Introduction to Reflection

Have you ever wondered why you can see your face in a mirror, or why the surface of a calm lake looks like a giant silver sheet? It all comes down to reflection. In this chapter, we will explore how light behaves when it hits a surface and how we can use ray diagrams to predict exactly where that light will go. Don't worry if you find drawing diagrams a bit fiddly at first—once you learn the simple "Golden Rule" of reflection, everything else falls into place!

What is Light?

Before we look at reflection, we need to remember a key fact from the "Properties of Waves" chapter: light is a transverse wave. This means the vibrations are at right angles to the direction the wave travels. Light waves also travel in straight lines, which is why we represent them using straight arrows called rays.

The Key Terms

To master reflection, you need to speak the language of a physicist. Here are the three most important parts of any reflection diagram:

  • The Incident Ray: This is the "incoming" ray of light that is heading towards the mirror or surface.
  • The Reflected Ray: This is the ray of light that "bounces off" the surface and moves away.
  • The Normal: This is an imaginary line drawn at a right angle (\(90^\circ\)) to the surface of the mirror at the point where the light hits it. We usually draw this as a dashed or dotted line.

The Law of Reflection

This is the most important rule in this chapter. Whether light is hitting a smooth mirror or a rough piece of paper, it always follows this law:

The angle of incidence is equal to the angle of reflection.

In mathematical symbols, we write this as:

\(i = r\)

Where:

  • \(i\) = the angle of incidence (the angle between the incident ray and the normal).
  • \(r\) = the angle of reflection (the angle between the reflected ray and the normal).

Common Mistake Alert! Always measure your angles from the normal, not from the surface of the mirror. This is the most common way students lose marks in exams!

Drawing Ray Diagrams

Ray diagrams are a way to show how light travels. To get full marks on a drawing question, follow these steps:

  1. Use a sharp pencil and a ruler. Precision is key!
  2. Draw the mirror surface (usually a straight line with little hatches on the back).
  3. Draw the normal at \(90^\circ\) to the mirror using a dashed line.
  4. Draw the incident ray hitting the mirror where the normal meets it.
  5. Use a protractor to measure the angle of incidence (\(i\)).
  6. Measure the same angle on the other side of the normal to draw your reflected ray (\(r\)).
  7. Add arrows to your rays to show which way the light is moving. Light never stands still!

Images in a Plane Mirror

When you look into a flat (plane) mirror, you see an image of yourself. This image has four specific characteristics that you need to memorize:

  1. It is Virtual: This means the light rays don't actually go through the mirror. The image "appears" to be behind the mirror, but there is no light actually there.
  2. It is Upright: The image is the right way up (your head is at the top).
  3. It is the Same Size: The image is not magnified or shrunk.
  4. It is Laterally Inverted: This is a fancy way of saying "swapped left-to-right." If you raise your right hand, your mirror image raises its left hand.
  5. Same Distance: The image appears to be the same distance behind the mirror as the object is in front of it.
Quick Tip: The "Virtual" Concept

Think of a virtual image like a "ghost" image. You can see it with your eyes, but if you put a piece of paper (a screen) behind the mirror where the image looks like it is, nothing would appear on the paper. Real images (like those on a cinema screen) can be projected; virtual images cannot.

Summary Checklist

Before moving on to Refraction, make sure you can:

  • State that light is a transverse wave.
  • State the law of reflection: \(i = r\).
  • Draw and label a ray diagram including the normal, incident ray, and reflected ray.
  • List the properties of an image in a plane mirror (virtual, upright, same size, same distance, laterally inverted).

Key Takeaway: Always draw your normal first, and always measure your angles from that normal line. If you do that, you've already mastered the hardest part of reflection!