The change in direction of a wave at a boundary so that it returns into the original medium. Reflection explains how mirrors form images, how we see most objects (they reflect light into our eyes), how radar and sonar detect targets, and how fibre optics keep light trapped inside a cable.
This concept is covered in depth in our reflections, symmetry, and congruent figures, with worked examples, practice problems, and common mistakes.
Definition
The change in direction of a wave at a boundary so that it returns into the original medium.
💡 Intuition
Like a ball bouncing off a wall—the wave reverses direction at the boundary.
🎯 Core Idea
The angle of incidence equals the angle of reflection, measured from the normal.
Example
Formula
Notation
\theta_i is the angle of incidence, \theta_r is the angle of reflection, and both are measured in degrees (or radians) from the normal to the reflecting surface.
🌟 Why It Matters
Reflection explains how mirrors form images, how we see most objects (they reflect light into our eyes), how radar and sonar detect targets, and how fibre optics keep light trapped inside a cable.
💭 Hint When Stuck
When solving a reflection problem, first draw the normal (a line perpendicular to the surface at the point where the wave hits). Then measure the angle of incidence from the incoming ray to the normal. The angle of reflection equals the angle of incidence, on the opposite side of the normal.
Formal View
Related Concepts
🚧 Common Stuck Point
Angles are measured from the normal (perpendicular), not from the surface.
⚠️ Common Mistakes
- Measuring angles from the surface instead of from the normal — the law of reflection uses angles measured from the perpendicular to the surface.
- Thinking that only mirrors reflect light — all surfaces reflect light; mirrors just do it in an orderly (specular) way, while rough surfaces scatter it (diffuse reflection).
- Forgetting that reflection does not change the speed or wavelength of a wave — only the direction changes.
Go Deeper
Frequently Asked Questions
What is Reflection in Physics?
The change in direction of a wave at a boundary so that it returns into the original medium.
What is the Reflection formula?
When do you use Reflection?
When solving a reflection problem, first draw the normal (a line perpendicular to the surface at the point where the wave hits). Then measure the angle of incidence from the incoming ray to the normal. The angle of reflection equals the angle of incidence, on the opposite side of the normal.
Prerequisites
Next Steps
Cross-Subject Connections
How Reflection Connects to Other Ideas
To understand reflection, you should first be comfortable with waves. Once you have a solid grasp of reflection, you can move on to refraction and diffraction.
Want the Full Guide?
This concept is explained step by step in our complete guide:
Symmetry, Rotational Symmetry, and Congruence →🧪 Interactive Playground
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