4. Light and Optics

4.1 Laws of Reflection

Scientist: Known since antiquity (Euclid described reflection geometrically c. 300 BCE); formalised in modern optics

Year / Era: Ancient origins; modern formal statement by the 17th century

Statement: (1) The angle of incidence is equal to the angle of reflection. (2) The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane.

Explanation: When light strikes a smooth (reflective) surface, it bounces off following these two precise geometric rules, which apply to plane mirrors, curved mirrors, and any reflective surface.

Application/Example: These laws govern the formation of images in plane and curved mirrors used in everyday objects such as vehicle rear-view mirrors (convex mirrors, which give a wider field of view), shaving/makeup mirrors (concave mirrors, which magnify), and periscopes.

4.2 Snell's Law of Refraction

Scientist: Willebrord Snellius (Snell); the sine-law relationship was also independently derived by René Descartes, and the phenomenon of refraction was studied earlier by Ibn Sahl

Year / Era: 1621 (Snell's formulation, published posthumously); Descartes 1637

Statement: When light passes from one transparent medium to another, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media, equal to the ratio of their refractive indices.

Explanation: Light bends (refracts) when it passes from one medium to another because its speed changes at the boundary. Light travelling from a less dense (optically 'rarer') medium like air into a denser medium like water or glass bends toward the normal, and vice versa when leaving the denser medium.

Formula: n1 sinθ1 = n2 sinθ2, where n1, n2 are the refractive indices of the two media and θ1, θ2 are the angles of incidence and refraction.

Application/Example: Snell's law explains why a straight stick appears bent when partly dipped in water, and is fundamental to the design of lenses in eyeglasses, cameras, microscopes, telescopes, and optical fibres used in telecommunications.

4.3 Laws of Photoelectric Effect

Scientist: The photoelectric effect was observed by Heinrich Hertz (1887); explained theoretically by Albert Einstein

Year / Era: Observed 1887 (Hertz); explained 1905 (Einstein) — this explanation earned Einstein the Nobel Prize in Physics in 1921

Statement: When light (electromagnetic radiation) of frequency above a certain minimum threshold value falls on the surface of a metal, electrons are emitted instantaneously from the metal surface. The kinetic energy of the emitted electrons depends on the frequency of the incident light, not its intensity, while the number of electrons emitted depends on the intensity of light.

Explanation: Classical wave theory of light could not explain why electron emission depended on frequency rather than intensity, and why there was a threshold frequency below which no electrons were emitted regardless of intensity. Einstein resolved this by proposing that light itself is composed of discrete energy packets called photons (building on Max Planck's quantum hypothesis), each carrying energy proportional to its frequency. This was a landmark result establishing the particle nature of light and the foundations of quantum mechanics.

Formula: E = hν = φ + KE(max), where h is Planck's constant, ν is frequency, φ is the work function of the metal, and KE(max) is the maximum kinetic energy of emitted electrons.

Application/Example: The photoelectric effect is the working principle behind solar cells (photovoltaic cells) that convert light directly into electricity, photodiodes and light sensors used in automatic street lights and burglar alarms, and photomultiplier tubes used in scientific instruments.