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Light & Sound Physics

Interactive exploration of wave phenomena, reflection, refraction, and total internal reflection with real-time simulations

🌈 Light Waves & Optics

3. Light

3.1 Nature of Light, Reflection, and Refraction

🎯 Key Learning Outcomes:

  • 3.14 Know that light waves are transverse waves and that they can be reflected and refracted
  • 3.16 Draw ray diagrams to illustrate reflection and refraction

Light waves are transverse waves. Like other waves, they exhibit several key behaviours:

  • Reflection: Light bounces off a surface.
  • Refraction: Light bends when it passes from one medium to another (e.g., air to glass) due to a change in its speed.

🅌 The Electromagnetic Spectrum

Light is part of the electromagnetic spectrum. Visible light is just a small portion:

Radio
Micro­wave
Infrared
Visible
UV
X-Ray
Gamma
← Low frequency / Long wavelength High frequency / Short wavelength →

3.2 The Law of Reflection

🎯 Key Learning Outcome:

  • 3.15 Use the law of reflection (the angle of incidence equals the angle of reflection)

When light reflects off a smooth surface, the angle of incidence (i) equals the angle of reflection (r). Both angles are measured from the normal — an imaginary line perpendicular to the surface.

⚡ Interactive Reflection Simulator

Angle of Incidence (i):
45°

3.3 Refraction and Refractive Index

🎯 Key Learning Outcomes:

  • 3.17 Practical: investigate the refraction of light using rectangular blocks, semi-circular blocks and triangular prisms
  • 3.18 Know and use the relationship: n = sin i / sin r (or n₁sinθ₁ = n₂sinθ₂)
  • 3.19 Practical: investigate the refractive index of glass using a glass block

When light enters a different medium at an angle, it changes speed and direction:

  • Entering a denser medium (air→glass) — slows down, bends towards the normal
  • Entering a less dense medium (glass→air) — speeds up, bends away from the normal

⚡ Interactive Refraction Simulator

Angle of Incidence (i):
45°
Medium 2:

Angle of refraction: —

n = sin(i) / sin(r)

General form: n₁ sin(θ₁) = n₂ sin(θ₂)

🧮 Snell’s Law Calculator

🔬 Practical: Investigating the Refractive Index of Glass (3.19)

Use a rectangular glass block, ray box, plain paper, protractor, and ruler to experimentally determine the refractive index.

🎯 Aim:

To determine the refractive index of a rectangular glass block.

🔧 Apparatus:
  • Rectangular glass block
  • Ray box with a single slit (or laser pointer)
  • Sheet of plain white paper (A4 or larger)
  • Sharp pencil, Protractor (full 360° preferred), Ruler (30 cm)
📋 Method:
  1. Place the glass block on paper, trace its outline. Label sides AB, BC, CD, DA.
  2. Remove the block. Choose a point O on side AB. Draw a normal at O, extending into the outline.
  3. Draw an incident ray at O making angle i (e.g., 30°) with the normal.
  4. Replace the glass block exactly on its outline.
  5. Shine a narrow ray along the drawn incident ray. Mark two points (P₁ and P₂) on the emergent ray.
  6. Remove the block. Draw the emergent ray through P₁ and P₂ to meet the outline at point E.
  7. Draw line OE (refracted ray inside glass). Measure angle r between OE and the normal.
  8. Calculate n = sin(i) / sin(r). Repeat for 4+ different angles. Average the results.
💡 Tips for Accuracy:
  • Use a sharp pencil — thick lines add uncertainty
  • Replace the block precisely on its outline each time
  • Mark emergent ray points as far apart as possible
  • Avoid very small (<20°) or very large (>70°) angles

3.4 Total Internal Reflection (TIR) and Critical Angle

🎯 Key Learning Outcomes:

  • 3.20 Describe the role of total internal reflection in transmitting information along optical fibres and in prisms
  • 3.21 Explain the meaning of critical angle c
  • 3.22 Know and use: sin c = 1/n (or sin c = n₂/n₁)

When light travels from a denser to a less dense medium, it bends away from the normal. The critical angle (c) is the angle of incidence at which the refracted ray travels along the boundary (r = 90°).

If the angle of incidence exceeds the critical angle, Total Internal Reflection (TIR) occurs — all light reflects back into the denser medium.

⚡ Interactive TIR Simulator

Angle of Incidence in Denser Medium:
30°

Glass (n₁ = 1.5) → Air (n₂ = 1.0)  •  Critical angle: 41.8°

Status: Normal refraction — Angle of refraction: —

sin(c) = n₂ / n₁    (where n₁ > n₂)

🧮 Critical Angle / Refractive Index Calculator

🌈 Prism Dispersion

When white light passes through a glass prism, different wavelengths (colours) refract by different amounts, separating the light into a spectrum. This is called dispersion.

⚡ Prism Dispersion Simulation

🚀 Applications of TIR

  • Optical Fibres: Light is sent down thin glass or plastic fibres. TIR keeps the light inside even when bent. Used for internet data and medical endoscopes.
  • Prisms: Right-angled prisms use TIR to turn light 90° or 180°, useful in periscopes and binoculars.

⚡ Optical Fibre — TIR in Action

Light bounces inside the fibre core via Total Internal Reflection, keeping the signal contained.

🔊 Sound Waves

4. Sound

🎯 Key Learning Outcome:

  • 3.23 Know that sound waves are longitudinal waves which can be reflected and refracted

Sound waves are longitudinal waves — the vibrations of particles are parallel to the direction of energy transfer. They consist of compressions (high pressure) and rarefactions (low pressure).

Like light, sound can be:

  • Reflected: Sound reflection is an echo. Hard, smooth surfaces are good reflectors.
  • Refracted: Sound bends when passing between media or when conditions change (e.g., temperature differences in air).

⚡ Longitudinal Sound Wave Simulator

Frequency:
0.8
Amplitude:
0.5

C = Compression (particles close together, high pressure)  •  R = Rarefaction (particles spread apart, low pressure)

🔊 Sound Reflection (Echo)

When sound hits a hard, flat surface, it bounces back. If the reflecting surface is far enough away (>17 m), you hear the echo as a separate sound.

Sound reflects off hard surfaces just like light reflects off mirrors. This principle is used in sonar and ultrasound imaging.

🌊 Wave Properties

Wave Fundamentals

All waves share common properties: amplitude, wavelength, frequency, and wave speed.

Wave Speed:   v = f × λ

where v = speed (m/s), f = frequency (Hz), λ = wavelength (m)

⚡ Interactive Transverse Wave Simulator

Amplitude (A):
50 px
Frequency (f):
1.0 Hz
Wavelength (λ):
150 px

Wave speed v = f × λ = —

The wave moves to the right. Adjust amplitude, frequency, and wavelength to see how they affect the wave. Notice: changing frequency or wavelength changes the wave speed.

🧮 Wave Speed Calculator (v = f × λ)

🧠 Knowledge Assessment

Knowledge Check: Light & Sound

1. The law of reflection states that:

2. If light travels from air (n≈1) into glass (n≈1.5), what happens?

3. Total Internal Reflection occurs when light travels from:

4. Sound waves are:

5. If n₁ = 2.0 and n₂ = 1.0, what is the critical angle? (sin 30° = 0.5)

6. Optical fibres work because of:

7. When white light passes through a prism, it separates into colours because:

8. The wave equation v = fλ tells us that:

© Light & Sound Physics — Interactive Learning Resource