1. A STUDY OF LIGHT

2. TOPICS History What is a scientific model
Why use a model to study light
Bases of models of light
What models are used
Particle model
Using the particle model
Direct evidence
Problems

3. History PYTHAGORAS (572BC – 490 BC) EMPEDOCLES (490 BC – 430 BC)
Believed that light traveled
as a stream of fast-moving
particles.
EMPEDOCLES
(490 BC – 430 BC)
Believed light traveled as
a wave-like disturbance.

4. PLATO
(424 BC – 346 BC)
Light emitted by “streamers” from the eyes.
4. EUCLID
(323 BC–283 BC)
Agreed with Plato
“How else can we explain that we do not see a needle on the floor until our eyes fall upon it?”

5. 5. ISAAC NEWTON
(1643 – 1727)
Corpuscular model of light
6. CHRISTIAAN HUYGENS
(1629 – 1695)
Wave model of light

6. What is a scientific model?
An educated guess about the structure of something.
- Based on known properties
- used to explain known properties in simple terms
-used to predict behaviors
- believed to be true and added/adjusted as new information becomes available

7. Why use a model to study light
Difficult to study since it travels at such a high speed.
Can only be noticed if it reflects off of something in its way.

8. An example of a scientific model:
E.g.. Bohr-Rutherford model of an atom
vs.
today’s understanding of an atom.

9. Properties of light Rectilinear Propagation Reflection – regular
- diffuse

10. Refraction
Partial Reflection

11. Snell’s Law
n1sinθ1 = n2sinθ2
Increased distance α decreased Intensity
Light has energy

12. Dispersion
White light can be separated into it’s component colors by a prism

14. Models of Light Particle Wave Wave-Particle Duality
Electro-Magnetic Wave

15. Particle (Corpuscular) Model
Newton proposed that light consisted of streams of tiny particle (corpuscles) emitted like bullets from the light source)

16. 1. Light of the same color is made up of identical particles
2. Light travels in straight lines (rectilinear propagation)
3. Particles of light are massless
4. Particles travel at very high speeds
5. Particles of light carry kinetic energy
6. Light spreads out in uniform distribution from the source
7.Particles of light experience perfectly elastic collisions

17. 7. Intensity of light = #particles/second
8. Light follows the inverse square law.
9. Speed of light is determined by – medium
- color
10. All colors travel at same speed in vacuum.
11. Different colors carry different amounts of energy.
12. Particles travel at increased speed in denser medium.

18. Using Particles to Explain Properties of Light

19. Snell’s Law

20. Light travels in straight lines Do Particles? (rectilinear propagation)
Consider: Throwing a ball to the ground straight down and it bounces back straight up.
Therefore: “particles” also travel in straight lines just like light

21. Particles of light are massless
Consider: any black object. It will absorb light “particles” without increasing mass
Therefore: light particles are virtually massless

22. Intensity of light = # of light particles/second
if we expose a piece of photographic paper to a short burst of light we will see:

23. Is this true for particles? Consider: a spray can of paint

24. If we spray just a short
burst, we get just a few
spots on the screen:
The longer we spray,
the more spots appear
until the whole area is
covered in paint:

25. Light Particles travel at very high speeds. Do real particles?

26. Light follows the inverse square law

27. Do particles follow the Inverse Square Law?
Consider: Butter Gun
                                                                                                             

28. All colors travel at same speed in vacuum.

29. Particles exert a force. Does Light exert a force?
Consider: a comet’s tail always points away from the sun.
Can we explain this in terms of light particles?

30. Problems with the Particle Model
Partial Reflection
Newton said that the particles arrived in “fits” at the boundary between two media.

31. 2. Snell’s Law

32. Particle Model predicts that the speed of light in water is 4
Particle Model predicts that the speed of light in water is 4.0 x 10 8 m/s
Armand Fizeau (1862) measured speed of light in water = 2.23 x 10 8 m/s

33. 3. Light can be polarized
If light is a particle then it should be able to pass through polarized plates