 10 cm; s = 20 cm; s’ = 20 cm: 1/20 + 1/20 = 1/10
   Refraction and the Lens-users Eq.
 1   1   1
   =   +                                     as s gets smaller,
 f   s   s'                                  s’ gets larger




                f                     f

                    s
                                             s’
Example: f = 10 cm; s = 13.3 cm; s’ = 40 cm: 1/13.3 + 1/40 = 1/10
    Refraction and the Lens-users Eq.
  1   1   1
    =   +                                     as s approaches f,
  f   s   s'                                  s’ approaches infinity




                  f                    f

                      s
                                                 s’
Example: f = 10 cm; s = 11.1 cm; s’ = 100 cm: 1/11.1 + 1/100 = 1/10
 Refraction and the Lens-users Eq.
Before we see what happens when s gets
 smaller than f, let’s use what we already
 know to see how the lens will work.
Refraction and the Lens-users Eq.
– Any ray that goes through the focal point
  on its way to the lens, will come out
  parallel to the optical axis. (ray 1)



        f              f
                                      ray
                                      1
Refraction and the Lens-users Eq.
– Any ray that goes through the focal point
  on its way from the lens, must go into the
  lens parallel to the optical axis. (ray 2)



        f               f
                                       ray
                                       1
                                    ray 2
   Refraction and the Lens-users Eq.
     – Any ray that goes through the center of
       the lens must go essentially undeflected.
       (ray 3)
object

                                  image
                             f               ray
             f
                                             1
                                                   ray 3

                                          ray 2
   Refraction and the Lens-users Eq.
     – Note that a real image is formed.
     – Note that the image is up-side-down.
object

                                image
                           f               ray
             f
                                           1
                                                 ray 3

                                        ray 2
      Refraction and the Lens-users Eq.
      – By looking at ray 3 alone, we can see
      by similar triangles that M = h’/h = -s’/s.
 object
  h                                 s
                                               image
                                    ’
                                        f       h’<0
           s      f
                                                                ray 3

Example: f = 10 cm; s = 40 cm; s’ = 13.3 cm:    note h’ is up-side-down
                                                and so is <0
M = -13.3/40 = -0.33 X
   Refraction and the Lens-users Eq.
 This is the situation when the lens is used
 in a camera or a projector. Image is REAL.
object

                               image
                          f               ray
           f
                                          1
                                                ray 3

                                       ray 2
   Refraction and the Lens-users Eq.
 What happens when the object distance, s,
  changes?

object

                               image
                          f               ray
           f
                                          1
                                                ray 3

                                       ray 2
    Refraction and the Lens-users Eq.
  Notice that as s gets bigger, s’ gets closer to f
   and |h’| gets smaller.

 object

                                                image
                                      f                    ray
                  f
                                                           1
                                                                 ray 3
Example: f = 10 cm; s = 100 cm; s’ = 11.1 cm:
M = -11.1/100 = -0.11 X                                 ray 2
                 Focusing
To focus a camera, we need to change s’ as s
 changes. To focus a projector, we need to
 change s as s’ changes. We do this by
 screwing the lens closer or further from the
 film or slide.
But what about the eye? How do we focus on
 objects that are close and then further away
 with our eyes? Do we screw our eyes in
 and out like the lens on a camera or
 projector?
                Focusing
But what about the eye? How do we focus on
 objects that are close and then further away
 with our eyes? Do we screw our eyes in
 and out like the lens on a camera or
 projector? - NO, instead our eyes
 CHANGE SHAPE and hence change f as s
 changes, keeping s’ the same!
 Refraction and the Lens-users Eq.
Let’s now look at the situation where
s < f (but s is still positive):



               s
           f              f
 Refraction and the Lens-users Eq.
We can still use our three rays. Ray one goes
through the focal point on the left side.
                                           ray 1



               s
           f              f
 Refraction and the Lens-users Eq.
Ray two goes through the focal point on the
right side (and parallel to the axis on the left).
                                                ray 1



                s
            f                f
                                                ray 2
 Refraction and the Lens-users Eq.
Ray three goes through the center of the lens
essentially undeflected.
                                            ray 1

   h’

                    s
           f              f
               s’                           ray 2

                                           ray 3
     Refraction and the Lens-users Eq.
   Notice that: s’ is on the “wrong” side, which
   means that s’ < 0 , and that |s’| > |s| so f > 0.
                                                                  ray 1

        h’

                            s
                   f                     f
                       s’                                        ray 2

                                                                ray 3

Example: f = 10 cm; s = 7.14 cm; s’ = -25 cm: 1/7.14 + 1/(-25) = 1/10
    Refraction and the Lens-users Eq.
  Notice that: h’ right-side-up and so h’ > 0.,
  M = h’/h = -s’/s . M > 0 (s’ < 0 but -s’ > 0).

       h’

