Lesson 5: Physics: Magnetism, EMI and Optics
A moving charge creates a magnetic field, and a changing magnetic field creates an electric field: that pair of facts links magnetism, electromagnetic induction and alternating current into one story. Optics then asks what light does at boundaries — reflecting, refracting, and forming images through lenses and mirrors.
This lesson covers the magnetic force on a current, Faraday's law, inductance in general terms, and the lens and mirror formulas, with the sign conventions that decide whether an image is real or virtual.
Magnetic force and electromagnetic induction
A straight conductor of length L carrying current I perpendicular to a field B experiences F = BIL. Faraday's law says the induced electromotive force equals the rate of change of magnetic flux, e = −N ΔΦ/Δt for N turns; Lenz's law supplies the minus sign, meaning the induced effect opposes the change that caused it.
A 0.4 m conductor carrying 2 A perpendicular to B = 0.5 T experiences F = 0.5 × 2 × 0.4 = 0.4 N.
A coil of 50 turns whose flux changes by 0.04 Wb in 0.2 s has induced emf of magnitude 50 × 0.04 / 0.2 = 50 × 0.2 = 10 V.
If the flux were increasing into the coil, Lenz's law says the induced current creates a field opposing that increase.
Lenses, mirrors and power
For a thin lens or a mirror, 1/v − 1/u = 1/f in the usual Cartesian sign convention, with u negative for a real object on the incident side; many textbooks write the lens formula as 1/v − 1/u = 1/f and the mirror formula as 1/v + 1/u = 1/f, so check which convention your question uses before substituting. Magnification is m = v/u for a lens (with sign conventions applied). Lens power in dioptres is P = 1/f, with f in metres.
A convex lens has f = 20 cm and an object is placed 30 cm away: 1/v = 1/20 + 1/( −30) in one common sign form, giving 1/v = (3 − 2)/60 = 1/60, so v = 60 cm on the opposite side.
Magnification magnitude is |v/u| = 60/30 = 2, so the image is twice the object's height.
A lens of focal length 0.25 m has power P = 1/0.25 = 4 dioptres.
A mirror of focal length 15 cm with an object 30 cm away gives 1/v = 1/15 − 1/30 = 1/30, so v = 30 cm: the image forms at the centre of curvature. Total internal reflection occurs only when light travels from a denser to a rarer medium at an angle above the critical angle.
Common pitfalls
Do not drop the sign in Faraday's law: the minus sign is Lenz's law, and direction questions test it directly.
Do not mix lens and mirror sign conventions: check whether the question's formula uses 1/v − 1/u or 1/v + 1/u before you substitute numbers.
Further study
- NCERT Class 12 Physics, chapters on moving charges and magnetism, electromagnetic induction, alternating current, and ray optics.
📝 Quiz
1. A 0.4 m conductor carrying 2 A perpendicular to B = 0.5 T experiences what force?
Show answer
b) F = 0.5 × 2 × 0.4 = 0.4 N.
2. A 50-turn coil whose flux changes by 0.04 Wb in 0.2 s has what induced emf magnitude?
Show answer
b) 50 × 0.04 / 0.2 = 10 V.
3. A lens of focal length 0.25 m has what power?
Show answer
b) P = 1/0.25 = 4 dioptres.
4. A convex lens (f = 20 cm) with an object at 30 cm forms an image at what distance?
Show answer
b) 1/v = 1/20 − 1/30 in magnitude form gives 1/60, so v = 60 cm.