Light – Reflection and Refraction

Reflection of light is the phenomenon where light bounces back after striking a surface. It follows two laws: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray, and normal all lie in the same plane. For example, a mirror reflects light to form images.

Refraction is the bending of light when it passes from one medium to another due to a change in its speed. This occurs because light travels at different speeds in different media. For instance, a straw appears bent in a glass of water due to refraction.

The focal length (f) of a spherical mirror is half its radius of curvature (R), given by the formula f = R/2. For example, if a mirror's radius of curvature is 20 cm, its focal length is 10 cm.

Concave mirrors curve inward and can form real or virtual images depending on the object's position. Convex mirrors curve outward and always form virtual, diminished images. Concave mirrors are used in torches, while convex mirrors are used as rear-view mirrors.

Convex mirrors provide a wider field of view and always give a diminished, erect image, making them ideal for seeing more traffic behind the vehicle. This enhances safety by allowing drivers to see a larger area than with a plane mirror.

The refractive index (n) is the ratio of the speed of light in vacuum (c) to its speed in the medium (v), given by n = c/v. For example, the refractive index of water is 1.33, meaning light travels slower in water than in air.

The lens formula is 1/f = 1/v - 1/u, where f is focal length, v is image distance, and u is object distance. It helps determine the position and nature of the image formed by a lens. For instance, if u = -20 cm and f = 10 cm, v can be calculated to find the image location.

Magnification (m) is the ratio of image height to object height or image distance to object distance (m = h'/h = v/u). A positive m indicates an erect image, while a negative m indicates an inverted image. For example, m = -2 means the image is inverted and twice the object's size.

The power of a lens (P) is the reciprocal of its focal length in meters, given by P = 1/f. It is measured in dioptres (D). A convex lens with f = 0.5 m has P = +2 D, indicating its converging ability.

A concave lens diverges light rays, causing them to appear to come from a point on the same side as the object. Since the rays do not actually meet, the image is always virtual, erect, and diminished, regardless of the object's position.

Concave mirrors are used in torches and headlights to produce parallel beams of light, in shaving mirrors for magnified images, and by dentists to examine teeth. They are also used in solar furnaces to concentrate sunlight for heating.

When light enters a glass slab, it bends towards the normal at the air-glass interface and away from the normal at the glass-air interface. The emergent ray is parallel to the incident ray but laterally displaced, demonstrating refraction without deviation in direction.

The principal focus is the point where parallel rays of light converge (convex lens) or appear to diverge (concave lens) after refraction. It determines the lens's focal length and is crucial for image formation calculations, such as using the lens formula.

Focus sunlight or a distant object onto a screen using the lens. Measure the distance between the lens and the sharp image formed on the screen. This distance equals the focal length, as parallel rays converge at the focus.

Light bends away from the normal when moving from a denser to a rarer medium, as its speed increases. For example, light exiting water into air bends away from the normal, making objects underwater appear shallower than they are.

Diamond has a high refractive index (2.42) because light travels much slower in diamond than in air. This high refractive index causes significant bending of light and intense dispersion, giving diamond its characteristic sparkle.

Real images are formed when light rays actually converge and can be projected on a screen, like those from a concave mirror. Virtual images appear to form where light rays seem to diverge and cannot be projected, like those in a plane mirror.

If the object is beyond 2F, the image is real, inverted, and diminished. Between F and 2F, the image is real, inverted, and enlarged. Inside F, the image is virtual, erect, and enlarged. At 2F, the image is real, inverted, and same-sized.

Snell’s law states 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: sin i / sin r = constant. This constant is the refractive index of the second medium relative to the first.

The pencil appears bent due to refraction. Light from the submerged part travels from water (denser) to air (rarer), bending away from the normal. This creates a visual displacement, making the pencil seem bent at the water's surface.

The optical centre is the midpoint of a lens where light passes undeviated. Rays passing through it do not bend, making it a reference point for ray diagrams. It helps in determining the path of light through the lens for image formation.

A concave lens diverges light rays before they enter the eye, shifting the focal point backward onto the retina. This compensates for the excessive converging power of a myopic eye, allowing distant objects to be seen clearly.

Lateral inversion is the left-right reversal of an image in a plane mirror. For example, raising your right hand appears as raising the left hand in the mirror. This occurs because the mirror reflects light symmetrically about the vertical axis.

Convex lenses converge light rays to form magnified, virtual images when the object is placed within their focal length. This magnification helps in viewing small details, making them ideal for reading small print or examining tiny objects.

The focal length (f) of a spherical mirror is half its radius of curvature (R), expressed as f = R/2. For example, a mirror with R = 30 cm has f = 15 cm. This relationship is crucial for mirror formula applications.