                           s
                  f                     f
                      s’
                                                ray 3
Example: f = 10 cm; s = 7.14 cm; s’ = -25 cm:
M = - (-25)/ 7.14 = 3.5 X
 Refraction and the Lens-users Eq.
This is the situation when the lens is used
as a magnifying glass! Image is VIRTUAL.
                                          ray 1

   h’

                   s
          f              f
              s’                          ray 2

                                         ray 3
 Refraction and the Lens-users Eq.
The same lens can be used as:
• a camera lens: s >> f, s > s’,
     M < 0, |M| < 1
• a projector lens: s > f, s’ > s,
     M < 0, |M| > 1
• a magnifying glass: s < f, s’ < 0,
      M > 0, M > 1
 Refraction and the Lens-users Eq.
Notes on using a lens as a magnifying glass:
• hold lens very near your eye
• want IMAGE at best viewing distance
   which has the nominal value of 25 cm
   so that s’ = -25 cm.
 Refraction and the Lens-users Eq.
Are there any limits to the magnifying power
  we can get from a magnifying glass?
 Refraction and the Lens-users Eq.
• Magnifying glass has limits due to size
• As we will see in a little bit, magnifying
   glass has limits due to resolving ability
• NEED MICROSCOPE (two lens system)
  for near and small things; need
  TELESCOPE (two lens system) for far
  away things.
            Telescope Basics
 Light from far away is almost parallel.

objective
lens                                       eyepiece




                                     fe
                fo
            Telescope Basics:
             Get More Light
 The telescope collects and concentrates light.

objective
lens                                     eyepiece




                                    fe
                fo
             Telescope Basics
 Light coming in at an angle, in is magnified
                               to out .
objective
lens                                       eyepiece



                                  x



                fo                    fe
               Magnification
 in = x/fo, out = x/fe; M = out/in = fo/fe

objective
lens                                          eyepiece



                                    x



                 fo                      fe
       Limits on Resolution
telescopes
  – magnification: M = out/in = fo /fe
  – light gathering: Amt D2
  – resolution: 1.22  = D sin(limit) so
    in = limit and out = 5 arc minutes
  so limit  1/D implies Museful = 60/in * D
              where D is in inches
  – surface must be smooth on order of 
       Limits on Resolution:
            calculation
    Mmax useful = out/in = eye/limit
         = 5 arc min / (1.22 *  / D) radians
= (5/60)*(/180) / (1.22 * 5.5 x 10-7 m / D)
= (2167 / m) * D * (1 m / 100 cm) * (2.54 cm / 1 in)
= (55 / in) * D
                 Example
What diameter telescope would you need to
 read letters the size of license plate numbers
 from a spy satellite?
                Example
• need to resolve an “x” size of about 1 cm
• “s” is on order of 100 miles or 150 km
• limit then must be (in radians)
     = 1 cm / 150 km = 7 x 10-8
• limit = 1.22 x 5.5 x 10-7 m / D = 7 x 10-8
 so D = 10 m (Hubble has a 2.4 m diameter)
  Limits on Resolution: further
            examples
• other types of light
  – x-ray diffraction (use atoms as slits)
  – IR
  – radio & microwave
• surface must be smooth on order of 
 Review of Telescope Properties
1. Magnification: M = fo/fe depends on the
   focal lengths of the two lenses.
2. Light gathering ability: depends on area
   of objective lens, so depends on diameter
   of objective lens squared (D2).
3. Resolution ability: depends on diameter
   of objective lens: Max magnification = 60
   power/in * D.
          Types of Telescopes
The type of telescope we have looked at so far, and
  the type we have or will have made in the lab is
  called a refracting telescope, since it uses the
  refraction of light going from air to glass and back
  to air. This is the type used by Galileo.
There is a second type of telescope invented by
  Newton. It is called the reflecting telescope since
  it uses a curved mirror instead of a curved lens for
  the objective. There are three main sub-types of
  reflectors that we’ll consider: Prime focus,
  Newtonian, and Cassegranian.
            Refracting Telescope

 Two lenses (as we had in the lab)

objective
lens                                      eyepiece




                                     fe
                fo
        Reflecting Telescope

Light from far away                     mirror
                                         focuses
                                            light




problem: how do we get to focused light without
  blocking incoming light?
         Reflecting Telescope
             Prime Focus
Light from far away
                                         mirror
                                          focues
                         eyepiece            light



Solution #1: If mirror is big enough (say 100
  to 200 inches in diameter), we can sit right in
  the middle and we won’t block much light -
  this is called the prime focus.
          Reflecting Telescope
           Newtonian Focus

Light from far away                     eyepiece
                                             primary
                                                mirror
                                                 focuses
                            mirror
                                                 light

Solution #2: Use secondary mirror to reflect light out
  the side of the telescope- this is called the Newtonian
  focus.
         Reflecting Telescope
         Cassegranian Focus

Light from far away                  primary mirror
                                             focuses
                                                light
                               mirror
                                            eyepiece

Solution #3: Use secondary mirror to reflect light
  out the back of the telescope- this is called the
  Cassegranian focus.

				
DOCUMENT INFO
kala22 kala22
About