CBSE Class 10 Physics Light Reflection and Refraction Notes Set 03

Reflection of Light

Light travels in straight lines at about \( 3 \times 10^8 \) m/s in vacuum and needs no medium. Opaque bodies cast shadows, and eclipses show that light moves in straight lines.
Laws of reflection: the incident ray, the normal and the reflected ray lie in one plane, and the angle of incidence equals the angle of reflection. A plane mirror always forms a virtual image.
Two plane mirrors inclined at angle \( \theta \) form \( N = 360^\circ/\theta - 1 \) images.

Spherical Mirrors

The pole is the middle of the mirror, the centre of curvature is the centre of the sphere, and the principal focus lies midway between them, so f = R/2.
Mirror formula: \( 1/v + 1/u = 1/f \). Magnification \( m = -v/u \). Use the sign convention consistently for concave and convex mirrors.

Refraction

A ray entering a glass slab obliquely bends so that \( \sin i / \sin r \) is constant, which is the refractive index (about 1.5 for glass and 1.33 for water). The incident ray, refracted ray and normal lie in one plane.
Critical angle: for a ray going from a denser to a rarer medium, the angle of incidence for which the refracted ray grazes the surface. It is about \( 42^\circ \) for glass and \( 49^\circ \) for water. Beyond it the ray is totally reflected, and a right-angled isosceles prism uses this to turn light through \( 90^\circ \) or \( 180^\circ \).

Prism and Colours

A prism deviates light towards its base at both faces. White light splits into violet, indigo, blue, green, yellow, orange and red, with violet bent most and red least. Red, green and blue are the primary colours of light.

Lenses

Lens formula: \( 1/v - 1/u = 1/f \). Magnification \( m = v/u \). Power \( P = 1/f \) with f in metres, in dioptres; a convex lens of focal length 25 cm has power +4 D, and a concave lens has negative power.