Physics: Light, Electricity and Magnetism
Physics in the Odisha HSC General Science paper tests light - reflection, refraction, lenses and the human eye - together with electricity, magnetic effects of current and sources of energy. Numericals are short, but sign and unit errors are heavily penalised, so state the sign convention you use before substituting values.
Mirror and lens formulae
For spherical mirrors, 1/v + 1/u = 1/f and magnification m = −v/u. For lenses, 1/v − 1/u = 1/f and m = v/u. Distances are measured from the pole of a mirror or the optical centre of a lens; distances measured against the incident light are taken as negative. The focal length of a convex mirror or lens is positive, of a concave one negative. Lens power P = 1/f with f in metres, unit dioptre (D): a focal length of 20 cm gives +5 D.
Electricity and magnetism
Ohm's law states V = IR at constant temperature. In series, R = R₁ + R₂ + …; in parallel, 1/R = 1/R₁ + 1/R₂ + …. The heating effect is H = I²Rt. Magnetic field lines run from north to south outside a magnet, never cross, and crowd where the field is strong. The right-hand thumb rule gives the field direction around a current-carrying conductor; Fleming's left-hand rule gives the force on a conductor placed in a magnetic field.
Worked example
A convex lens of focal length 10 cm has an object placed 15 cm in front of it. Find the image position and magnification. Here u = −15 cm and f = +10 cm. So 1/v = 1/f + 1/u = 1/10 − 1/15 = (3 − 2)/30 = 1/30, giving v = +30 cm. Then m = v/u = 30/(−15) = −2. The image is real, inverted and twice the object's height, formed 30 cm on the far side of the lens.
Traps that cost marks
Mixing the mirror and lens formulae; dropping the minus sign in magnification; converting cm to m in one step but not another; writing 1/R = R₁ + R₂ for a parallel combination; confusing conventional current direction with electron flow.