Refraction and Dispersion of Light

Grade 12 · Physics

Semester 1 | Period 1 | Week 2

Download the Lessonotes Mobile Liberia app for faster lesson access on Android and iPhone.

Get it on Google Play Get it on the Apple App Store

Subject: Physics

Semester: 1

Period: 1

Week: 2

School Name:

Teacher’s Name:

Subject: Physics

Grade Level: Grade 12

Week & Period: Week 2, Period I

Date:

Topic: Refraction and Dispersion of Light
Sub-topic (Week 2): Refractive Index – Snell’s Law and Speed of Light

Learning Objectives:

By the end of the lesson, learners should be able to:

  1. Define refractive index.
  2. State and apply Snell’s Law in solving problems.
  3. Relate refractive index to the speed of light in different media.
  4. Perform calculations involving refractive index.
  5. Demonstrate understanding through class experiments.

 

Previous Knowledge:

Students are familiar with the laws of refraction and basic ray diagrams from Week 1.

 

Instructional Materials:

  • Glass and perspex blocks
  • Light box or torch
  • White cardboard
  • Protractors and rulers
  • Scientific calculators
  • Water tank or beaker
  • Stopwatch
  • Data chart for speed of light in different materials

 

Anticipation (Warm-up):

Pose this question:
“Why does light travel faster in air than in water or glass?”
Link to students’ prior experience with refraction and introduce the concept of refractive index.

Types of Refractive Index:

  1. Absolute refractive index: Medium compared to vacuum.
  2. Relative refractive index: One medium compared to another.

 

This formula is used to calculate the refractive index using angles of incidence and refraction.

Experiment: Measuring Refractive Index of Glass Using Snell’s Law

Apparatus:

  • Ray box or torch
  • Protractor
  • Glass block
  • Ruler
  • White paper
  • Pencil

Procedure:

  1. Trace the outline of a rectangular glass block.
  2. Draw a normal and an incident ray at a known angle (30°, 45°, etc.).
  3. Shine the light and trace the refracted and emergent rays.
  4. Measure the angle of refraction.
  5. Use Snell’s Law to calculate the refractive index.

 

Learners’ Activities:

  • Work in groups to carry out the experiment.
  • Record angles and compute refractive index.
  • Compare results with theoretical values for water, glass, and plastic.

 

Assessment:

Classwork Questions:

  1. Define refractive index.
  2. State Snell’s Law.
  3. A light ray passes from air into water. If the angle of incidence is 40° and the angle of refraction is 30°, find the refractive index.
  4. What is the speed of light in a medium with refractive index 1.6?

Homework:

  • Research and list refractive indices of 5 materials.
  • A material has a refractive index of 1.45. What is the speed of light through it?

 

Expanded Notes and Real-Life Applications:

  • Lenses in glasses and microscopes rely on refractive index to function.
  • Fiber optics use the concept of total internal reflection, which depends on refractive index.
  • Diamonds sparkle due to their high refractive index.

 

Differentiation:

  • Use colored ray diagrams for visual learners.
  • Let advanced students derive the formula for refractive index from velocity relations.
  • Group weaker learners with stronger ones during experiments.

 

Teacher's Reflection:

  • Were students able to relate refraction to light speed changes?
  • Did they interpret Snell’s Law correctly in calculations?
  • Were experiments engaging and did they build deeper understanding?