Introduction: The Magic of Light

Welcome to one of the most visual chapters in Physics! In this section, we explore how light bounces, bends, and changes direction. These principles aren't just for textbooks—they explain why you can see yourself in a mirror, why a straw looks "broken" in a glass of water, and how glasses help people see clearly. Don't worry if it seems like a lot of diagrams at first; once you master the Normal line, everything else clicks into place!

Note: This chapter is part of Paper 1. It builds on what you know about waves (Topic 4) and focuses specifically on how light interacts with different materials.

1. Reflection: Bouncing Back

Reflection happens when light hits a surface and bounces off it. To understand this, we always draw a Ray Diagram.

Key Terms for Ray Diagrams

  • The Normal: An imaginary dashed line drawn at \(90^{\circ}\) (perpendicular) to the surface where the light hits. Always measure your angles from this line!
  • Angle of Incidence (\(i\)): The angle between the incoming ray and the normal.
  • Angle of Reflection (\(r\)): The angle between the reflected ray and the normal.

The Law of Reflection: The angle of incidence is always equal to the angle of reflection (\(i = r\)).

Specular vs. Diffuse Reflection

Why can you see your face in a mirror but not in a piece of white paper?

  • Specular Reflection: Happens on smooth surfaces (like mirrors). All light rays reflect at the same angle, creating a clear image.
  • Diffuse Reflection: Happens on rough surfaces. The light rays reflect in many different directions because the surface is uneven. This is why you can see the object, but not a reflection of yourself in it.

Quick Tip: In an exam, if you are asked to draw a reflection ray diagram, use a ruler and a sharp pencil. Ensure your angles look equal!

2. Refraction: Bending Light

Refraction is the change in direction of a wave when it moves from one medium (material) to another. This happens because the wave changes speed.

How it works:

  • When light enters a more dense medium (like from air into glass), it slows down and bends towards the normal.
  • When light enters a less dense medium (like from glass into air), it speeds up and bends away from the normal.

Higher Tier Only: Refraction happens because the wavelength changes when the speed changes, but the frequency stays the same.

Core Practical 5.9: Investigating Refraction

You need to know how to investigate light entering a rectangular glass block:

  1. Place a glass block on paper and trace around it.
  2. Shine a thin ray of light (using a ray box) into the block at an angle.
  3. Mark the path of the incident ray and the ray that comes out the other side (the emergent ray).
  4. Remove the block and join the dots to show the path of the light inside the block.
  5. Draw the normal at the point where the light entered.
  6. Measure the angle of incidence and the angle of refraction using a protractor.

Common Mistake: Students often measure the angle between the light and the glass surface. Always measure between the light and the normal line!

3. Total Internal Reflection (TIR)

Sometimes, light doesn't refract out of a material at all—it stays trapped inside! This is called Total Internal Reflection.

Two conditions for TIR:

  1. The light must be travelling from a more dense medium to a less dense medium (e.g., from glass to air).
  2. The angle of incidence must be greater than the critical angle.

The Critical Angle: This is the specific angle of incidence where the light refracts exactly at \(90^{\circ}\) along the boundary. If you increase the angle any further, the light reflects back inside.

4. Lenses and Images

Lenses use refraction to change the path of light to form an image. There are two main types you need to know:

1. Converging (Convex) Lenses

  • Shape: Thicker in the middle than at the edges.
  • Effect: They bring parallel rays of light together at a point called the principal focus.
  • Uses: Magnifying glasses, correcting long-sightedness.

2. Diverging (Concave) Lenses

  • Shape: Thinner in the middle (caves inward).
  • Effect: They make parallel rays of light spread out (diverge).
  • Uses: Correcting short-sightedness.

Lens Power

The "strength" of a lens is called its power. A powerful lens is more curved and bends light more strongly.

The formula for power is:
\(P = \frac{1}{f}\)

Where:

  • \(P\) is Power (measured in dioptres, \(D\)).
  • \(f\) is the focal length (the distance from the lens to the principal focus, measured in metres, \(m\)).

Real vs. Virtual Images

  • Real Image: Formed when light rays actually meet. These images can be projected onto a screen (like a cinema screen).
  • Virtual Image: Formed when light rays appear to come from a point, but don't actually meet. You cannot project these onto a screen (like your reflection in a mirror or the image seen through a magnifying glass).

5. Colour and Materials

How we see objects depends on how they absorb, transmit, or reflect light.

Types of Materials:

  • Transparent: Transmits (lets through) almost all light. You can see through it clearly.
  • Translucent: Scatters light as it passes through. You can see light, but not a clear image.
  • Opaque: Does not let any light through. It absorbs or reflects all of it.

Why do things look coloured?

White light is actually a mixture of all the colours of the visible spectrum. When white light hits an opaque object:

  • The object reflects certain wavelengths (colours).
  • The object absorbs all other wavelengths.
  • Example: A red apple reflects red light into your eyes and absorbs all other colours.

Higher Tier Tip: If an object is black, it has absorbed all wavelengths of visible light. If it is white, it has reflected all wavelengths equally.

Summary Checklist

  • Can you draw a reflection diagram where \(i = r\)?
  • Do you know the difference between specular and diffuse reflection?
  • Can you explain why light bends toward the normal when entering glass?
  • Do you know the two conditions for Total Internal Reflection?
  • Can you calculate lens power using \(P = \frac{1}{f}\) (remembering to use metres)?
  • Can you describe the difference between a real and a virtual image?