Hello IGCSE Physicist! Welcome to the Chapter on Light

The chapter on Light is one of the most fascinating parts of Physics! Light allows us to see the world, powers our communication systems, and helps doctors scan our bodies. Since Light is a type of wave, understanding its behavior requires remembering the core wave concepts you've already studied.
Don't worry if concepts like Refraction or Lenses seem complicated; we will break them down step-by-step using clear analogies. Let's get started!

I. Review: Light as a Transverse Wave

Before diving into mirrors and lenses, remember what light actually is:

  • Light is a form of energy transferred by an electromagnetic wave.
  • Light waves, like all electromagnetic waves, are transverse waves.
  • This means the direction of vibration (oscillation) is at right angles (perpendicular) to the direction the wave is travelling (the direction of propagation).
  • Waves transfer energy from one place to another without transferring matter.

Quick Key Takeaway:

Light is energy travelling as a transverse wave at an incredibly high speed.

II. Reflection of Light

Reflection is simply the bouncing of light off a surface. The simplest type of reflection happens on a smooth, shiny surface, like a plane mirror.

Key Terms for Reflection (3.2.1 Core 1)

  • Normal: An imaginary line drawn perpendicular (at 90°) to the reflecting surface at the point where the ray hits.
  • Angle of Incidence (\(i\)): The angle between the incident ray (incoming light ray) and the Normal.
  • Angle of Reflection (\(r\)): The angle between the reflected ray (outgoing light ray) and the Normal.

The Laws of Reflection (3.2.1 Core 3)

These are two fundamental rules that govern how light reflects:

  1. The angle of incidence is equal to the angle of reflection.
    $$(i = r)$$
  2. The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane. (Imagine them all flat on one piece of paper).

Memory Aid: Think of a pool shot—the angle the ball hits the cushion is the same angle it leaves the cushion.

Images Formed by a Plane Mirror (3.2.1 Core 2)

When you look in a flat bathroom mirror (a plane mirror), the image you see has specific characteristics:

  • Same Size: The image is the same size as the object.
  • Same Distance: The image appears to be the same distance behind the mirror as the object is in front of it.
  • Upright: It is not upside down.
  • Laterally Inverted: Left and right are swapped (like in an ambulance sign).
  • Virtual: This is the most important term! A virtual image is one that cannot be projected onto a screen because the light rays do not actually pass through the image location; they only appear to come from there.

Quick Key Takeaway:

Reflection follows a simple rule: \(i=r\). Plane mirrors form virtual, same-sized images.

III. Refraction of Light, Critical Angle, and TIR

What is Refraction? (3.2.2 Core 3)

Refraction is the bending of light as it passes from one transparent medium (like air) into another (like glass or water). This happens because light changes its speed when moving between materials of different densities.

Analogy: Imagine driving a car from a smooth road (Air/Fast) onto mud (Glass/Slow) at an angle. The wheel that hits the mud first slows down, causing the car to turn (bend).

Refraction Rules:
  • When light travels from a less dense medium (like air) to a more dense medium (like glass), it slows down and bends TOWARDS the Normal. (\(r < i\))
  • When light travels from a more dense medium (like glass) to a less dense medium (like air), it speeds up and bends AWAY from the Normal. (\(r > i\))

Refractive Index (\(n\)) (3.2.2 Supplement 6, 7)

The refractive index (\(n\)) measures how much a material slows down light and how much it refracts the light.

  • Definition: The refractive index is the ratio of the speed of a wave in two different regions.
  • Snell's Law (Extended): We calculate \(n\) using the angles of incidence (\(i\)) and refraction (\(r\)).
    $$n = \frac{\sin i}{\sin r}$$
  • Note: A higher refractive index means the light slows down more and bends more. Glass usually has \(n \approx 1.5\).

Critical Angle (\(c\)) and Total Internal Reflection (TIR) (3.2.2 Core 4, 5)

If light tries to move from a dense medium (like glass) to a less dense medium (like air), something special happens if the angle is too large.

1. Critical Angle (\(c\))

The critical angle is the specific angle of incidence (\(i\)) in the denser medium for which the angle of refraction (\(r\)) is exactly \(90^\circ\). The refracted ray skims along the boundary.

Relationship (Extended): You can calculate the refractive index using the critical angle: $$n = \frac{1}{\sin c}$$

2. Total Internal Reflection (TIR)

When the angle of incidence (\(i\)) is made even larger than the critical angle (\(c\)), the light stops refracting entirely. Instead, it is completely reflected back into the denser medium. This is Total Internal Reflection (TIR).

Two Conditions for TIR:

  1. Light must travel from a denser medium towards a less dense medium.
  2. The angle of incidence (\(i\)) must be greater than the critical angle (\(c\)). $$(i > c)$$

Real-World Use: TIR is essential for optical fibres (3.2.2 Supp 9). These thin strands of glass carry information (like internet data or phone calls) by trapping light signals using continuous internal reflections.

Quick Key Takeaway:

Refraction is bending due to speed change. If light tries to escape a dense material at too steep an angle (\(i > c\)), it is perfectly trapped by TIR.

IV. Thin Lenses

Lenses use refraction to change the path of light rays and form images. There are two main types.

Types of Lenses (3.2.3 Core 1)

  1. Converging Lens (Convex):

    Thicker in the middle than at the edges. When parallel rays hit, the lens bends them inwards to meet at a single point (the focus).

  2. Diverging Lens (Concave):

    Thinner in the middle than at the edges. When parallel rays hit, the lens bends them outwards (they diverge). The rays appear to come from a focal point behind the lens.

