WebLab STEM

Physics · 4. Waves and optics

Refraction and Snell’s law

Measure incidence and refraction angles and determine refractive index from evidence.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

Can a transparent material be identified from the path of a light ray?

Curriculum coverage

Vietnam 2018 and Cambridge: refraction, refractive index and total internal reflection.

Model scope and limitations

Transparent uniform media; dispersion is ignored unless wavelength is explicitly selected.

Learning objectives

  • Apply n₁sinθ₁ = n₂sinθ₂.
  • Measure angles from the normal, not the boundary.
  • Use a sinθ₁–sinθ₂ graph to determine refractive index.

Prerequisite knowledge

  • Sine and angle measurement
  • Incident ray, normal and refracted ray

Core knowledge

Snell’s law

n₁sinθ₁ = n₂sinθ₂, with every angle measured from the normal.

Direction of bending

Light bends towards the normal when entering a higher-index medium.

Total internal reflection

It occurs only from higher to lower index when incidence exceeds the critical angle.

Worked example

From air into glass of n = 1.50 at 30°: sin r = sin30°/1.50 = 0.333, so r ≈ 19.5°.

Misconceptions to avoid

  • Incidence and refraction angles are measured from the normal, not the boundary.
  • Total internal reflection is possible only from higher to lower refractive index.

Virtual experiment procedure: Refraction and lenses

  1. Choose two media and record their indices.
  2. Vary incidence from 10° to 60° and record refraction each time.
  3. Calculate the sine values and test their ratio or graph gradient.

Safety and cautions

  • Never look directly into a laser in a real lab.
  • The ray model does not fully represent intensity and scattering.

Evidence to collect

  • A table of i, r, sin i and sin r for at least five incidence angles.
  • A sin i versus sin r graph and the refractive index inferred from its gradient.
Open the virtual experiment

Diagnostic check

1. The angle of incidence is measured between the ray and which line?

  1. The normal
  2. The boundary
  3. The screen horizontal
Show answer and explanation

Correct answer: The normal

All Snell-law angles are measured from the normal.

2. Light entering glass from air usually bends in which direction?

  1. Towards the normal
  2. Away from the normal
  3. Always straight
Show answer and explanation

Correct answer: Towards the normal

Glass has a higher index than air, so the ray bends towards the normal.

3. Which quantity is dimensionless?

  1. Refractive index
  2. Wavelength
  3. Speed of light
Show answer and explanation

Correct answer: Refractive index

Refractive index is a ratio of speeds and has no unit.

Application practice

1. From air n₁ = 1.00 into glass n₂ = 1.50 at θ₁ = 30°. What is sinθ₂?

  1. 0.333
  2. 0.750
  3. 1.000
Show answer and explanation

Correct answer: 0.333

sinθ₂ = 1.00 × 0.5 / 1.50 ≈ 0.333.

2. What is required for total internal reflection?

  1. Higher to lower n and incidence above critical angle
  2. Lower to higher n
  3. Any normally incident ray
Show answer and explanation

Correct answer: Higher to lower n and incidence above critical angle

Both propagation direction and incidence angle must satisfy the conditions.

3. A plot of sinθ₁ against sinθ₂ is a straight line through the origin. What does this show?

  1. The quantities are proportional as predicted by Snell
  2. Index changes with angle
  3. The angles must be equal
Show answer and explanation

Correct answer: The quantities are proportional as predicted by Snell

For two fixed media, sinθ₁/sinθ₂ is constant.

Academic references

  1. OpenStax Physics — peer-reviewed high-school physics
  2. BIPM — SI Brochure, 9th edition

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer