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Physics 1230: Light and Color

Chuck Rogers, Charles.Rogers@colorado.eduRyan 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, Charles.Rogers@colorado.eduRyan 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, Charles.Rogers@colorado.eduRyan 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

Gas discharges!

Which type of light has the highest energy per photon?

A. Red light

B. Blue light

C. X-ray light

D. Radio

E = hf

High frequency light photons have higher energy

E = hf

High ELow E

This is why we limit our exposure to UV and X-rays

High frequency light has higher energy

The latest in lighting: LED

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

Basic reflection: Light on mirror

Flashlight shining at mirror

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.

Good place to stop!