Today’s Topic: The Law of Reflection Learning Goal:
Students will be able to explain the laws that govern the reflection of light.
Please take out your notebooks and something to write with, as we are taking traditional notes today. Homework
Complete The Law of Reflection Worksheet (Due Tuesday 5/26)
Complete the Doppler Effect Worksheet (Three Days Late) Reflection What is the definition of a reflection in terms of Physics?
When a wave reaches a boundary between two media, usually some or all of the wave bounces back into the first medium. Wave Boundary (wall)
Reflection Reflection
Why is light “weird”? Because it behave with wave-like and particle-like properties.
As a result, light will reflect when it hits the boundary between two media.
When light hits a reflective surface, (like metal or a mirror) the reflected wave has almost the full intensity of the incoming wave. Reflection Let’s think about objects bouncing.
This tennis ball represents a photon/waves when they reflect off of an object.
When the object travels in one direction, how do we expect the object to reflect? Reflection How will the ball reflect when moving in two directions?
Using these tennis balls, I would like you to test your prediction.
Does your prediction match? What happens to the reflection angle if you change the angle in which you toss the ball? The Law of Reflection The direction of light movement can be thought of as a straight line – or rays.
If you create a straight line perpendicular from a reflecting surface, it is called the “normal”. Normal
Mirror The Law of Reflection The ball you just tossed can be thought of as the “incident ray” and you have tossed it with a certain angle of incidence from the normal.
Incident ray Angle of Normal incidence
Mirror The Law of Reflection How does this angle you toss it compare to the angle the ball makes when it bounces up from the table? They’re equal!
Incident ray Reflected ray Angle of Angle of reflection incidence
Mirror The Law of Reflection The Law of Reflection states that the angle of incidence and the angle of reflection are equal to each other.
Incident ray Reflected ray Angle of Angle of reflection incidence
Mirror The Law of Reflection Angle of Incidence = Angle of Reflection
θi = θr
Incident ray Reflected ray Angle of Angle of reflection incidence
Mirror Specular Reflection When we have more than one light ray striking a flat reflective surface, we get the following situation:
This is called Specular Reflection.
A clear reflection is produced, because all of the light is hitting one spot.
Other Reflection However, this is not very realistic, as most surfaces – up close – are not flat.
Realistically, surfaces look more like this.
However, the Law of Reflection still holds true for incoming waves!
Other Reflection
How is this different than specular reflection?
The reflected waves are all over the place!
This is called diffuse reflection.
Diffuse Reflection
Although each ray obeys the law of reflection, the many angles of the surface cause the incident light rays to reflect in many different directions.
Diffuse Reflection
Here are two pieces of glass.
Which piece will produce normal reflections and which will produce diffuse reflection?
Diffuse Reflection All rough objects diffuse light. Even smooth objects, like a piece of paper, diffuse light.
Under a microscope, paper looks like that. Now you can understand why light rays will diffuse when they contact that. Diffusion Uses Identify what common object in your house diffuses light.
Hint: it is located very close to light sources in your house.
Lampshades! Wet/Dry Asphalt at Night Another instance of diffuse reflection is seen at night after rain has fallen.
Normally, light will diffuse when a car’s headlights hit the asphalt: Wet/Dry Asphalt at Night After it rains, however, light will bounce off the wet roadway and cause specular reflection.