Physics · Wave Optics · NEET
All directions, not none. Unpolarised light (like sunlight or a bulb) has an electric field that keeps changing its direction very fast and randomly, but it always stays in the plane perpendicular (at right angles) to the direction the light travels. So at any instant it points somewhere, but over a tiny time it has pointed everywhere in that transverse plane equally. NCERT (Points to Ponder) says: the electric vector takes all possible directions in the transverse plane, rapidly and randomly, during a measurement.
It filters. A polaroid does not add anything new; it blocks all the vibration directions except one. NCERT says a polaroid has long chain molecules aligned in one direction, and the electric vectors along that molecule direction get absorbed. Only the component along the pass-axis (the special allowed direction) passes through. So the light coming out is now polarised, but the polaroid simply removed the other components.
Because unpolarised light has equal energy shared over all directions in the transverse plane. On average, half of the electric-field vibration lines up with the pass-axis and half does not, so half is transmitted. NCERT states directly: the intensity coming out of a single polaroid is half of the incident intensity. Note this exact 'half' rule applies only to unpolarised input. If the input is already polarised, use Malus's law I = I0 cos^2(theta) instead.
No. Only transverse waves can be polarised. Sound in air is a longitudinal wave (the air particles vibrate back and forth ALONG the direction the sound travels), so there is no sideways direction to restrict. Light is a transverse wave (electric field vibrates at right angles to the ray), which is exactly why light CAN be polarised. This is why polarisation is treated as the direct proof that light is transverse.
The electric field vector. A light wave is an electromagnetic wave, and NCERT says the electric field associated with a propagating light wave is always at right angles to the direction of propagation. When we polarise light, we fix the direction of this electric field. We usually talk only about the electric field (not the magnetic field) because the electric field is the part that interacts with detectors, our eyes, and photographic film.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Polarised light has its electric field vibrating in one fixed direction; unpolarised light has it vibrating in all transverse directions randomly.
Natural light from the sun, a bulb, or a flame is unpolarised. It becomes polarised only after passing through a polaroid, or after reflection or scattering under the right conditions.
Yes. After one polaroid, the light is linearly (plane) polarised along the pass-axis, and its intensity drops to half of the incident value.
Light is a transverse wave, so it has vibration directions perpendicular to its travel that can be restricted. Sound in air is longitudinal, with no such sideways direction, so it cannot be polarised.
A polaroid is a thin sheet of long chain molecules aligned in one direction. It absorbs the electric-field component along the molecules and lets the perpendicular component (pass-axis) through.
