physics 1230: light and color - university of colorado … · 1.5 2 2.5 3 3.5 s score exam 2...
TRANSCRIPT
1
Physics 1230: Light and Color
Chuck Rogers, [email protected] Heinemann, [email protected]
www.colorado.edu/physics/phys1230
Projects
First list of proposed project topics and group
members is due tomorrow (Friday).
Exam 2 solutions are on D2L.
Good Job! 82+/-8 of 100 points
HWK 6 is due at 5PM today.
2
0
0.5
1
1.5
2
2.5
3
3.5
Nu
mb
er
of
Stu
den
ts
Score
Exam 2 HistogramAverage: 82+/-8 of 100 points
3
Physics 1230: Light and Color
Chuck Rogers, [email protected] Heinemann, [email protected]
www.colorado.edu/physics/phys1230
Lecture 9
Refraction and how light behaves at
boundaries between materials.
Where are we at?
Unit 1: What is light?Unit 2: Light as a waveUnit 3: Color in nature and technology (light sources and the spectrum)Unit 4: ReflectionUnit 5: RefractionUnit 6: LensesUnit 7: Eye and cameraUnit 8: Color perceptionUnit 9: Visual perception, illusion, artUnit 10: TBD
incident
Transparent material (e.g., water, glass)
What happens to light when it hits a boundary between materials?
Why does the light ray bend?
“Light slows down inside materials”: Light waves travel more slowly when they are inside stuff. That makes the light wave bend.
Here is one way to say it:
Index of refraction, n:
OR c c
n vv n
MaterialRefractive
Index
Air 1.0008
Water 1.330
Glass 1.5
Diamond 2.417
Ruby 1.760
But,… I thought light traveled at 3x108 m/s always!
That is the speed of light in VACUUM, where everyone always agrees on the speed! Inside stuff, light slows down (always slows down).
Index of refraction, n:
OR c c
n vv n
MaterialRefractive
Index
Air 1.0008
Water 1.330
Glass 1.5
Diamond 2.417
Ruby 1.760
A light beam goes from air (n = 1) to glass (n = 1.5) and is observed to bend
v c n
The speed of the light inside the glass is:
A) 3x108 m/s. Duh!
B) 4.5x108 m/s. Duh!
C) 2x108 m/s. Duh!
D) Something else (duh!).
A light beam goes from air (n = 1) to glass (n = 1.5) and is observed to bend
For a wave, we know that speed, frequency,
and wavelength are all related by: v fFrequency can’t be different inside glass, so
if the wave is slower, what happens to inside the glass compared to in the air?
A) Nothing, wavelength is fixed too
B) Must get larger
C) Must get smaller
Index of Refraction
If the wavelength in air is 600 nm, what will be the
wavelength in the glass? Assume the index of
refraction of glass is 1.5
A) 800 nm
B) 400 nm
C) 1200 nm
D) 200 nm
Index of Refraction
If the wavelength in air is 600 nm, what will be the
wavelength in the glass? Assume the index of
refraction of glass is 1.5
A) 800 nm
B) 400 nm
C) 1200 nm
D) 200 nmYep! Wavelength
got smaller.
Why does the light ray bend?
“Light slows down inside materials”: Light waves travel more slowly when they are inside stuff. That makes the light wave bend.
Why does the light ray bend?
Yeah, OK… but,
• Wavefronts illustrate peaks and troughs in wave
• Right part of the wavefront hits the medium first and is slowed down first.
• Causes the wave to bend.
Wavefronts bend when hit slower medium
slower medium
faster medium
Refraction…
… is the bending of light rays due to the slowing of light in a medium.
PhET bending light
θI
θII
I
II
In which medium is light traveling slower?A. Medium IB. Medium IIC. Light travels the same speed in both
θI
θII
I
II
In which medium is light traveling slower?A. Medium IB. Medium IIC. Light travels the same speed in both
θI
θII
I
II
In which medium is light traveling slower?A. Medium IB. Medium IIC. Light travels the same speed in both
θI
θII
I
II
In which medium is light traveling slower?A. Medium IB. Medium IIC. Light travels the same speed in both
Try sketching this: Draw refracted rays from bottom of pencil, towards the observer
Where does this ray appear to have come from?
The observer will see the underwater part of body being
a) Shorter than it really is;
b)Taller than it really is;
c) Of natural size;
Feet look like they’re here
The observer will see the underwater part of body being
a) Shorter than it really is;
b)Taller than it really is;
c) Of natural size;
38
Air
Water
Refracted
Reflectedinternally
Case 1near normal incidence (light comes out)
Case 2, far from normal incidence(internal reflection)
There is a critical angle at which this happens
Refraction out OR Total internal reflection!
Light coming out of water: 2 possibilities
θ>θcritical
I
II
Total Internal Reflection!
Law that allows quantitative understanding of all these angles:Snell’s Law – we won’t cover, involves ratios of Sine of angles
Feet look like they’re here
The observer will see the underwater part of body being
a) Shorter than it really is;
b)Taller than it really is;
c) Of natural size;
Feet look like they’re here
• If the critical angle condition is satisfied, will the snorkler see the upper part of the swimmer’s body?a) Yes;
b) No.
Feet look like they’re here
• If the critical angle condition is satisfied, will the snorkler see the upper part of the swimmer’s body?a) Yes;
b) No.
Legs up and down!
46
Total internal reflection makesfiber optic communication possible
In a high “n” material, light bounces around inside and doesn’t exit until the end of the fiber. Demo: lucite light pipe water pipe
Prisms demonstrate refraction and dispersion
48
Reflection at a transparent surface occurs because the n values are different. Only a few percent of the light is reflected this way.
Prisms demonstrate refraction and dispersion
49
Dispersion = different colors refracted at different angles.
Different colors bend at different angles. Why could this be?
A. Different colors travel at different speeds through the material
B. Different colors have different values of “n” in the material
C. Both A and B
D. None of the above
51
color n (index of refraction)
(blue) 1.523 (bent more)
(yellow) 1.517
(deep red) 1.514 (bent less)
Ordinary glass
Called “dispersion”
Both “n” and speed varies with color
52
180 degree rainbow is possible. Double rainbow (woah!) is possible.
Both together is very rare.
Rainbow: Dispersion via water droplets
Raindrop
Dispersion occurs
here during refraction
white lightcomes in
Reflections
Dispersion occurs
here during refraction
A spectrum ofcolors comes out
Rainbow: Colors spread within the raindrop
Raindrop
Dispersion occurs
here during refraction
white lightcomes in
Reflections
Dispersion occurs
here during refraction
A spectrum ofcolors comes out
Rainbow: Colors spread within the raindrop
58
big
raindrops
Sun
(behind you)
this ray not seen
this ray not seen
these rays are seen
How we see a rainbow
• Light travels in straight lines at speed “c”
• Unless it enters another material (in which case it changes direction and slows down)
• Light of different colors bends different amounts
So…
62
Physics 1230: Light and Color
Chuck Rogers, [email protected] Heinemann, [email protected]
www.colorado.edu/physics/phys1230
Projects
First list of proposed project topics and group
members is due Friday.
Exam 2 Good Job!