Natural Phenomena
What this domain tests
Natural Phenomena covers light - reflection, refraction, lenses and the human eye - together with electricity and magnetic effects of current: Ohm's law, resistor networks, heating effect, field lines and induction.
Core rules
- Law of reflection: angle of incidence equals angle of reflection. A plane-mirror image is virtual, behind the mirror, same size and laterally inverted.
- Mirror formula 1/v + 1/u = 1/f with m = –v/u; lens formula 1/v – 1/u = 1/f with m = v/u.
- Power of a lens P = 1/f in metres, measured in dioptres (D); a converging (convex) lens has positive power.
- Refractive index n = c/v; light bends toward the normal on entering a denser medium.
- The cornea provides most refraction; the lens fine-focuses by accommodation. Myopia is corrected by a concave lens, hypermetropia by a convex lens.
- Ohm's law V = IR at constant temperature. Series: R = R₁ + R₂, same current. Parallel: 1/R = 1/R₁ + 1/R₂, same voltage.
- Heating effect H = I²Rt, used in fuses and filaments. Right-hand thumb rule gives the field direction around a straight wire; Fleming's left-hand rule applies to motors, the right-hand rule to generators.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across 12 V. Voltage is the same: I₁ = 12/6 = 2 A and I₂ = 12/3 = 4 A, so total current is 6 A. Equivalent resistance = 12/6 = 2 Ω, matching 1/6 + 1/3 = 1/2.
Traps
- Distances measured opposite to the incident light are taken as negative.
- Convert focal length to metres before finding power: 50 cm gives 0.5 m, so P = 2 D.
- Series divides voltage; parallel divides current.
- Myopia needs a concave lens, not convex - a frequent one-mark loss.