A STUDY OF LIGHT
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
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.
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. ISAAC NEWTON (1643 – 1727) Corpuscular model of light 6. CHRISTIAAN HUYGENS (1629 – 1695) Wave model of light
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
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.
An example of a scientific model: E.g.. Bohr-Rutherford model of an atom vs. today’s understanding of an atom.
Properties of light Rectilinear Propagation Reflection – regular - diffuse
Refraction Partial Reflection
Snell’s Law n1sinθ1 = n2sinθ2 Increased distance α decreased Intensity Light has energy
Dispersion White light can be separated into it’s component colors by a prism
Models of Light Particle Wave Wave-Particle Duality Electro-Magnetic Wave
Particle (Corpuscular) Model Newton proposed that light consisted of streams of tiny particle (corpuscles) emitted like bullets from the light source)
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
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.
Using Particles to Explain Properties of Light
Snell’s Law
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
Particles of light are massless Consider: any black object. It will absorb light “particles” without increasing mass Therefore: light particles are virtually massless
Intensity of light = # of light particles/second if we expose a piece of photographic paper to a short burst of light we will see:
Is this true for particles? Consider: a spray can of paint
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:
Light Particles travel at very high speeds. Do real particles?
Light follows the inverse square law
Do particles follow the Inverse Square Law? Consider: Butter Gun
All colors travel at same speed in vacuum.
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?
Problems with the Particle Model Partial Reflection Newton said that the particles arrived in “fits” at the boundary between two media.
2. Snell’s Law
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
3. Light can be polarized If light is a particle then it should be able to pass through polarized plates