Refraction and Dispersion of Light

Grade 12 · Physics

Semester 1 | Period 1 | Week 3

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Subject: Physics

Semester: 1

Period: 1

Week: 3

School Name:

Teacher’s Name:

Subject: Physics

Grade Level: Grade 12

Week & Period: Week 3, Period I

Date:

Topic: Refraction and Dispersion of Light
Sub-topic (Week 3): Real and Apparent Depth, Critical Angle, and Total Internal Reflection

Learning Objectives:

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

  1. Differentiate between real and apparent depth.
  2. Define and compute the critical angle.
  3. Explain and illustrate the concept of total internal reflection.
  4. Solve problems involving real depth, apparent depth, and critical angle.
  5. List applications of total internal reflection.

 

Previous Knowledge:

Learners are familiar with Snell’s Law, refractive index, and how light bends when it travels through different media.

 

Instructional Materials:

  • Glass blocks
  • Water tank/beaker
  • Coin and transparent container
  • Torch/ray box
  • White paper and rulers
  • Calculator
  • Optical fiber demo (optional)

 

Anticipatory Set (Warm-up):

Drop a coin into a transparent container filled with water and ask learners:
“Why does the coin look closer than it really is?”
Introduce the idea of apparent depth and link it to refraction.

Building Knowledge (Main Lesson):

Real and Apparent Depth

When light rays pass from a denser to a rarer medium, the object appears shallower due to the bending of rays.

 

Total Internal Reflection (TIR)

Occurs when:

  1. Light travels from denser to rarer medium, and
  2. Angle of incidence > critical angle

Applications of TIR:

  • Optical fibers
  • Periscopes
  • Diamond brilliance
  • Medical endoscopy

Experiment: Observing Total Internal Reflection

Apparatus:

  • Glass block
  • Ray box
  • Protractor
  • Paper and pencil

Procedure:

  1. Place the glass block on paper.
  2. Direct a light ray at increasing angles toward the interface.
  3. Measure angles where no light exits—observe internal reflection.
  4. Find the angle at which light no longer refracts but reflects completely.

Observation:
At the critical angle, the light grazes the surface. Beyond this, all light reflects back internally.

 

Learners’ Activities:

  • Perform calculations using apparent depth and critical angle formulas.
  • Use the glass block to observe TIR.
  • Draw ray diagrams showing real and apparent depth.
  • Discuss where and how TIR is used in daily life.

 

Assessment

Classwork:

  1. Define apparent depth.
  2. A tank is 1.5 m deep. What is the apparent depth if the refractive index is 1.25?
  3. What is the critical angle for a glass-air boundary (n=1.5n = 1.5n=1.5)?
  4. Explain two uses of total internal reflection.

Homework:

  • Draw a diagram showing total internal reflection inside an optical fiber.
  • Explain why diamonds sparkle using TIR.
  • A diver underwater sees the sun directly overhead. Explain why they cannot see beyond a certain angle.

 

Expanded Notes and Applications:

  • In a swimming pool, the bottom appears closer due to apparent depth.
  • Optical fibers use total internal reflection to guide light over long distances.
  • The sparkle of diamonds is due to their high refractive index and multiple internal reflections.

 

Differentiation (Support & Challenge):

  • Use visual ray-tracing demos for visual learners.
  • Provide scaffolded problem-solving steps for beginners.
  • Allow advanced learners to explore fiber-optic and laser diagrams.

 

Teacher's Reflection:

  • Did students distinguish between real and apparent depth effectively?
  • Were they able to calculate and interpret critical angle accurately?
  • Was the link between ray behavior and optical technologies clear?