Lesson Note on Light Waves: Refraction, Snell’s Law and Total Internal Reflection for SSS 2

Class: Senior Secondary School 2 (SS2 / SSS2)
Term: 1st Term
Week: 9
Age: 16 years
Duration: 45 minutes
Subject: Physics
Curriculum Theme: Waves
Previous Lesson: Light Waves: Reflection, Mirrors and Periscope Construction.
Topic: LIGHT WAVES
Subject Matter: Refraction of light through rectangular glass block (bending of light), laws of refraction (Snell’s law), real and apparent depth (effects in water), total internal reflection (conditions), critical angle (meaning and determination)

Specific Objectives

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

Cognitive Domain:

• Define refraction of light.
• State the laws of refraction, including Snell’s law.
• Explain the concepts of real and apparent depth.
• State the conditions for total internal reflection.
• Define critical angle and its relationship with refractive index.

Affective Domain:

• Appreciate the practical applications of refraction and total internal reflection in daily life.
• Show interest in conducting experiments related to light.

Psychomotor Domain:

• Trace the path of light through a rectangular glass block and measure angles.
• Verify Snell’s law using recorded values from an experiment.
• Illustrate real and apparent depth with simple diagrams.
• Draw diagrams to explain total internal reflection and critical angle.

Social Domain:

• Collaborate in groups to perform experiments on refraction.

Reference Materials

The following resources were used in planning this lesson:

• Senior Secondary Schools Education Curriculum
• State Unified Scheme of Work
• New School Physics for Senior Secondary Schools

Instructional Materials

The teacher will teach this lesson with the aid of:

• Textbook
• Equipped laboratory (rectangular glass block, optical pins, drawing board, plain sheets, protractor, ruler, water, beaker)
• Charts showing diagrams of refraction, real/apparent depth, total internal reflection, and critical angle.

Rationale for the Lesson

This lesson helps pupils understand how light bends when passing through different materials, which explains common observations like objects appearing shallower in water. It also introduces principles that are fundamental to technologies like optical fibers and periscopes.

Prerequisite/Previous Knowledge

Pupils have prior knowledge of light as a form of energy, its propagation, and reflection from plane surfaces.

Lesson Content/Board Summary

LIGHT WAVES: REFRACTION

Refraction of Light

Refraction is the bending of light as it passes from one transparent medium to another of different optical density. This change in direction occurs because the speed of light changes as it moves between media.

Refraction through a Rectangular Glass Block

When a ray of light enters a rectangular glass block obliquely from air, it bends towards the normal. When it emerges from the glass block back into the air, it bends away from the normal. The emergent ray is parallel to the incident ray but laterally displaced.

Laws of Refraction (Snell’s Law)

There are two laws of refraction:

• First Law: The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.
• Second Law (Snell’s Law): For any two given media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This constant is called the refractive index (n) of the second medium with respect to the first.

Formula:

n = sin(i) / sin(r)

Where:

• n = refractive index
• i = angle of incidence
• r = angle of refraction

Worked Example 1: Calculating Refractive Index

A ray of light passes from air into a glass block. If the angle of incidence is 45° and the angle of refraction is 28°, calculate the refractive index of the glass.

Step 1: Write down the formula.

n = sin(i) / sin(r)

Step 2: Substitute the given values.

n = sin(45°) / sin(28°)

Step 3: Calculate the sine values and divide.

n = 0.7071 / 0.4695

n ≈ 1.506

The refractive index of the glass is approximately 1.51 (to 3 significant figures).

Real and Apparent Depth

When an object is viewed in a denser medium (like water) from a rarer medium (like air), it appears to be shallower than its actual depth. The actual depth is called the real depth, and the perceived depth is called the apparent depth.

Formula:

n = Real Depth / Apparent Depth

Where:

• n = refractive index of the denser medium

Worked Example 2: Calculating Real Depth

A fish in a pond appears to be 1.5 m deep when viewed vertically from above. If the refractive index of water is 1.33, calculate the real depth of the fish.

Step 1: Write down the formula.

n = Real Depth / Apparent Depth

Step 2: Rearrange the formula to find Real Depth.

Real Depth = n × Apparent Depth

Step 3: Substitute the given values.

Real Depth = 1.33 × 1.5 m

Step 4: Calculate the real depth.

Real Depth = 1.995 m

The real depth of the fish is approximately 2.0 m.

Total Internal Reflection (TIR)

Total Internal Reflection is the phenomenon where a ray of light traveling from an optically denser medium to an optically rarer medium, at an angle of incidence greater than the critical angle, is completely reflected back into the denser medium.

Conditions for Total Internal Reflection:

• Light must travel from an optically denser medium to an optically rarer medium.
• The angle of incidence in the denser medium must be greater than the critical angle.

Critical Angle (c)

The critical angle is the angle of incidence in the optically denser medium for which the angle of refraction in the optically rarer medium is 90°.

