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Physics 1230: Light and Color
Chuck Rogers, [email protected] Henley, Valyria McFarland, Peter Siegfried
physicscourses.colorado.edu/phys1230
Congratulations on completing Exam 1. Great job!
Exam 1 Average: 76 +/- 12 of 100 points
Group Exam Average: 86 +/- 10 of 100 points.
Come down and collect your Exam 1.
Get a diffraction grating if needed.
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Physics 1230: Light and Color
Chuck Rogers, [email protected] Henley, Valyria McFarland, Peter Siegfried
physicscourses.colorado.edu/phys1230
Exam 1 Questions??
1. Write your question on a separate page.
2. Attach to otherwise untouched exam.
3. Bring it to Prof. Rogers and I will have a look.
4. Try to ask questions by Thurs. next week.
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Physics 1230: Light and Color
Chuck Rogers, [email protected] Henley, Valyria McFarland, Peter Siegfried
physicscourses.colorado.edu/phys1230
Lecture 11:
Atoms and light: Luminescence.
Start discussing about Reflection and Images
Last Time: There are two main ways to make light
1. Incandescence: Heat an object up, and it glows. (“Blackbody radiation”)
2. Luminescence: For example discharge lamp
Last Time: IncandescenceHot things glow = incandescence
• Current runs through the filament
• This makes the filament hot
• Electrons jiggle in hot things emit light
PhET radio waves
Heating objects causes their
electrons to jiggle and they emit
light.
Last Time:
Wien’s Law:
Brightest color wavelength multiplied by
the absolute temperature is a constant.
32.9 10 meters KelvinbrightestT
Tell me the brightest color of a hot object
and I can tell you how hot it is!
• Wien’s Law tells about brightest wavelength.
But other waves are created too.
• Hot objects radiate at all wavelengths.
Last Time: Key points about atoms
• Electrons are “bound” to atoms
• There are only certain amounts of energy the electrons can have (energy levels)
• The more energy the electron has, the further away it is from nucleus (higher energy level)
8These are both simplified models of atom
Atoms can absorb energy (e.g., light)
• Then, the electron moves up an energy level
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+
E = hfPhoton energy is proportional
to frequency of the light.
We say a PHOTON is absorbed.
Atoms can emit energy (e.g. light)
Spacing between energy levels leads to the color emitted: E = hf
One color (f) only
We say a PHOTON is emitted
Which type of light has the highest energy per photon?
A. Red light
B. Blue light
C. X-ray light
D. Radio
E = hf
Another example of luminescence:
Lasers:Light Amplification by Stimulated Emission
You don’t have to know how they work.
But very important part of modern technology.In the category of luminescence.
Where are we in the course?
Unit 1: What is light?
Unit 2: Light as a wave – finishing polarization
Unit 3: Light sources and spectra
Unit 4: Reflection
Unit 5: Refraction
Unit 6: Lenses
Unit 7: Eye and camera
Unit 8: Color perception
Unit 9: Visual perception, illusion, art
Unit 10: TBD
Breathe in… Breathe out.
• Light travels in straight lines from a source
• It travels in all directions from each spot on an extended source
• We can predict all sorts of useful phenomena by drawing “light ray diagrams”: Pinhole camera, Shadows in sun
Recall: Light as a ray
What if light interacts with a material – reflects off,
transmits through – how do we extend ray ideas?
What can happen to incoming light
Or any partial combination of these things
Glass
Silver, water
Black pavement
Important parameters:• Frequency of light• Nature of material
Equal angle rule
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qrqi
Normal
Mirror
qi = angle of incidence
qr = angle of reflection
qi = qr is specular reflection
A normal is a line
perpendicular to
the surface.
Incident RayReflected Ray
Light on mirror versus white card
Flashlight shining at mirror
Flashlight shining at white paper card
Does light still obey the law of reflection in the case of the white paper card??
Specular or diffuse?
Diffuse reflection
(paper)
Specular reflection
(mirror)
Diffuse transmission
(wax paper)
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Law of reflection?
Reflection can be “Specular” (mirror-like)
But most things reflect diffuselyStill follows law of reflection at each little interface
Law of reflection?
Which best describes the images: A) smooth water is specular and wavy water is diffuseB) wavy water is specular and smooth water is diffuse
Activities: How do we use law of reflection to understand mirrors?
Get together with a few group members, Delegates get activity sheets (one per member),mirror, etc.
Activities: How do we use law of reflection to understand mirrors?
Part 1: Where is the image?Click “A” when you have finished page 1.
The eye/brain interprets these rays as?
Images
A) A white wallB) A single white lineC) A point of lightD) Something else
There are some light rays. Some reach the eye.