CBSE Class 10 Physics Light Reflection and Refraction Notes Set 01

Reflection of Light

Light is an electromagnetic wave and needs no medium. A ray is the path of light. Laws of reflection: (1) the angle of incidence equals the angle of reflection, i = r; (2) the incident ray, reflected ray and normal at the point of incidence lie in the same plane. Angles are measured from the normal. A ray falling along the normal retraces its path.
A plane mirror forms a virtual, erect image, the same size as the object, as far behind the mirror as the object is in front, with left and right interchanged. Its magnification is 1 and its radius of curvature is infinite.

Spherical Mirrors

A concave mirror reflects from the inner curved surface and is converging. A convex mirror reflects from the outer surface and is diverging. Terms: pole P (centre of the mirror), centre of curvature C, radius of curvature R, principal axis (line through P and C), principal focus F, focal length f. The focal length is half the radius: \( f = R/2 \).
Rays parallel to the axis meet at F after reflection from a concave mirror, and appear to come from F for a convex mirror. A ray through C returns along itself, and a ray through F becomes parallel to the axis.

Sign Convention and Mirror Formula

Measure distances from the pole. Distances along the direction of the incident light are positive, against it negative. Heights above the axis are positive. For a concave mirror f is negative; for a convex mirror f is positive.
Mirror formula: \( \dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f} \). Magnification: \( m = \dfrac{h'}{h} = -\dfrac{v}{u} \). A negative m means an inverted (real) image; a positive m means an erect (virtual) image.

A convex mirror always forms a virtual, erect, diminished image between P and F. Uses: concave mirrors in shaving mirrors, doctors' head mirrors, torches and headlights, and solar furnaces; convex mirrors as rear-view mirrors because they give an erect image and a wide field of view.

Refraction of Light

Refraction is the bending of light when it passes from one medium to another because its speed changes. A ray bends towards the normal on entering a denser medium and away from it on entering a rarer one. Frequency never changes; speed and wavelength change together.
Laws: (1) the incident ray, refracted ray and normal lie in one plane; (2) Snell's law, \( \dfrac{\sin i}{\sin r} = \text{constant} = n \) for given media and colour.
Absolute refractive index \( n = \dfrac{c}{v} \) (no unit). The relative index of medium 2 with respect to medium 1 is \( n_{21} = n_2/n_1 \), and \( n_{12} = 1/n_{21} \). In a denser medium light travels slower and its wavelength is shorter by a factor n.
Effects of refraction: a straight stick dipped in water looks bent, and a pool looks shallower than it is. A glass slab shifts a ray sideways, but the emergent ray is parallel to the incident ray.

Lenses

A convex (converging) lens is thicker at the centre; a concave (diverging) lens is thinner at the centre. The optical centre is the point through which a ray passes without deviation. Focal length f is the distance from the optical centre to the principal focus.
Lens formula: \( \dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f} \). Magnification \( m = \dfrac{v}{u} = \dfrac{h'}{h} \). For a convex lens f is positive; for a concave lens it is negative.

A concave lens always forms a virtual, erect, diminished image between the lens and F. Only a convex lens can form a real image on a screen. Cutting a lens along its principal axis does not change the focal length, only the brightness of the image.

Power of a Lens

Power \( P = \dfrac{1}{f} \) with f in metres; the unit is the dioptre (D). A convex lens has positive power and a concave lens negative power. Lenses in contact add their powers: \( P = P_1 + P_2 \). Separated by distance d: \( P = P_1 + P_2 - dP_1P_2 \). A myopic eye is corrected with a concave lens; a hypermetropic eye with a convex lens.