About this experiment
Send a light ray toward the boundary between two transparent materials. Adjust the angle and refractive indices to compare reflection, refraction and total internal reflection. Both angles are measured from the normal, the line perpendicular to the interface. Ray widths provide a schematic indication of power, while the numerical readouts give the calculated fractions.
Learning goals
- Apply the law of reflection and Snell law at a flat boundary
- Predict whether the transmitted ray bends toward or away from the normal
- Identify the critical angle and conditions for total internal reflection
How to use it
- 1Use air above glass and increase the incidence angle. Compare incident and refracted angles.
- 2Swap the media. Observe how the refracted ray moves away from the normal.
- 3Choose the total internal reflection preset, then lower the angle through the critical angle.
- 4Set both refractive indices equal. Check the ray direction and reflected power.
The model
Assumptions
- Both media are homogeneous, isotropic, nonmagnetic and transparent, with no absorption.
- The interface is flat and much larger than the wavelength. Diffraction and surface roughness are ignored.
- Refractive indices are real and independent of wavelength. Fresnel power fractions assume unpolarized light.
Try this in class
- Q1Why does no critical angle exist when light travels from air into glass?
- Q2For glass with index 1.50 into air, estimate the critical angle and test your estimate.
- Q3At normal incidence, does zero refraction angle imply zero reflected power?
Frequently asked questions
Why does the reflected ray remain when there is refraction?
At a boundary between different refractive indices, part of the incident power is generally reflected and part transmitted. The Fresnel equations determine those fractions.
What happens exactly at the critical angle?
The limiting transmitted direction lies along the interface at 90 degrees from the normal. For a lossless boundary, normal transmitted power tends to zero. Above the critical angle there is no propagating transmitted ray in this model.