Unit 2: How Does Physics Help Us Understand the World?
Core concepts
Unit 2 widens the lens to waves, light, and the physics of motion in fields. You need the wave equation v = fλ, the ideas of reflection, refraction, diffraction and interference, and how the electromagnetic spectrum is ordered by frequency and wavelength. Understand Snell's law, total internal reflection and the critical angle, and how lenses form real and virtual images using ray diagrams and the thin-lens equation.
For mechanics in fields, distinguish mass from weight, describe projectile motion as independent horizontal and vertical components, and analyse uniform circular motion using centripetal acceleration directed toward the centre. Understand that a net force is required to change direction even at constant speed. Gravitational field strength near Earth is approximately 9.8 N kg⁻¹.
Traps examiners set
- Confusing the direction of light bending: toward the normal when entering a denser medium.
- Forgetting that frequency is unchanged on refraction while speed and wavelength change.
- Treating a virtual image as if it could be projected onto a screen.
- Assuming a projectile's horizontal velocity changes during flight - it does not, ignoring air resistance.
- Thinking centripetal force is an extra outward or inward force rather than the net force.
Worked example
Light travels from water (n = 1.33) into air. Find the critical angle.
At the critical angle, the refracted ray grazes the surface at 90°. Snell's law: n₁ sin θc = n₂ sin 90° = 1. So sin θc = 1/1.33 ≈ 0.752, giving θc ≈ 48.8°. Total internal reflection occurs only when light travels from the denser to the less dense medium and the angle exceeds this value.
Rule of thumb
For any wave problem, write v = fλ, identify which quantity is fixed across a boundary (frequency), and then track the others.