W.2 Sound and Light

UNIT SYLLABUS

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

Index of Refraction

n = \frac{c}{v}

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.

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’.

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

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

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