Light
Light travels in straight lines and bends at boundaries between materials, following predictable rules we can use to correct vision and design optical instruments.
Quick Overview
- Reflection — Light bounces off surfaces; angle in equals angle out (i = r)
- Refraction — Light bends when crossing between different transparent materials; more optical density = stronger bend toward normal
- Refractive Index — Measures how much a material slows light down; n = sin i / sin r or n = c / v
- Total Internal Reflection — All light bounces back inside a denser material when angle exceeds the critical angle
- Converging Lenses — Convex; bring parallel rays to a focus; form real (inverted) or virtual (upright) images depending on object distance
- Diverging Lenses — Concave; spread rays apart; always form virtual, upright, diminished images
- Vision Correction — Converging lens fixes long-sightedness; diverging lens fixes short-sightedness
- Dispersion — White light splits into spectrum (ROYGBIV) because different colours refract by different amounts
Reflection of Light
Reflection is what happens when light bounces off a surface. Understanding the rules of reflection allows us to predict where light will go and design mirrors and optical instruments.
Ray Diagrams & Key Terms
In optics, we use ray diagrams to show the path light takes. The normal is an imaginary line drawn at right angles (90°) to the surface. We measure all angles from the normal, not from the surface itself.
💡 Key Point: The incident ray and reflected ray are always on opposite sides of the normal. If the surface is smooth (like a mirror), reflection is regular and creates a clear image. If rough, rays scatter in all directions (diffuse reflection).
Plane Mirrors
A plane mirror is a flat, smooth, polished surface. When an object is placed in front, light rays reflect according to the law of reflection. By tracing where the reflected rays appear to come from, we find the virtual image.
The image in a plane mirror is:
• Same size as the object — magnification = 1
• Same distance behind the mirror as the object is in front
• Virtual — rays don't actually pass through it; it appears to be behind the mirror
• Upright — not inverted
⚠️ Virtual Ray Concept: When light reflects from a mirror, your eye sees rays coming toward you. If you trace these rays backward behind the mirror (where they don't actually go), they meet at the virtual image location.
❓ A person stands 1.5 m in front of a plane mirror. Where is their image, and what are three properties of this image?
❓ The law of reflection states i = r. Explain what these symbols mean and describe what happens when a light ray hits a mirror at 35° to the normal.
Refraction of Light
Refraction is the bending of light when it passes from one transparent material into another. The amount of bending depends on how much the two materials differ in optical density.
Refractive Index