Class: Senior Secondary School 2 (SS2 / SSS 2)
Term: 1st Term
Week: 8
Age: 16 years
Duration: 45 minutes
Subject: Physics
Curriculum Theme: Optics
Previous Lesson: Light Waves: Meaning, Sources, Shadows and Pinhole Camera.
Topic: LIGHT WAVES
Subject Matter: Reflection of light at plane and curved surfaces (mirrors), laws of reflection (incident angle equals reflected angle), formation of images by plane and curved mirrors (ray diagrams), application of plane and curved mirrors (daily uses), focal length of a concave mirror (determination)

Specific Objectives

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

Cognitive Domain:

Define reflection of light.
State the laws of reflection.
Describe the formation of images by plane and curved mirrors.
Explain the applications of plane and curved mirrors.
State a method for determining the focal length of a concave mirror.

Affective Domain:

Appreciate the various applications of mirrors in daily life.
Show interest in performing experiments related to reflection.

Psychomotor Domain:

Draw accurate ray diagrams for image formation by plane and curved mirrors.
Construct a simple periscope.
Carry out a simple experiment to determine the focal length of a concave mirror.

Collaborate effectively with peers during practical activities and discussions.

Reference Materials

The following resources were used in planning this lesson:

Senior Secondary Schools Education Curriculum
State Unified Scheme of Work
New School Physics by P.N. Okeke and M.W. Anyakoha
https://www.physicsclassroom.com/class/refln/Lesson-1/The-Law-of-Reflection
https://www.khanacademy.org/science/physics/light-waves/reflection-and-refraction/a/specular-vs-diffuse-reflection

Instructional Materials

The teacher will teach this lesson with the aid of:

Plane mirror
Concave mirror
Convex mirror
Ray box
Pins
Screen
Ruler
Protractor
Materials for periscope construction (e.g., cardboard, two small plane mirrors)

Rationale for the Lesson

Understanding reflection of light helps pupils comprehend how mirrors work and their various uses in daily life, from seeing themselves to advanced optical instruments. This knowledge is fundamental for further studies in optics and practical applications in technology.

Prerequisite/Previous Knowledge

Pupils are expected to have prior knowledge of light as a form of energy, its propagation, and basic concepts of rays and beams.

Lesson Content/Board Summary

LIGHT WAVES: REFLECTION

1. Reflection of Light

Reflection of light is the bouncing back of light rays when they strike a surface. The surface can be smooth (specular reflection) or rough (diffuse reflection).

2. Laws of Reflection

The reflection of light follows two fundamental laws:

First Law: The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
Second Law: The angle of incidence (i) is equal to the angle of reflection (r). That is, i = r.

NOTE: The normal is an imaginary line perpendicular to the reflecting surface at the point of incidence.

3. Image Formation by Plane Mirrors

A plane mirror forms an image that is:

Virtual (cannot be formed on a screen)
Erect (upright)
Laterally inverted (left appears right, and vice versa)
Same size as the object
Same distance behind the mirror as the object is in front

Example: Ray Diagram for a Plane Mirror

Step 1: Draw the plane mirror and the principal axis. Place the object (e.g., an arrow) in front of the mirror.

Step 2: Draw two incident rays from the top of the object to the mirror. One ray perpendicular to the mirror (reflects back along itself). Another ray at an angle.

Step 3: Draw the reflected rays according to the laws of reflection (angle of incidence = angle of reflection).

Step 4: Extend the reflected rays backward (as dotted lines) behind the mirror. The point where they appear to intersect forms the image.

4. Image Formation by Curved Mirrors

Curved mirrors can be concave (converging) or convex (diverging).

a. Concave Mirrors

A concave mirror curves inwards. Its focal point (F) and center of curvature (C) are in front of the mirror.

Rules for Drawing Ray Diagrams for Concave Mirrors:

A ray parallel to the principal axis passes through the principal focus (F) after reflection.
A ray passing through the principal focus (F) becomes parallel to the principal axis after reflection.
A ray passing through the center of curvature (C) reflects back along the same path.
A ray directed towards the pole (P) reflects such that the angle of incidence equals the angle of reflection.

Image Characteristics for Concave Mirrors (Summary):

Object at infinity: Real, inverted, highly diminished, at F.
Object beyond C: Real, inverted, diminished, between F and C.
Object at C: Real, inverted, same size, at C.
Object between C and F: Real, inverted, magnified, beyond C.
Object at F: Real, inverted, highly magnified, at infinity.
Object between F and P: Virtual, erect, magnified, behind the mirror.

Example: Ray Diagram for Concave Mirror (Object beyond C)

Step 1: Draw the concave mirror, principal axis, P, F, and C. Place the object beyond C.

Step 2: Draw a ray from the top of the object parallel to the principal axis. It reflects through F.

Step 3: Draw a second ray from the top of the object passing through F. It reflects parallel to the principal axis.

Step 4: The intersection of the two reflected rays forms the image. Describe the image (real, inverted, diminished, between F and C).

b. Convex Mirrors

A convex mirror curves outwards. Its focal point (F) and center of curvature (C) are behind the mirror.

Rules for Drawing Ray Diagrams for Convex Mirrors:

A ray parallel to the principal axis appears to diverge from the principal focus (F) after reflection.
A ray directed towards the principal focus (F) becomes parallel to the principal axis after reflection.
A ray directed towards the center of curvature (C) reflects back along the same path.

Image Characteristics for Convex Mirrors (Summary):

Convex mirrors always form virtual, erect, and diminished images, regardless of the object’s position. The image is always located between F and P behind the mirror.

Example: Ray Diagram for Convex Mirror (Object at any position)

Step 1: Draw the convex mirror, principal axis, P, F, and C (behind the mirror). Place the object in front of the mirror.

