Light, Sound and Electricity
Light, sound and electricity is the largest physics block: mirror and lens calculations, sound, and Ohm's law circuits, often with a heating or magnetic effect item.
Reflection and mirrors
Angle of incidence = angle of reflection. A plane mirror gives a virtual, erect, laterally inverted image the same size as the object. Concave mirrors converge (torches, dentists); convex mirrors diverge and give a wide field of view (vehicle mirrors). Mirror formula: 1/v + 1/u = 1/f, with f = R/2; magnification m = −v/u.
Refraction and lenses
Light bends towards the normal on entering a denser medium. Snell's law: sin i / sin r = constant (refractive index); water is about 1.33, glass about 1.5. A convex lens converges; 1/v − 1/u = 1/f, and power P = 1/f in metres, in dioptres. A prism disperses white light into seven colours because each colour has its own refractive index.
Sound
Sound needs a material medium, so it cannot travel in a vacuum. It is longitudinal in air. Speed is highest in solids, lowest in gases. Frequency determines pitch; amplitude determines loudness; the audible range is 20 Hz to 20,000 Hz. An echo is reflected sound; the minimum distance for a distinct echo is about 17 m.
Electricity
Current = charge/time, in amperes. Ohm's law: V = IR at constant temperature. Resistors in series add; in parallel, 1/R = 1/R₁ + 1/R₂. Heating effect: H = I²Rt. A fuse is a safety device connected in series; an electromagnet uses the magnetic effect, and Fleming's left-hand rule gives the direction of force on a current-carrying conductor.
Worked example
Two 6 Ω resistors in parallel across 12 V. 1/R = 1/6 + 1/6 = 1/3, so R = 3 Ω. Then I = V/R = 12/3 = 4 A.
Traps
Using the mirror formula for a lens; ignoring the sign convention when substituting u; saying sound travels fastest in air; forgetting to invert 1/R for a parallel combination.