MYP Physics

W.2 Sound and Light

Exploring properties of sound, electromagnetic spectrum, reflection, refraction, and image formation in lenses.

Questions to explore

? Why does a straw look bent in a glass of water?
? How does the same family of waves both cook food and diagnose disease?

💡 Key ideas, explained simply

🔦 Reflection and refraction

Reflection bounces a wave off a surface — the angle in equals the angle out (both measured from the *normal*, the line at 90° to the surface).

Refraction bends a wave as it changes speed entering a new material. Light slows down going into glass, so it bends *towards* the normal — which is why a straw looks broken in water.

🎵 Pitch, loudness and the EM family

For sound, higher frequency = higher pitch, and bigger amplitude = louder.

Light is part of the electromagnetic spectrum — a family from radio waves to gamma rays. They all travel at the same speed in a vacuum ( $3\times10^8\text{ m/s}$ ) but differ in frequency and energy.

The electromagnetic spectrum (low → high frequency)

Wave	Everyday use	Watch out
Radio	TV & radio broadcasts	Very low energy
Microwave	Cooking, satellite, Wi-Fi	Heats water in food
Infrared	Remote controls, thermal imaging	Felt as heat
Visible	Seeing, photography	—
Ultraviolet	Sterilising, detecting fakes	Causes sunburn
X-ray	Medical & security imaging	Ionising — damages cells
Gamma	Sterilising, cancer treatment	Most ionising

📖 Key terms

Normal: A construction line drawn at 90° to a surface, used to measure angles.
Angle of incidence: Angle between the incoming ray and the normal.
Refraction: Bending of a wave as it changes speed between materials.
Refractive index $n$: How much a material slows light, $n = \frac{c}{v}$ .
Pitch: How high or low a sound is — set by frequency.
EM spectrum: The full family of electromagnetic waves, radio to gamma.

1 Key Formulas

Index of Refraction

n = \frac{c}{v}

✏️ Worked example

Refractive index

Light travels at $2.0\times10^8\text{ m/s}$ in glass. Find the refractive index. ( $c = 3.0\times10^8\text{ m/s}$ )

1

Write the formula

$n = \dfrac{c}{v}$
2

Substitute

$n = \dfrac{3.0\times10^8}{2.0\times10^8}$

✓

n = 1.5 — glass slows light to two-thirds of its vacuum speed.

🔬

Interactive Lab

Sound Wave Lab

See how a longitudinal sound wave compresses and stretches the medium it travels through.

→

🌈 Physics around you

A rainbow is refraction in action: raindrops bend and split sunlight into its colours because each colour slows down by a slightly different amount. The same effect makes a diamond sparkle.

🎯 Nail it in the exam

Reflection and Refraction Diagrams

MYP exams heavily assess your ability to draw and interpret ray diagrams.

Reflection (plane mirror):

- Draw normal (dashed line) perpendicular to the mirror at the point of incidence.

- Angle of incidence $i$ = angle of reflection $r$ , both measured from the normal.

- Image is virtual, upright, same size, laterally inverted, and as far behind the mirror as the object is in front.

Refraction:

- Ray bends towards the normal when entering a denser medium (air → glass), away from the normal when leaving (glass → air).

- Include normal lines and label angles.

Snell’s Law may be introduced: $n_1 \sin\theta_1 = n_2 \sin\theta_2$ , but simpler index calculations ( $n = c/v$ ) are standard.

Pro Exam Strategy

Use a ruler and a sharp pencil for all diagrams.
Draw arrows on rays to show direction of travel.
If a question asks ‘Explain why the light bends’, mention change in speed, not change in density.
Total internal reflection occurs only when going from denser to rarer medium, with angle of incidence greater than the critical angle.

Electromagnetic Spectrum – Order and Uses

Learn the order: Radio, Microwaves, Infrared, Visible, Ultraviolet, X‑rays, Gamma rays (increasing frequency/energy, decreasing wavelength).

Common past paper connections:

- Radio: communication.

- Microwaves: satellite communication, cooking.

- Infrared: thermal imaging, remote controls.

- Visible: seeing, photography.

- Ultraviolet: sunbeds, detecting forged banknotes.

- X‑rays: medical imaging, security.

- Gamma rays: sterilising medical equipment, cancer treatment.

A ‘discuss’ question might ask you to weigh risks and benefits of a particular wave (e.g., X‑rays in medicine).

Pro Exam Strategy

All EM waves travel at the same speed in vacuum.
The higher the frequency, the more dangerous the radiation (ionising).
Linking wave property to use is a frequent explain question – e.g., ‘Microwaves are used because they are absorbed by water molecules in food’.

🧠 Check your understanding

Tap an answer to see if you're right — and why.

Q1. Which of the following electromagnetic waves has the highest frequency?

Radio waves
Microwaves
Visible light
Gamma rays

Q2. What is the normal line in reflection and refraction diagrams?

A line parallel to the surface
A line perpendicular to the surface at the point of incidence
The incident ray itself
The reflected ray itself

📝 Exam-style questions

Try each one, then reveal the model answer.