The wave model explains two things you can see. In reflection, wave fronts bounce off a surface and the angle of reflection equals the angle of incidence. In refraction, light slows down in a denser material such as glass or water, so the wave fronts bunch up and the direction bends towards the normal; light going straight in at 90° does not bend.
A straw in a glass of water looks snapped at the surface. The straw is fine. What is the light doing?
The last lesson said light is a wave. Here the wave model explains two things you can see every day.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: into the glass
ExploreTry it on the board and compare what happens with your prediction.
Explore: wave fronts at a boundary
The blue lines are the crests of the waves. Watch what happens to the gap between them as they enter the glass.
Try it yourselfYour edits are saved in this browser.
Set the angle of incidence to 60°. What is the angle of refraction?
Use the board. This ticks when you get there.
Set the angle to 0°, straight in. Does the ray bend?
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: reflection and refraction
In your book: draw a ray going from air into glass with the normal, the angle of incidence and the angle of refraction. Add three wave fronts in each, closer together in the glass, and write one sentence saying why the ray bends.
Elaborate TypicalUse the same idea in a new context.
Elaborate: mirror, glass, diamond
Try it yourselfYour edits are saved in this browser.
On the mirror, set 50°. What is the angle of reflection?
Use the board. This ticks when you get there.
Now choose Diamond at 60°. Then try Glass at the same angle.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: fibre optics
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Reflection: the angle of reflection equals the angle of incidence, both measured from the normal.
Refraction: light slows down in a denser material, so the wave fronts bunch up and the direction bends towards the normal. Going out, it bends away.
Light going straight in (along the normal) slows down but doesn't bend.
The more the light slows, the more it bends.
For teachers
Students use the wave model to explain reflection and refraction: wave fronts bounce at equal angles, and slow down and bend towards the normal in a denser material (AC9S9U04, node 4; builds on AC9S5U03). About 25 minutes.
I can say why light bends when it goes into glass.
I can say what happens to a ray that goes straight in.
I can use the law of reflection.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It follows What is the electromagnetic spectrum? and comes before Wave or particle?
Students met reflection and refraction as rules for rays in Year 5. Here the wave model gives the reason. The blue lines are wave fronts (crests); the orange arrows are rays, at right angles to the fronts.
Board notes: the wave fronts' spacing in the second material is the incident spacing divided by the refractive index (1.33 water, 1.5 glass, 2.4 diamond); the ray's angle follows Snell's law. The light is a single colour, so dispersion isn't shown. Partial reflection at the glass is drawn as a dotted line. The index and Snell's law are not named for students.
Running the lesson
Engage: tally the predictions. Ask the 'away' group which end of the front gets into the glass first.
Explore: ask what happened to the gap between fronts.
Explain: why it bends and the law of reflection are the success criteria.
In your own words: the labelled ray diagram with fronts.
The hard part (on purpose)
Connecting a change of speed to a change of direction. When a student says 'the glass bends it', ask what happens to one end of the wave front when it slows down first.
When students get stuck
A student measures the angle from the surface. Ask: Which line are both angles measured from on the board? Fix: The normal, the dashed line at right angles to the surface.
A student says light speeds up in glass. Ask: Are the fronts closer together or further apart in the glass? Fix: Closer: the waves are slower there.
A student says a ray along the normal bends. Ask: Which end of the front enters the glass first? Fix: Neither: the whole front enters together, so it slows without turning.
Stretch and support
Stretch: Water bends light less than glass. Is light faster or slower in water than in glass? (Faster: water slows it down less.)
Support: Give a marching-band picture: a line of people walks from pavement onto sand at an angle and slow down one by one. Students draw the line turning.
Where it leads
Next: Wave or particle? weighs up the wave and particle models.
In class: a laser pointer through a tank of water, a clear block or a semicircle; a coin in a cup that appears when water is poured in; a mirror and a ruler for the law of reflection.
What each step is for
Engage: into the glass: Students guess the ray bends away, or by a rule they can't justify. Slowing down in the denser material, and the wave fronts changing direction as a result, is the explanation the wave model gives.
Explore: wave fronts at a boundary: Seeing the wave fronts, not just the ray, shows why the bend happens: the fronts are closer together in the glass, so the direction must change.
Explain: reflection and refraction: Applying the wave model to refraction, then the law of reflection, then the everyday straw, meets the node's second success criterion.
Elaborate: mirror, glass, diamond: Comparing a mirror with glass, then glass with diamond, shows reflection as a bounce, refraction as a bend, and the amount of bending as the change in speed.
Elaborate: fibre optics: Total internal reflection is the limit of refraction; fibre optics shows how an idea from the wave model carries the internet.