Grade 8 · Waves, Light and Sound · Wave Behaviour
Light: Reflection and Refraction
Why light bounces, bends and makes a straw look broken.
Learning objectives
- State the law of reflection.
- Explain refraction using a change in speed.
- Draw ray diagrams for reflection and refraction.
NGSS alignment
HS-PS4-3
Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model.
MS-PS4-2
Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.
MYP criteria
- Criterion A — Knowing and Understanding
- Criterion B — Inquiring and Designing
ENGAGE
The broken straw
A straight straw in a glass of water appears to bend sharply at the water surface.
Think about it
- Is the straw actually bent?
- What happens to light as it crosses the surface?
EXPLAIN
Bouncing and bending
Law of reflection
The angle of incidence equals the angle of reflection, both measured from the normal — a line drawn at 90 degrees to the surface.
Refraction happens because light changes speed when it enters a new material. Light slows entering glass or water, so the ray bends towards the normal. Leaving into air it speeds up and bends away from the normal.
| Situation | Effect on the ray |
|---|---|
| Air into water or glass | Slows, bends towards the normal |
| Water or glass into air | Speeds up, bends away from the normal |
| Entering along the normal | Speed changes, direction does not |
- Smooth surfaces give specular reflection and a clear image
- Rough surfaces scatter light in all directions, called diffuse reflection
- Refraction explains lenses, spectacles and why a pool looks shallower than it is
Common misconception
Light does not bend because water is heavier. It bends because its speed changes at the boundary.
INVESTIGATION
Testing the law of reflection
research question
Is the angle of reflection always equal to the angle of incidence?
hypothesis
The two angles will be equal for every angle tested, within measurement uncertainty.
variables
Independent: angle of incidence (10 to 70 degrees in steps of 10). Dependent: angle of reflection. Controlled: same mirror, same ray box and slit, same paper.
method
Draw a normal on paper and place a plane mirror on the line. Direct a single ray at each chosen angle, mark the incoming and outgoing rays, then measure both angles with a protractor.
safety
Do not look directly into the ray box. It becomes hot.
analysis
Tabulate the angles, calculate the difference for each pair, and discuss whether differences are within your measurement uncertainty of plus or minus 1 degree.
Watch
Geometric Optics: Crash Course Physics
CrashCourse · 10 min
Ray diagrams for reflection and refraction.
Before you watch: Why does a straw look bent in water?
- State the law of reflection.
- Which way does a ray bend entering glass?
Interactive simulation · PhET
Bending Light
Shine a ray from air into water and use the protractor to measure both angles.
While you explore
- Which way does the ray bend entering water?
- What happens when the ray travels along the normal?
Key vocabulary
- Normal
- An imaginary line at right angles to a surface.
- Refraction
- Light bending as it enters a new material.
Practice questions
0/2 correct
Level 1 · Criterion A
A ray hits a mirror at 30 degrees to the normal. The angle of reflection is:
Level 2 · Criterion A
Light entering water from air bends towards the normal because it:
MYP criterion tasks
Level 3 · Criterion B
Design an investigation to find whether the angle of refraction depends on the angle of incidence for a glass block.
Reflect & track
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