Demo Equipment

Pinhole
- LED light source, pinhole

Notes: ray optics / diagrams

Rays represent the path of light.
As long as they travel through a homogenous, clear substance, they are straight.
If they hit an opaque substance, they are absorbed, or reflect.
If they hit a boundary with a second clear substance, they bend (refract) AND reflect.

Analogy

Imagine a laser which can fire a very brief burst of light.
Small bundle of light travels in a straight line at constant speed.
... forever, until the substance it travels through changes.
A ray is the path of the bundle.

Reflection

Specular - mirror reflection.
A single ray, pointed at a mirror, reflects to a single outgoing ray.
Diffuse - ordinary reflection.
A single ray, pointed at an ordinary object, reflects in many directions.

Ray diagram

Goal is to describe behavior of rays, or draw rays which make it to an observer in order to explain what they see.
Rays start at a light source or object of interest, and are able to travel in any direction from every point on the source.
Screens = opaque, diffuse reflectors.
Observer - only rays that reach the observer are seen.

Diagram ray behaviors

Ray traveling through clear substance (air)
Rays reaching gray surface - some terminate, some reflect in all directions.
Rays reaching glass surface - some reflect, some refract.

Demo: patterned light source

I have a patterned light source.
If I shine it at the wall, what will be seen?

Observation

There is a patternless spot of light on the wall.

Explanation - why they see pattern

Diagram: Light source, observer, wall
Start several rays from top of light source.
Emphasize the single ray which makes it to the observer.
Repeat for middle, bottom.

Explanation - why the wall reflection is patternless

Diagram: Light source, wall, observer.
Rays the observer sees should come from the wall.
Start several rays from top → wall. Emphasize the couple which reach the observer.
Repeat for middle and bottom.
Note: Every point on the wall receives SOME light from every point on the source.
This is what removes the pattern.

Worksheet: Pinholes

Draw rays which start at the light source and reach the observer.
Use the rays reaching the observer to describe what they see.

1 - observer facing pinhole

Only rays which start on the portion of the light source behind the pinhole make it to the observer.
Observer only sees rays from one direction → small, bright point of light.

2 - observer facing wall

Some rays from each point on the light source reach the wall.
Each of the rays reaching the wall reflects in every direction.
One of each of the reflected rays reaches the observer.
All rays cross at the pinhole.
Observer sees the pattern on the wall, but it is upside down.