Light Rays

Overview

This lesson needs a strong emphasis on drawing discipline. Students should leave able to draw a normal, measure angles from the normal rather than the mirror surface, and describe the image formed by a plane mirror using the standard language that will be reused in later optics lessons.

Key knowledge and explanations

• Introduce the ray model for light and remind students that rays show the direction of travel.
• Define normal, angle of incidence, and angle of reflection using one large labelled diagram.
• State and use the law of reflection, making it explicit that both angles are measured from the normal.
• Describe the image in a plane mirror as virtual, the same size as the object, and the same distance behind the mirror as the object is in front.
• Model one simple construction where students trace reflected rays backwards to locate the image.

Lesson flow

1. Start with a short retrieval task on straight-line travel and ask students how they know where an image in a mirror appears to be.
2. Demonstrate reflection with a ray box and plane mirror, then sketch the setup step by step so the class can copy the standard diagram layout.
3. Practise measuring and drawing angles of incidence and reflection, then move on to image questions where students predict the image position before drawing it.
4. Finish with a mix of quick diagram checks and one written explanation of why the mirror image is described as virtual.

Checks for understanding

• Give one hinge question where students must choose the correct angle measurement from four mirror diagrams.
• Ask students to state three characteristics of a plane-mirror image without looking at notes.
• Use a short construction task where they mark the image position of an object in front of a plane mirror.

Common mistakes or misconceptions

• Many students measure the angle from the mirror surface rather than the normal. Keep returning to the same labelled diagram and insist that the normal is drawn first.
• Students often place the image on the mirror surface instead of behind it. Use equal object-distance and image-distance markings to reinforce the geometry.
• Some think the image is physically on the mirror. Revisit the meaning of virtual by asking whether the image could be caught on a screen.

Follow-up

• Set a short ray-diagram sheet so students repeat the same drawing routine enough times to make it secure.
• Carry forward the vocabulary of normal and incidence angle directly into the refraction lesson, where the same conventions still apply.