What is light?

Links and useful resources

Lesson-specific resource links

Lightning Round Questions

Design a voltage divider that will give you 3.5v from a 5v source.
What was the Meiji restoration? (A rapid Japanese move to industrialize in order to protect their culture)
Draw a right triangle and label one of the right angles as $θ$ . Which two sides make the ratio called $\cos θ$ ?
Give an example of an equation with infinite solutions.
What is the formula for the gravitational force of attraction between any two masses?
Give an example of a physical model, a computer model, and a mathematical model.

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

Demonstration

Green laser
Red laser
Aluminum foil
sharp needle
sharp razor blade

Make a needle-width hole in the foil and shine the lasers through. Record what you see.
Make a tiny pinhole in the foil and shine the lasers through it one at a time. Has anything changed? Is this what you would expect to see?
Make an extremely fine slice in the foil and do the same experiment.
Make two extremely fine, parallel slices in the foil. How has the pattern changed?

Concept summary and connections

Light as a wave
Light as a particle
Light as a ray
water-tub experiments
diffraction
interference
wavelength and color (visible light spectrum)

Light is a complex physical entity that's difficult to explain, because it seems to behave as a completely different kind of thing when you change the circumstances of the experiment. At the scale of things we interact with from day to day, you can think of light as a "ray" - it follows perfectly straight paths and casts sharp shadows just like a Hollywood laser gun. However, when you start making the features smaller, you suddenly get to a point where the behavior completely changes and it acts like ripples in a three-dimensional pond! It no longer follows straight paths, but wraps around things and spreads in every direction. Finally, if you turn the intensity WAAAAY down, the light comes out in single pieces! If it was just a wave, you'd expect the brightness of the patterns it makes to just get gradually dimmer as you reduce the intensity. What actually happens is that you start to see that what looked like a wave was actually a spray of particles, but each particle acts like a wave! The single "photons" interact with the slit experiments just as if they knew whether or not there is a second slit, even though each particle only goes through one slit or the other!

To really start to understand this, we'd have to learn a lot about quantum mechanics, which is outside of the wheelhouse for this course. We'll be focusing on the ray and wave models, mostly because they are things we can actually do our own experiments with. Just know that at the bottom, it's weirder than you thought.

Lesson content with examples

Diffraction: Waves that approach a slit or hole that's smaller than the wavelength will spread out in a circular arc as they pass through the gap.
Interference: When two or more waves interact with each other, there are places where the total energy is higher (constructive), and other places where they cancel each other out (destructive). This is called interference, and with light it shows up as "fringes" in a diffraction pattern.
- The places of constructive interference depend on the path length difference of the two waves: $Δ r = m λ$ , where $m = {0, 1, 2, \dots}$ . In other words, where the difference in path length is equal to a multiple of the wavelength of the light, you'll get a bright spot where they are both at their maximum.
- Destructive interference also depends on the path length, but with a half-wave added so that the peaks and troughs align: $Δ r = (m + \frac{1}{2}) λ$ , where $m = {0, 1, 2, \dots}$ . When the path difference is equal to the a multiple of the wavelength plus an extra half wave, they cancel each other out.
The spacing of the fringes depends on the spacing of the slits and the wavelength of the light: $y_{m} = \frac{m λ L}{d}$ where $m = {0, 1, 2, \dots}$ , d is the slit separation, and L is the distance from the slits to the viewing screen. So larger wavelength and screen distance increases the fringe spacing, as does smaller distance between slits.

Media resources

Guided practice

If you have a green laser and a red laser, which one will have the most space between fringes in a double-slit experiment?
What is "white light"?
Why do you think humans have visual receptors for the colors we see?

Homework

#hw (physics) (in class) Make a foil pinhole, single slit, and double slit experimental setup and investigate what green and red lasers do when shined through them. 2025-03-21