CBSE Class 10 Science Light Reflection and Refraction Notes Set 04

Light

Light is a form of energy that enables us to see. It behaves as a ray (reflection), a wave (interference, diffraction) and a particle (photoelectric effect). The speed of light in vacuum is about 3 × 10⁸ m/s. Light travels in a straight line (rectilinear propagation), taking the path of least time (Fermat's principle). When light falls on another medium it may be absorbed, reflected or refracted.

Reflection and Plane Mirror

Laws of reflection: (1) the incident ray, the reflected ray and the normal lie in the same plane; (2) the angle of incidence equals the angle of reflection (∠i = ∠r). A ray that is reversed retraces its path (principle of reversibility).

A plane mirror forms an image that is virtual, erect, laterally inverted and as far behind the mirror as the object is in front. An image is real if the rays actually meet (it can be shown on a screen) and virtual if they only appear to meet.

Spherical Mirrors

A concave mirror curves inwards; a convex mirror curves outwards. Rules for ray diagrams: a ray parallel to the principal axis passes through (or appears to come from) the focus; a ray through the centre of curvature retraces its path; a ray through the focus goes out parallel to the axis.

Images by a Concave Mirror

A convex mirror always forms a virtual, erect and diminished image behind the mirror. Concave mirrors are used in torches, shaving mirrors and solar cookers; convex mirrors as rear-view mirrors because they give a wider field of view.

Mirror Formula and Magnification

Sign convention: distances are measured from the pole; those along the direction of the incident light are positive and those against it are negative. Heights above the principal axis are positive and below are negative.

Mirror formula: 1/v + 1/u = 1/f. Magnification: m = h'/h = −v/u. A negative m means a real, inverted image; a positive m means a virtual, erect image.

Refraction

The bending of light when it passes from one transparent medium to another is refraction. Laws: (1) the incident ray, the refracted ray and the normal lie in the same plane; (2) sin i / sin r is constant for a given pair of media and a given colour of light (Snell's law). Light bends towards the normal going from a rarer to a denser medium and away from the normal going from a denser to a rarer medium. Through a rectangular glass slab the emergent ray is parallel to the incident ray but is laterally displaced.

Refractive index: n = sin i / sin r = speed of light in medium 1 / speed of light in medium 2. The refractive index with respect to vacuum is the absolute refractive index.

Total internal reflection: when light goes from a denser to a rarer medium and the angle of incidence is greater than the critical angle, it is reflected back into the denser medium. Mirages and optical fibres are examples.

Spherical Lenses

A convex lens is thicker in the middle and converges light; a concave lens is thinner in the middle and diverges light. Terms: optical centre (P), principal axis, centres of curvature, principal focus (F) and focal length. Rules: a ray parallel to the principal axis passes through (or appears to come from) the focus; a ray through the optical centre goes straight without deviation.

Lens formula: 1/v − 1/u = 1/f. Magnification: m = h'/h = v/u. Power of a lens: P = 1/f (f in metres); the SI unit is the dioptre (D). A convex lens has positive power and a concave lens negative power. Lenses are used in spectacles, magnifying glasses, microscopes, binoculars and telescopes.