Critical Angle

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In this optics activity, learners examine how a transparent material such as glass or water can actually reflect light better than any mirror. Learners discover that all they need to do is look at it from the proper angle, which is known as the critical angle. Use this activity to introduce learners to other key vocabulary and principles related to optics including reflection, refraction, the incident beam, and total internal reflection. This activity also explains how total internal reflection helps transmit telephone messages along optical fibers.

Quick Guide

Preparation Time:
10 to 30 minutes

Learning Time:
10 to 30 minutes

Estimated Materials Cost:
1 cent - $1 per group of students

Age Range:
Ages 8 - 18

Resource Type:


Materials List (per group of students)

  • A light source with a well-difined beam (i.e. a laser)
  • A rectangular aquarium filled with water
  • A few drops of milk (or some powdered milk) to add to aquarium water to make the beam visible


  • Engineering and Technology
    • Engineering
      • Electrical Engineering
    • Technology
      • Information and Communication
  • Life Sciences
    • Human Senses and Perception
      • Vision
  • Mathematics
    • Geometry
      • Plane Geometry
  • Physical Sciences
    • Vibration and Waves
      • Light and Optics
    • Light and Optics
      • Lenses and Mirrors
      • Reflection and Refraction
    • States of Matter
      • Liquids
  • The Nature of Science
    • The Scientific Process
      • Conducting Investigations


To use this activity, learners need to:

  • see
  • touch

Learning styles supported:

  • Involves hands-on or lab activities


This resource is part of:

Access Rights:

  • Free access



Funding Sources:

  • National Science Foundation
  • California Department of Education
  • NEC Foundation of America