Optics: Ray diagrams

Links and useful resources

Lightning Round Questions

What did Muhammad Ali Pasha do in 19th century Egypt (not the boxer)? (industrialized and modernized many parts of the nation).
What are zaibatsu? (powerful Japanese family business organizations that controlled whole sections of industry)
What shape do Kepler's laws specify for planetary orbits?
What is the formula for the gravitational force of attraction between any two masses?
What is a hypothesis?
What is the density of water?
What is the acceleration due to gravity on the moon as a fraction of Earth's?

gr7: [lightning:: 3]
gr10: [lightning:: 1]

Lesson content with examples

Light rays are not real, they're just a useful model!

A light ray is an imaginary thing that represents how light seems to work in everyday life:

It travels in straight lines when it's in a single medium
It continues forever unless stopped
It reflects at predictable angles.

When you use the ray model of light to figure out what optical systems will do, the objects in the system are the sources for the rays. You'll pick a point on the object and trace various rays through the system to see what happens.

When the rays from one point all converge on another point, an image will form. This is a real image, meaning that if you put a screen there, you'd see a picture of the scene projected onto it.
When the rays of light seem to come from a point that isn't really there, you get a virtual image. Putting a screen there would show nothing special, but if you look through the system, it will seem like the scene is reproduced out where the rays converge.
Images are created by reflections and refractions of light rays.
Your eye sees an image when diverging rays from an object are focused onto a real image on your retina by the lens in your eye.

By tracing the rays through the system and finding the real and virtual images, you can figure out things like magnification, inversion, and other stuff that happens in the system. This is how you design things like telescopes and microscopes.

Pinhole cameras

A pinhole camera is just about the simplest interesting optical device. The idea is simple: you make a hole so small that it restricts all of the light rays coming through to basically a single point. That means that, no matter what angle or distance, the light rays that pass through at a given angle all came from the same place. That means that light passing through a pinhole will always make a real image on a screen behind it.

This is what we did with the small mirror during the eclipse, and it's why all of the light spots through the leaves in the bamboo patch during the eclipse were mini-pictures of the eclipse: they were all pinhole cameras!

Media resources

Youtube search for "Rays travel in straight lines forever."

Guided practice

Draw the light rays passing through a pinhole camera. Why does it always form an image? Why does a larger hole not form an image like that?
Light hits a mirror at a 30-degree angle. What angle does the reflected light make with the mirror's surface?
Two people are standing 10 feet apart with their backs to the same wall. A mirror is mounted to the opposite wall so that each person can see the other person in the mirror. Where is the mirror relative to the people?

Homework

#hw (phys) Three question on the lesson page about rays 2025-03-26

You made a pinhole camera with a 15-cm long chamber. You want to take a photo of your friend, who is 2 m tall. How far should they be from the aperture of the camera in order to make a 5 cm image?
In treasure hunter movies, sometimes the heros enter a dark chamber where the treasure is, and they angle a mirror that suddenly hits a dozen other mirrors and lights the entire space up like it's daylight. Why is this ridiculous?
Why is it silly to think that you'd see a laser beam shining in space if you aren't in its direct path (like the laser guns in Star Wars, for example)?