Reflection of Light by Plane and Spherical Mirrors

Grade 11 · Physics

Semester 2 | Period 6 | Week 35

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Subject: Physics

Semester: 2

Period: 6

Week: 35

School Name:

Teacher’s Name:

Subject: Physics

Grade Level: Grade 11

Week & Period: Week 35, Period VI

Date:

Topic: Reflection of Light by Plane and Spherical Mirrors
Sub-topic: Laws of Reflection, Image Formation in Plane and Spherical Mirrors

Learning Objectives:

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

  1. State and explain the laws of reflection.
  2. Describe the nature and characteristics of images formed by plane and spherical mirrors.
  3. Draw ray diagrams to show image formation in concave and convex mirrors.
  4. Apply the mirror formula and magnification equations in solving numerical problems.

 

Previous Knowledge:

Students already understand that light travels in straight lines and can be reflected.

 

Instructional Materials:

  • Plane mirror
  • Concave and convex mirrors
  • Light rays (pins and paper or laser pointers)
  • Protractor
  • Mirror stands
  • Whiteboard with ray diagram samples

 

Anticipation (Warm-Up) – 5 minutes:

Ask:

  • “Why do we see our reflection in mirrors?”
  • “Can a mirror make you look taller or shorter?”

Let students examine their reflection in a flat and curved mirror and observe the differences.

 

Building Knowledge (Main Lesson) – 25 minutes

  1. Laws of Reflection:
  • Law 1: The incident ray, the reflected ray, and the normal all lie in the same plane.
  • Law 2: The angle of incidence iii is equal to the angle of reflection r.

       i=r

  1. Types of Reflection:
  • Regular (Specular): On smooth surfaces like mirrors.
  • Diffuse: On rough surfaces like paper.

 

  1. Plane Mirrors:
  • Image is virtual, upright, same size, and laterally inverted.
  • Image appears to be the same distance behind the mirror as the object is in front.

 

  1. Spherical Mirrors:
  • Concave Mirror (converging): Curved inward; can produce real or virtual images depending on object position.
  • Convex Mirror (diverging): Curved outward; always forms virtual, diminished, and upright images.

 

  1. Mirror Formula & Magnification:
  • Mirror Formula:

      

Where:

    • f = focal length
    • v = image distance
    • u = object distance
  • Magnification:

 

Sign Conventions (Concave is positive, Convex is negative)

 

  1. Ray Diagrams:
  • For Concave Mirrors:
    • Object at infinity: Real, inverted, at focus.
    • Object beyond center (C): Real, inverted, between F and C.
    • Object at center (C): Real, inverted, same size at C.
    • Object between C and F: Real, inverted, enlarged.
    • Object at focus (F): No image (rays parallel).
    • Object between F and pole (P): Virtual, upright, enlarged.
  • For Convex Mirrors:
    • Always forms virtual, diminished, and upright images behind the mirror.

 

Learners’ Activities:

  • Use ray boxes and mirrors to trace incident and reflected rays.
  • Draw ray diagrams for various object positions.
  • Calculate focal lengths and magnifications using the mirror formula.

 

Sample Problem:

Q: An object is placed 30 cm in front of a concave mirror of focal length 15 cm. Find the position and nature of the image.

Solution:

 

Consolidation (Review and Assessment) – 10 minutes

Oral Questions:

  • What are the two laws of reflection?
  • Describe the nature of images formed by convex mirrors.
  • What is the mirror formula?

Homework:

  1. Draw ray diagrams for object:
    • (a) at the center of a concave mirror
    • (b) between focus and pole
  2. State two uses of convex mirrors in everyday life.

 

Notes – Detailed and Explained:

  • Plane mirrors reflect light symmetrically, preserving the shape and size of images.
  • Concave mirrors can enlarge images and are used in shaving mirrors and solar cookers.
  • Convex mirrors give a wide field of view and are used in vehicle side mirrors.
  • The mirror equation helps locate image positions for precise applications.

 

Expanded Notes / Instructions:

  • Encourage practice with ray diagrams and solving using sign conventions.
  • Allow learners to explore the effect of distance on image formation.
  • Explain that real images can be projected on a screen; virtual images cannot.

 

Differentiation:

  • Ray diagrams for visual learners.
  • Practical demonstrations for kinesthetic learners.
  • Formula recitations and oral quizzes for auditory learners.

 

Teacher’s Reflection:

  • Did learners understand the concept of real and virtual images?
  • Were learners able to apply the mirror formula correctly?
  • How did students respond to practical and diagrammatic activities?