Relationship between Critical Angle and Refractive Index:

sin(c) = 1 / n

Where:

• c = critical angle
• n = refractive index of the denser medium with respect to the rarer medium

Worked Example 3: Calculating Critical Angle

The refractive index of diamond is 2.42. Calculate the critical angle for light traveling from diamond to air.

Step 1: Write down the formula.

sin(c) = 1 / n

Step 2: Substitute the refractive index of diamond.

sin(c) = 1 / 2.42

Step 3: Calculate the value of sin(c).

sin(c) ≈ 0.4132

Step 4: Find the critical angle c.

c = sin⁻¹(0.4132)

c ≈ 24.4°

The critical angle for diamond is approximately 24.4°.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils and asks them to recall what they know about light and its properties, specifically reflection. The teacher then shows an object partially immersed in water and asks pupils to describe what they observe.
Pupils’ Activity: Pupils respond to the questions and describe the apparent bending of the object in water.
Learning Point: Pupils recall previous knowledge and are introduced to the concept of light bending.

Step 2: Refraction of Light and Laws of Refraction

Time: 10 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher defines refraction of light and explains the two laws of refraction, including Snell’s law (n = sin i / sin r). The teacher uses a chart to illustrate the path of light through a rectangular glass block, showing incident, refracted, and emergent rays, as well as the angles.
Pupils’ Activity: Pupils listen attentively, observe the chart, and copy notes as instructed by the teacher.
Learning Point: Pupils understand the definition and laws of refraction.

Step 3: Practical Verification of Snell’s Law

Time: 10 minutes
Teaching Skill: Practical Demonstration/Guidance
Teacher’s Activity: The teacher guides pupils to set up the experiment using a rectangular glass block, optical pins, drawing board, and plain paper. The teacher demonstrates how to trace the path of light and measure the angles of incidence and refraction for different angles.
Pupils’ Activity: Pupils observe the demonstration and, in groups, attempt to trace light rays and measure angles, recording their observations.
Learning Point: Pupils gain practical experience in verifying Snell’s law.

Step 4: Real and Apparent Depth

Time: 5 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher explains the concepts of real and apparent depth using a beaker of water and a coin. The teacher states the formula relating real depth, apparent depth, and refractive index.
Pupils’ Activity: Pupils observe the demonstration, ask questions for clarification, and copy the formula.
Learning Point: Pupils understand why objects appear shallower in water and the formula to calculate it.

Step 5: Total Internal Reflection (TIR)

Time: 5 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher defines total internal reflection and explains the two conditions necessary for it to occur, using diagrams on a chart.
Pupils’ Activity: Pupils listen, observe the diagrams, and take notes on the definition and conditions for TIR.
Learning Point: Pupils grasp the concept and conditions for total internal reflection.

Step 6: Critical Angle

Time: 5 minutes
Teaching Skill: Explanation/Formula Derivation
Teacher’s Activity: The teacher defines the critical angle and derives its relationship with the refractive index (sin c = 1/n), using a diagram to show light refracted at 90 degrees.
Pupils’ Activity: Pupils listen, observe the derivation, and copy the definition and formula for the critical angle.
Learning Point: Pupils understand the critical angle and its mathematical relationship with refractive index.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Define refraction of light.
2. State the two laws of refraction.
3. Explain the terms “real depth” and “apparent depth.”
4. List the two conditions necessary for total internal reflection to occur.
5. What is the critical angle? How is it related to the refractive index of a medium?

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 0 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the key points of the lesson, emphasizing the importance of refraction, real/apparent depth, total internal reflection, and critical angle in everyday phenomena and technology.
Pupils’ Activity: Pupils listen and ask any remaining questions.
Learning Point: Pupils consolidate their understanding of the lesson’s main concepts.

Lesson Keywords

• Refraction – The bending of light as it passes from one transparent medium to another.
• Snell’s Law – The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media.
• Refractive Index – A measure of how much a medium can bend light, defined as the ratio of the speed of light in vacuum to its speed in the medium.
• Real Depth – The actual depth of an object in a denser medium.
• Apparent Depth – The perceived shallower depth of an object when viewed from a rarer medium.
• Total Internal Reflection – Complete reflection of a light ray back into the denser medium when the angle of incidence exceeds the critical angle.
• Critical Angle – The angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°.

Differentiation

For pupils who grasp concepts quickly, the teacher can provide additional problems involving calculations of refractive index, real/apparent depth, or critical angle. For pupils needing more support, the teacher can provide simplified diagrams and allow more time for practical activities, offering one-on-one guidance.

Note for teachers using this lesson plan

Ensure that all practical demonstrations are conducted safely and that pupils have clear instructions before attempting any hands-on activities. Emphasize the real-world applications of these concepts to enhance pupil engagement.