Light

When light hits a smooth surface: Angle of incidence = Angle of reflection (both measured from the normal). Incident ray, reflected ray and normal all lie in the same plane. Regular reflection: smooth mirror → clear image. Diffuse reflection: rough surface → scattered light, no clear image but whole surface visible. Plane mirror: image is virtual, erect, same size, laterally inverted, at same distance behind mirror.

Concave mirror: curved inward (reflecting surface inside). Can form real, inverted images (when object far) or virtual, magnified (close up). Used in: torches (light converges), doctors (ear/dental examination), solar cookers. Convex mirror: curved outward (reflecting outside). Always forms virtual, erect, diminished image. Used in: rear-view mirrors (wide field of view).

Refraction: bending of light as it passes from one medium to another. Happens because light changes speed. From rarer to denser medium (air to glass): bends towards normal (slows down). From denser to rarer (glass to air): bends away from normal. Real-life refraction: pencil in water appears bent, coins in water appear raised, twinkling of stars.

Convex lens: converges light, thicker in middle. Forms real inverted image (far objects) or virtual magnified (close — magnifying glass). Used in cameras, projectors, spectacles for far-sightedness. Concave lens: diverges light, thinner in middle. Forms virtual, erect, diminished image. Used for near-sightedness. Human eye: cornea+lens focuses image on retina. Retina converts to nerve signals.