Key Terms for Lenses (3.2.3 Core 2)

  • Principal Axis: The imaginary line passing straight through the centre of the lens.
  • Principal Focus (F or Focal Point): The point on the principal axis where parallel rays meet after passing through a converging lens, or appear to diverge from after passing through a diverging lens.
  • Focal Length (\(f\)): The distance between the centre of the lens and the principal focus.

Image Formation by Lenses (3.2.3 Core 3, 4, 5)

Images are described using four characteristics:

  • Size: Enlarged, Diminished, or Same Size.
  • Orientation: Upright or Inverted.
  • Type: Real (rays actually cross, can be projected) or Virtual (rays only appear to cross, cannot be projected).
Converging Lens Applications (3.2.3 Supp 7)

A converging lens can produce both real and virtual images, depending on where the object is placed.

Example: The Magnifying Glass.
When the object is placed closer to the lens than the focal length (\(f\)), the converging lens acts as a magnifying glass, producing a Virtual, Upright, and Enlarged image.

Lenses for Correcting Vision (3.2.3 Supplement 8)

Lenses are used to correct common vision problems:

  • Long-sightedness (difficulty seeing near objects) is corrected using a Converging (convex) lens.
  • Short-sightedness (difficulty seeing distant objects) is corrected using a Diverging (concave) lens.

Quick Key Takeaway:

Lenses use refraction to focus (converge) or spread (diverge) light. A key practical use is vision correction.

V. Dispersion of Light

Splitting White Light (3.2.4 Core 1)

When white light passes through a glass prism, it splits into its constituent colours. This process is called dispersion.

Why does this happen?
White light is a mixture of all visible colours. Each colour has a slightly different wavelength. When light enters glass, its speed changes depending on its wavelength. Since different colours refract by different amounts, they separate.
The speed of light depends on the colour/frequency in glass, but NOT in a vacuum!

The Visible Spectrum (3.2.4 Core 2)

The order of the colours produced, from least refracted (longest wavelength/lowest frequency) to most refracted (shortest wavelength/highest frequency):

Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV)

  • Red is refracted least.
  • Violet is refracted most.
Monochromatic Light (3.2.4 Supplement 3)

Light of a single frequency (and therefore a single wavelength, like a pure red laser) is called monochromatic light. This light will not be dispersed by a prism.

Quick Key Takeaway:

Dispersion separates white light into the spectrum (ROYGBIV) because different colours refract by different amounts in the glass.

VI. The Electromagnetic Spectrum (EM Spectrum)

Visible light is only a tiny part of a much larger family of waves called the Electromagnetic (EM) Spectrum. These are all transverse waves and all share certain properties.

Shared Properties of EM Waves (3.3 Core 2, Supplement 6)

  1. All EM waves are transverse waves.
  2. All travel at the same speed in a vacuum (and approximately the same speed in air).
  3. This speed is the Speed of Light, \(c\):
    $$c = 3.0 \times 10^8 \text{ m/s}$$
  4. They obey the wave equation: $$(c = f\lambda)$$

Did you know? Since \(c\) is constant, if the wavelength (\(\lambda\)) increases, the frequency (\(f\)) must decrease, and vice versa.

The Order of the Spectrum (3.3 Core 1)

The EM spectrum is arranged by wavelength (and frequency/energy). You must know the order:


Radio waves
Microwaves
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays

Mnemonic Trick: Romans Men In Very Unique X-ray Gardens

As you move from Radio waves toward Gamma rays:

  • Wavelength (\(\lambda\)): Decreases (gets shorter)
  • Frequency (\(f\)) and Energy: Increases (gets higher)

Uses and Dangers of EM Regions (3.3 Core 3, 4)

Radio Waves
  • Use: Radio and television transmissions, RFID.
  • Danger: Generally low energy, so they are the least dangerous.
Microwaves
  • Use: Satellite television, mobile phones, microwave ovens (heating water molecules in food).
  • Danger: Can cause internal heating of body cells. This is why strict safety controls are needed on mobile phone transmitters and microwave ovens.
  • Communication Note (3.3 Core 5): Microwaves are used for satellite communication.
Infrared (IR)
  • Use: Electric grills (toasters), remote controllers, intruder alarms, thermal imaging, heating water (solar panels).
  • Danger: Excessive exposure can cause skin burns (you feel IR as heat).
Visible Light
  • Use: Vision, photography, illumination, optical fibres (cable TV, high-speed broadband).
Ultraviolet (UV)
  • Use: Security marking (makes fluorescent ink glow), detecting fake bank notes, sterilising water.
  • Danger: Causes damage to surface cells and eyes, leading to sunburn, skin cancer, and cataracts.
X-rays
  • Use: Medical scanning (imaging bones and teeth), security scanners (baggage).
  • Danger: Highly penetrating; cause mutation or damage to cells in the body. Shielding is required (like lead aprons).
Gamma Rays
  • Use: Sterilising food and medical equipment, detection and treatment of cancer.
  • Danger: Highest energy waves; cause serious mutation or damage to cells. Requires heavy shielding (like thick concrete or lead).

Quick Key Takeaway:

All EM waves travel at \(3.0 \times 10^8 \text{ m/s}\) in a vacuum. As you move from Radio to Gamma, wavelength decreases, and energy (and danger) increases.

Final Summary of Light and Waves

You have now mastered how light behaves: reflecting off mirrors, bending (refracting) through lenses and glass blocks, and sometimes being perfectly trapped (TIR). You also know that visible light is just one part of the huge Electromagnetic Spectrum, which includes everything from radio waves to dangerous gamma rays! Keep practicing those ray diagrams and definitions—you've got this!