Step 2: Draw a ray from the top of the object parallel to the principal axis. It reflects as if coming from F (draw dotted line to F).

Step 3: Draw a second ray from the top of the object directed towards F. It reflects parallel to the principal axis (draw dotted line to F).

Step 4: The intersection of the extended reflected rays (dotted lines) behind the mirror forms the image. Describe the image (virtual, erect, diminished, between F and P).

5. Applications of Plane and Curved Mirrors

Plane Mirrors: Used in dressing tables, periscopes, kaleidoscopes, and as rearview mirrors in some older vehicles.
Concave Mirrors: Used as shaving mirrors, dentist mirrors, headlamps of cars, searchlights, solar furnaces, and in reflecting telescopes.
Convex Mirrors: Used as rearview mirrors in vehicles (to provide a wider field of view), security mirrors in shops, and street light reflectors.

6. Determination of Focal Length of a Concave Mirror

The focal length (f) of a concave mirror can be determined by:

Distant Object Method: Place the concave mirror facing a distant object (e.g., a distant tree or building). Adjust a screen in front of the mirror until a sharp, inverted image of the distant object is formed on the screen. The distance between the mirror and the screen is approximately the focal length of the mirror. This is because rays from a distant object are considered parallel and converge at the focal point.
Using the Mirror Formula: The mirror formula is 1/f = 1/u + 1/v, where ‘u’ is the object distance and ‘v’ is the image distance. By taking several readings of u and v, plotting a graph of 1/u against 1/v, or calculating f for each pair, the focal length can be determined.

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 asks pupils what they see when they look into a mirror and what happens when light hits a shiny surface.
Pupils’ Activity: Pupils respond by saying they see their reflection and that light bounces back.
Learning Point: Pupils recall prior knowledge about mirrors and light.

Step 2: Explanation of Reflection and Laws

Time: 10 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher defines reflection, explains the types of reflection, and demonstrates the laws of reflection using a ray box, plane mirror, and protractor.
Pupils’ Activity: Pupils observe the demonstration, participate in measuring angles, and state the laws of reflection.
Learning Point: Pupils understand the definition and laws of reflection.

Step 3: Image Formation by Plane Mirrors

Time: 7 minutes
Teaching Skill: Guided Practice
Teacher’s Activity: The teacher guides pupils to draw ray diagrams for image formation by plane mirrors, highlighting the characteristics of the image.
Pupils’ Activity: Pupils draw ray diagrams and identify the characteristics of the image formed by a plane mirror.
Learning Point: Pupils can illustrate and describe image formation by plane mirrors.

Step 4: Image Formation by Concave Mirrors

Time: 7 minutes
Teaching Skill: Guided Practice
Teacher’s Activity: The teacher explains the rules for drawing ray diagrams for concave mirrors and guides pupils through drawing a ray diagram for an object placed beyond C, summarizing other image characteristics.
Pupils’ Activity: Pupils draw the ray diagram for a concave mirror and note the different image characteristics.
Learning Point: Pupils understand image formation principles for concave mirrors.

Step 5: Image Formation by Convex Mirrors

Time: 5 minutes
Teaching Skill: Guided Practice
Teacher’s Activity: The teacher explains the rules for drawing ray diagrams for convex mirrors and guides pupils through drawing a ray diagram, summarizing the image characteristics.
Pupils’ Activity: Pupils draw the ray diagram for a convex mirror and note the image characteristics.
Learning Point: Pupils understand image formation principles for convex mirrors.

Step 6: Applications of Mirrors and Focal Length Determination

Time: 3 minutes
Teaching Skill: Discussion/Explanation
Teacher’s Activity: The teacher discusses various applications of plane and curved mirrors and explains the distant object method for determining the focal length of a concave mirror.
Pupils’ Activity: Pupils identify uses of mirrors and listen to the explanation of focal length determination.
Learning Point: Pupils know the practical uses of mirrors and a method for determining focal length.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

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

Define reflection of light.
State the two laws of reflection.
List three characteristics of an image formed by a plane mirror.
Mention two applications of a concave mirror.
Describe how you would determine the focal length of a concave mirror using a distant object.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 3 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the key points of the lesson and assigns homework, which includes practicing ray diagrams for different mirror types and object positions.
Pupils’ Activity: Pupils listen to the summary and copy the homework assignment.
Learning Point: Pupils consolidate their learning and prepare for further practice.

Lesson Keywords

Reflection – The bouncing back of light when it strikes a surface.
Incident ray – The ray of light falling on the surface.
Reflected ray – The ray of light bouncing off the surface.
Normal – An imaginary line perpendicular to the surface at the point of incidence.
Angle of incidence – The angle between the incident ray and the normal.
Angle of reflection – The angle between the reflected ray and the normal.
Plane mirror – A flat reflecting surface.
Concave mirror – A spherical mirror with the reflecting surface curved inwards.
Convex mirror – A spherical mirror with the reflecting surface curved outwards.
Focal length – The distance between the pole and the principal focus of a spherical mirror.
Ray diagram – A diagram showing the path of light rays to determine image formation.

Differentiation

For pupils who grasp concepts quickly, the teacher can provide additional challenges, such as asking them to derive the mirror formula or investigate applications in optical instruments. For struggling pupils, the teacher will provide one-on-one assistance, simplified diagrams, and repetitive practice on basic definitions and ray tracing.

Note for teachers using this lesson plan

Ensure that practical demonstrations are clear and visible to all pupils. Encourage active participation during ray diagram construction. Emphasize the importance of drawing accurate diagrams with labels. For focal length determination, ensure safety when handling mirrors and ensure pupils understand the concept of a distant object.