Physics 102: Lecture 18, Slide 1
Snell’s Law, Total InternalReflection, Brewster’s Angle,Dispersion, Lenses
Physics 102: Lecture 18
Physics 102: Lecture 18, Slide 2
Snell’s Law: A Quick Review
n1
n2
When light travels from one medium to another thespeed changes v=c/n, but the frequency is constant.So the light bends:
n1 sin(1)= n2 sin(2)
1
2
Physics 102: Lecture 18, Slide 3
Total Internal Reflection
normal
2
1
n2
n1
Recall Snell’s Law: n1 sin(1)= n2 sin(2)
(n1 > n2 2 > 1 )
1 = sin-1(n2/n1) then 2 = 90
c
Light incident at a larger anglewill only have reflection (i = r)
i
r
“critical angle”
For water/air:
n1=1.33, n2=1
1 = sin-1(n2/n1)
= 48.80
Physics 102: Lecture 18, Slide 4
Physics 102: Lecture 18, Slide 5
Fiber Optics
Telecommunications
Arthoscopy
Laser surgery
Total Internal Reflection onlyworks if noutside < ninside
At each contact w/ the glass air interface, if the lighthits at greater than the critical angle, it undergoes totalinternal reflection and stays in the fiber.
ninside
noutside
Physics 102: Lecture 18, Slide 6
Can the person standing on the edge of the pool beprevented from seeing the light by total internalreflection ?
1) Yes2) No
Preflight 18.1
Physics 102: Lecture 18, Slide 7
ACT: Refraction
•As we pour more water into bucket, whatwill happen to the number of people whocan see the ball?
1) Increase2) Same3) Decrease
Physics 102: Lecture 18, Slide 8
Physics 102: Lecture 18, Slide 9
Brewster’s angle
When angle betweenreflected beam and refractedbeam is exactly 90 degrees,reflected beam is 100%horizontally polarized !
Reflected light is usually unpolarized (mixture ofhorizontally and vertically polarized). But…
n1 sin B = n2 sin (90-B)
n1 sin B = n2 cos (B)
horiz. and vert.polarized
B
B
90º-B
90º
horiz.polarizedonly!
n1
n2
Physics 102: Lecture 18, Slide 10
ACT: Brewster’s Angle
When a polarizer is placed between the light sourceand the surface with transmission axis aligned asshown, the intensity of the reflected light:
(1) Increases(2) Unchanged(3) Decreases
T.A.
Physics 102: Lecture 18, Slide 11
Polarizing sunglasses are often considered to bebetter than tinted glasses because they…
Preflight 18.3, 18.4
•block more light
•block more glare
•are safer for your eyes
•are cheaper
Polarizing sunglasses (when worn by someonestanding up) work by absorbing light polarized inwhich direction?
•horizontal
•vertical
Physics 102: Lecture 18, Slide 12
Physics 102: Lecture 18, Slide 13
Dispersion
prism
White light
Blue light getsdeflected more
nblue > nred
The index of refraction n depends on color!
In glass: nblue = 1.53nred = 1.52
Physics 102: Lecture 18, Slide 14
Skier sees blue coming upfrom the bottom (1), andred coming down from thetop (2) of the rainbow.
Rainbow:
Preflight 18.5
Wow lookat thevariation inindex ofrefraction!
Which is red?
Which is blue?
Blue light is deflected more!
Physics 102: Lecture 18, Slide 15
LIKE SO!
In second rainbowpattern is reversed
Physics 102: Lecture 18, Slide 16
Physics 102: Lecture 18, Slide 17
Flat Lens (Window)
n1
n2
Incident ray is displaced, butits direction is not changed.
t
1
1
If 1 is not large, and if t issmall, the displacement,d, will be quite small.
d
Physics 102: Lecture 18, Slide 18
1) Rays parallel to principal axis pass through focal point.
2) Rays through center of lens are not refracted.
3) Rays through F emerge parallel to principal axis.
Assumptions:
• monochromatic light incident on a thin lens.
• rays are all “near” the principal axis.
Converging Lens Principal Rays
F
F
Object
P.A.
Image is: real, inverted and enlarged (in this case).
Image
Physics 102: Lecture 18, Slide 19
Converging Lens
All rays parallel to principal axis pass through focalpoint F.
Double Convex
P.A.
F
nlens > noutside
F
• At F
• Inside F
• Outside F
P.A.
F
F
Preflight 18.6
A beacon in a lighthouseproduces a parallel beam of light. Thebeacon consists of a bulb and aconverging lens. Where should the bulbbe placed?
Physics 102: Lecture 18, Slide 20
Physics 102: Lecture 18, Slide 21
Object
Image
This could be used ina camera. Big objecton small film
Inverted
Reduced
Real
Past 2F
Image
Object
This could be used asa projector. Smallslide on big screen
Inverted
Enlarged
Real
Between
F & 2F
Image
Object
This is a magnifyingglass
Upright
Enlarged
Virtual
Inside F
Physics 102: Lecture 18, Slide 22
ACT: Converging Lens
Which way should you move object so imageis real and diminished?
(1)Closer to lens
(2)Further from lens
(3)Converging lens can’t create realdiminished image.
F
F
Object
P.A.
Physics 102: Lecture 18, Slide 23
1) Rays parallel to principal axis pass through focal point.
2) Rays through center of lens are not refracted.
3) Rays toward F emerge parallel to principal axis.
Diverging Lens Principal Rays
F
F
Object
P.A.
Only 1 case for diverging lens:
Image is always virtual, upright, and reduced.
Image
Physics 102: Lecture 18, Slide 24
Physics 102: Lecture 18, Slide 25
Which way should you move object so imageis real?
1)Closer to lens
2)Further from lens
3)Diverging lens can’t create real image.
ACT: Diverging Lenses
F
F
Object
P.A.
Physics 102: Lecture 18, Slide 26