Quantum Theory of Light

Nature of Light

• At the beginning of the 20th century, the wave theory of light was found to be inadequate in explaining the interaction of light with matter.
• Light exhibits both wave-like and particle-like properties.
• The modern quantum theory of light reconciles these properties.

Reflection of Light

• A highly polished surface, such as a mirror, reflects most of the light falling on it.
• The laws of reflection of light state that:
• The angle of incidence is equal to the angle of reflection.
• The incident ray, the normal to the mirror at the point of incidence, and the reflected ray all lie in the same plane.
• These laws are applicable to all types of reflecting surfaces, including spherical surfaces.

Mirrors and Reflection

• A small source of light casts a sharp shadow of an opaque object, indicating that light travels in straight lines.
• When an opaque object on the path of light becomes very small, light bends around it, exhibiting diffraction.
• In optics, light is thought of as a wave to explain phenomena such as diffraction.

Rays and Mirrors

• A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror.
• A ray passing through the principal focus of a concave mirror or a ray directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis.
• A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path.