Module 2.4-2.5 Optics, wave motion and sound

Knowledge levels

Sub-module	A	B1	B2 / B2L	B3
2.4 Optics (light)	—	2	2	—
2.5 Wave motion and sound	—	2	2	—

Only B1, B2 and B2L candidates are examined on these sub-modules.

Nature and speed of light

• Light is electromagnetic radiation visible to the eye, wavelength roughly 400 nm (violet) to 700 nm (red). It behaves both as a wave and as particles (photons).
• Speed of light in a vacuum c ≈ 3 × 10⁸ m/s (299 792 km/s). It is slower in denser media.
• Refractive index n = c / v (speed in vacuum ÷ speed in the medium). Glass ≈ 1.5; water ≈ 1.33.

Reflection

• Law of reflection: angle of incidence = angle of reflection, both measured from the normal (a line perpendicular to the surface), and incident ray, reflected ray and normal lie in one plane.
• Plane mirror: image is virtual, upright, same size, laterally inverted and as far behind the mirror as the object is in front.

Spherical mirrors

• Concave (converging) mirror: parallel rays meet at the principal focus F; focal length f = R/2 (R = radius of curvature). Can form real, inverted images. Used in landing lights and reflectors (lamp at the focus gives a parallel beam).
• Convex (diverging) mirror: always forms a virtual, upright, smaller image; wide field of view (inspection mirrors, wide-angle mirrors).