Refraction of light
Why can light change direction when it enters another medium?
Use an interface, normal, angles, and refractive index to predict how light bends, then explore total internal reflection.
Learning goals
- Measure angles of incidence and refraction from the normal.
- Predict bending toward or away from the normal using refractive indices.
- State the direction and angle conditions for total internal reflection.
Before you begin
- Read angles and recognise perpendicular lines.
- Know that rays can represent light travelling through a uniform medium.
Interactive science experiment
- 1Interface
- 2Direction
- 3Refractive index
- 4Total internal reflection
Preparing the interactive model…
Qualitative teaching model: values and diagrams compare trends; they are not precision measurements.
Observe, then explain
Work through four steps. Each button moves the interactive model to the matching observation state.
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The interface separates two media. The normal is perpendicular to it at the point of incidence. Measure every ray angle from the normal, not the interface.
Your task: Identify the interface, point of incidence, and normal; then set incidence to 0°.
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Light bends toward the normal on entering a higher-index medium and away on entering a lower-index medium. Along the normal, its direction does not change.
Your task: Predict the direction from air into glass, then check it in the model.
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Snell’s law, n₁sinθ₁ = n₂sinθ₂, connects the two indices and the two angles; each θ is measured from the normal.
Your task: Change incidence and record refraction; check whether the angles differ by a fixed amount.
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From a higher- to a lower-index medium, incidence beyond the critical angle produces no transmitted refracted ray: total internal reflection.
Your task: Reverse the medium direction and increase the angle to find where the refracted ray just disappears.
Build an explanation
A speed change causes the turn
Light changes speed at an interface. At oblique incidence, one side of a wavefront changes speed first, turning the direction. Source-set frequency stays constant while wavelength changes with speed.
Be precise about “denser”
Refractive index describes light propagation and is not the same as mass density. Predict bending by comparing indices, not by whether a material feels heavy.
Check your understanding
Printable worksheet
Complete the worksheet first. Answers and explanations are on a separate page.
Physics
Refraction of light
Why can light change direction when it enters another medium?
- From which line is the angle of incidence measured?
- Light enters higher-index glass obliquely from air. How does it usually bend?
- When can total internal reflection occur?
Answer page
Refraction of light
- The normal
The angle of incidence lies between the incident ray and the normal at the point of incidence.
- Toward the normal
In the higher-index, lower-speed medium, the refracted angle is smaller than the incident angle.
- From high to low index beyond the critical angle
Total internal reflection requires travel from higher to lower index and incidence above the critical angle.