Physics - waves, light, and sound
What the exam tests
Wave properties, reflection, refraction, lenses and sound. Questions often use ray diagrams and the relationship between speed, frequency and wavelength.
Core rules
- Wave speed: v = fλ. Frequency is set by the source and does not change when a wave enters a new medium.
- Reflection: angle of incidence equals angle of reflection, measured from the normal.
- Refraction: light bends toward the normal when entering a denser, slower medium, and away when leaving.
- Sound needs a medium, travels faster in solids, and cannot travel through a vacuum. Higher frequency means higher pitch; larger amplitude means louder sound.
- A convex lens can form a real, inverted image or a virtual, magnified image; a concave lens forms a virtual, diminished image.
Worked example
A sound wave has frequency 170 Hz and speed 340 m/s. λ = v/f = 340/170 = 2.0 m. An echo from a wall 85 m away travels 170 m there and back, so the delay is 170/340 = 0.50 s. For light entering water, the frequency stays the same but the speed and wavelength decrease.
Traps
- For echoes, double the distance before dividing by speed.
- Do not say frequency changes on refraction; speed and wavelength change.
- Measure angles from the normal, not from the surface.
- A virtual image cannot be projected onto a screen.