Lesson focus: refractionWhen light changes direction as it passes from one material into another, because its speed changes. occurs when a wave changes speed at a boundaryThe place where two materials meet, such as where air meets glass. and therefore changes direction unless it travels along the normalAn imaginary line drawn at right angles (90°) to a surface where a ray of light hits it..
Predict and explain ray direction when light crosses air, glass or water boundaries.
Build on earlier learning
Investigating reflectionReflection changed direction while light remained in one medium. Refraction follows light across a boundary where speed changes.
You will be successful when you can…
- Define refraction as a change in direction caused by speed change.
- Use a normal to describe bending toward or away from it.
- Predict rays entering and leaving glass or water.
- Explain normal incidence and apparent depth.
Words that carry the science
- refraction
- A change in wave direction caused by a change in speed at a boundary.
- boundary
- The interface between two media.
- normal
- A line perpendicular to the boundary at the point where the ray crosses.
- angle of refraction
- The angle between the refracted ray and the normal.
- optical density
- A description related to how much light slows in a medium, not simply its mass density.
- normal incidence
- A ray meeting a boundary along the normal, with incidence angle zero.
Connect speed change to bending at a boundary
01Changing speed changes direction
Light slows when it enters glass or water from air. At a non-zero incidence angle, one side of the wavefront changes speed first, turning the ray. This change in direction is refractionWhen light changes direction as it passes from one material into another, because its speed changes..
Entering a slower optical medium bends the ray toward the normal. Entering a faster medium bends it away from the normal.
02The normal provides the reference
Angles are measured from the normalAn imaginary line drawn at right angles (90°) to a surface where a ray of light hits it., as in reflection. In a parallel-sided block, the ray bends toward the normalAn imaginary line drawn at right angles (90°) to a surface where a ray of light hits it. on entry and away on exit, emerging parallel to the original ray but shifted sideways.
At normal incidenceWhen a light ray hits a surface straight on, along the normal, so it does not bend., the ray slows or speeds up but does not bend because both sides of the wavefront cross together.
03Apparent position follows backward ray tracing
Light from an underwater object refracts away from the normal as it enters air. The eye traces the rays backward in straight lines, so the object appears shallower than its true position.
This is an apparent-position effect. The object has not moved, and the ray does not curve gradually in each uniform medium.
Refraction through a block
Light slows down as it enters glass or water, so it bends towards the normal. It speeds up as it leaves and bends away, coming out parallel to the ray that went in.
Controls
At 0° the ray goes straight through: it slows down but does not change direction.
Predict a ray through a glass block
A ray enters a parallel-sided glass block from air at 40° to the normal. Describe its path.
- Light slows on entering glass from air.
- The refracted ray bends toward the normal, so its angle in glass is less than 40°.
- At the second face it speeds up as it leaves glass for air and bends away from the normal.
- Because the faces are parallel, the emerging ray is parallel to the incident ray but laterally displaced.
How to make the reasoning visible: The ray changes direction at each boundary because its speed changes, while the parallel faces restore the original travel angle.
Predict four boundary crossings
Assume light travels slower in glass and water than in air.
| From | To | Speed change | Direction relative to normal |
|---|---|---|---|
| Air | Glass | Slows | Toward normal |
| Glass | Air | Speeds up | Away from normal |
| Air | Water | Slows | Toward normal |
| Water | Air along normal | Speeds up | No directional change |
Why does the final row not bend even though speed changes?
Explain why a coin under water appears shallower.
Check your understanding
Answer from memory, then check against the suggested answer.
What causes refraction?
How does light bend when it enters glass from air?
What happens to direction at normal incidence?
A ray leaves glass for air at a non-zero angle. Predict its speed and direction changes.
Explain why “light bends toward the denser material” is an unsafe general rule.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A ray of light enters a parallel-sided glass block from air and then leaves it.
Explain what happens to the ray as it enters the glass.
(2)Describe what happens as the ray leaves the block, and the direction of the final ray.
(2)Total for this question: 4 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., connect each bending direction to a named speed change.