Lab 11: Snell's Law and Refractive Index

Learning Objectives

  • Verify Snell's Law using incident and refracted angle measurements.
  • Determine the refractive index of a glass or acrylic block.
  • Trace light rays accurately and reduce angular measurement error.
  • Interpret the relationship between light speed, refractive index, and bending of light.

This placeholder is prepared for a complete optics laboratory experiment. It can be adapted for a ray box, laser pointer, semicircular acrylic block, rectangular glass block, protractor, or simulation-based setup.

Snell's Law

Light changes direction when it passes between media with different refractive indices.

n1sin⁡θ1=n2sin⁡θ2n_1\sin\theta_1 = n_2\sin\theta_2

Variables

SymbolDescriptionUnit
n1n_1refractive index of first mediumdimensionless
n2n_2refractive index of second mediumdimensionless
θ1\theta_1angle of incidence measured from the normaldegrees
θ2\theta_2angle of refraction measured from the normaldegrees

Refractive index from measured angles

For light entering a material from air, this approximation is commonly used when air has refractive index close to 1.

n≈sin⁡θisin⁡θrn \approx \frac{\sin\theta_i}{\sin\theta_r}

Suggested Apparatus

ApparatusPurpose
Ray box or laser pointerProduces a narrow light beam.
Glass or acrylic blockTransparent material being tested.
White paperSurface for ray tracing.
ProtractorMeasures incident and refracted angles.
Ruler and pencilDraws normals and ray paths.

Placeholder procedure outline

  1. Place the transparent block on white paper and trace its outline.
  2. Draw a normal line at the point of incidence.
  3. Direct the light ray into the block at a chosen incident angle.
  4. Mark the incoming and outgoing ray paths.
  5. Remove the block and draw the refracted path inside the block.
  6. Measure the incident and refracted angles from the normal.
  7. Repeat for several incident angles.
  8. Compute refractive index and compare trials.

Data Table Placeholder

TrialIncident angle, degreesRefracted angle, degreessin⁡θi\sin\theta_isin⁡θr\sin\theta_rRefractive index
1
2
3
4

Laser safety

If a laser pointer is used, never point it at anyone's eyes or at reflective surfaces that can redirect the beam toward people.

To complete this lab

Add ray-tracing diagrams, graphing instructions for sin⁡θi\sin\theta_i versus sin⁡θr\sin\theta_r, sample calculation, and post-lab questions about total internal reflection.

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