Chapter 9 Light Reflection Refraction

The laws of reflection govern how light bounces off a surface. The first law states that the angle of incidence (angle between the incident ray and normal) equals the angle of reflection (angle between the reflected ray and normal). The second law states that the incident ray, reflected ray, and normal at the point of incidence all lie in the same plane. These laws apply to both plane mirrors and curved mirrors. In a plane mirror, the image is virtual (cannot be projected on a screen), erect, of the same size, and laterally inverted (left-right reversed). Concave mirrors follow these laws too — they form different types of images (real or virtual) depending on the object's position. Concave mirrors are used in torches and headlights because they converge reflected light into a beam.

A concave mirror (converging mirror) has its reflecting surface curved inward, like the inside of a spoon. It can form both real and virtual images depending on where the object is placed. When the object is beyond the centre of curvature, a real, inverted, and diminished image is formed. When the object is between the pole and focus, a virtual, erect, and magnified image is formed. Concave mirrors are used in torches, searchlights, solar furnaces, and as shaving or make-up mirrors. A convex mirror (diverging mirror) has its reflecting surface curved outward. It always forms a virtual, erect, and diminished image regardless of the object's position. This gives a wider field of view, which is why convex mirrors are used as rear-view mirrors in vehicles and as security mirrors in shops.

Refraction is the bending of light when it travels from one medium to another with a different optical density. This happens because light changes speed at the interface of two media. The laws of refraction (Snell's Law) state that the incident ray, refracted ray, and normal at the point of incidence lie in the same plane, and that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media (n = sin i / sin r). When light travels from air (less dense) to water (more dense), it slows down and bends toward the normal. When it enters air from water, it speeds up and bends away from the normal. This is why a pencil half-dipped in water appears bent, why pools look shallower than they are, and why stars appear to twinkle.

A convex lens (converging lens) is thicker at the centre and brings parallel rays of light to a point called the principal focus. The image formed depends on object position. If the object is beyond 2F, the image is real, inverted, and diminished (used in cameras). At 2F, the image is real, inverted, and same size. Between F and 2F, the image is real, inverted, and magnified (projectors). Within F, the image is virtual, erect, and magnified (magnifying glass). A concave lens (diverging lens) is thinner at the centre and always diverges light, forming a virtual, erect, and diminished image regardless of object position. Ray diagrams use three standard rays: one parallel to principal axis (refracts through focus), one through optical centre (goes straight), and one aimed at focus (emerges parallel). These diagrams help determine the nature and position of the image.

The mirror formula relates the object distance (u), image distance (v), and focal length (f) of a curved mirror: 1/v + 1/u = 1/f. The sign convention used is the New Cartesian Convention where distances measured in the direction of incident light are positive and against it are negative. The lens formula is similar: 1/v – 1/u = 1/f, where the same sign convention applies. Magnification for a mirror is m = –v/u and for a lens m = v/u. If magnification is positive, the image is erect; if negative, it is inverted. The power of a lens (P = 1/f, measured in dioptres) indicates how strongly it converges or diverges light. Positive power means converging (convex lens) and negative means diverging (concave lens). These formulas are crucial for solving numerical problems in Chapter 9.