TG POLYCET Physics: Light, Electricity and Everyday Applications
TG POLYCET Physics draws on Telangana SSC physical science: reflection at curved surfaces, refraction at plane and curved surfaces, lenses, the human eye, electric current and electromagnetism. Numerical questions usually test one formula plus a sign convention, so precise substitution matters more than lengthy derivation.
Core rules
- Mirror formula: 1/f = 1/v + 1/u. Distances are measured from the pole; use the sign convention consistently. Magnification m = −v/u = hᵢ/h₀.
- Lens formula: 1/f = 1/v − 1/u; magnification m = v/u. Power P = 1/f, with f in metres; the unit is the dioptre (D).
- Ohm's law: V = IR. Series: R = R₁ + R₂ + … Parallel: 1/R = 1/R₁ + 1/R₂ + …
- Electrical power: P = VI = I²R = V²/R. Energy consumed = power × time.
- Electromagnetism: right-hand thumb rule gives the field direction around a current-carrying conductor; Fleming's left-hand rule applies to a motor, Fleming's right-hand rule to a generator.
- Human eye: near point about 25 cm, far point infinity. Myopia is corrected by a concave lens, hypermetropia by a convex lens; presbyopia is age-related loss of accommodation.
Worked example
A concave mirror of focal length 15 cm has an object 10 cm in front of it. Take f = −15 cm and u = −10 cm. From 1/v = 1/f − 1/u = (−1/15) − (−1/10) = −1/15 + 1/10 = 1/30, so v = +30 cm. The positive v means a virtual image behind the mirror; m = −v/u = −30/(−10) = +3. The image is virtual, erect and three times magnified.
Traps
Using the lens formula for a mirror; forgetting that concave-mirror focal length is negative in the standard convention; leaving f in centimetres when calculating power; adding parallel resistances directly instead of using reciprocals; confusing left-hand and right-hand rules.