Historical Theories of Light and Optical Phenomena

Newton's Corpuscular Theory of Light

This theory, proposed by Isaac Newton (1642-1726), posits that light consists of tiny particles of matter, called corpuscles, emitted at high speed from luminous bodies in a straight line. The direction of propagation of these particles is known as a ray of light. Key principles of Newton's theory include:

• Rectilinear Propagation: Light travels in a straight line because the corpuscles move at high speed.
• Reflection: It is known that light reflects when hitting a mirror. Newton explained this phenomenon by stating that light particles are perfectly elastic, and therefore, reflection adheres to the laws of elastic collision.
• Refraction: The change in light speed and propagation direction when passing through media of different densities was challenging to explain with particle theory. However, Newton hypothesized that the surface separating two media with different refractive indices exerted a pull on light particles, thereby increasing the normal component of their velocity while the tangential component remained unchanged.

Huygens' Wave Theory of Light

This physical concept was developed by the Dutch scientist Christiaan Huygens. It proposes that light propagates as mechanical waves emitted by a light source. Light required a highly elastic, all-pervading, impalpable medium for propagation, even in a vacuum, which was named the ether. Unlike particle theory, light energy is not concentrated in each particle but is spread throughout the wavefront. The wavefront is perpendicular to the direction of propagation. The wave theory effectively explains light phenomena through a geometrical construction known as Huygens' Principle. According to this theory, light travels faster in less dense media. Despite its merits, Huygens' theory was largely overlooked for a century due to Newton's significant authority.

Laws of Reflection

Reflection is the phenomenon where light, upon striking a surface, returns to the medium from which it originated. The laws governing reflection are:

• First Law: The incident ray (I), the normal (n), and the reflected ray (r) all lie in the same plane.
• Second Law: The angle of incidence (i) equals the angle of reflection (r): i = r.

Laws of Refraction

Refraction is the phenomenon where a light ray deflects as it passes from one transparent medium into another of different optical density. The laws governing refraction are:

• First Law: The incident ray, the refracted ray, and the normal all lie in the same plane.
• Second Law (Snell's Law): The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, known as the refractive index of the second medium with respect to the first.