Refraction

• Refraction is the change in direction and speed of a wave due to its transition from one medium to another.
• Snell's Law offers a mathematical relation between angles of incidence and refraction for a wave impinging on an interface between two media with different indices of refraction. It's expressed as

  n1sinθ1 = n2sinθ2

  where

  n1

  and

  n2

  are the refractive indices of the first and second mediums, and

  θ1

  and

  θ2

  are the angles of incidence and refraction, respectively.
• The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium. It's a dimensionless quantity which helps in determining how much a light ray will bend when transitioning from one medium to another.
• A convex lens or a positive lens causes light rays to converge or come together at a single point and is used for magnifying purposes, while a concave lens or a negative lens causes light rays to diverge or spread out.
• Total internal reflection (TIR) is a phenomenon which occurs when a propagating wave strikes a medium boundary at an angle larger than a certain critical angle with respect to the normal to the surface and the wave is entirely reflected back into the medium.

Reflection

• Reflection is the change in direction of a wave at a boundary between two different media, so that the wave returns into the medium from which it originated.
• Law of reflection states that the angle of incidence is equal to the angle of reflection, measured from the normal (perpendicular) to the reflective surface.
• In specular reflection, light from a single incoming direction is reflected into a single outgoing direction. This is typical of smooth, shiny surfaces like mirrors.
• Diffuse reflection is when light hits an object and reflects in many different directions. This happenes on rough surfaces and is the reason why we can see non-shiny objects from different angles.

Absorption

• Absorption of light is the way in which the energy of a photon is taken up by matter, typically the atoms and molecules of a physical material.
• The absorption of light waves may generate heat, as it increases the energy of the absorbing object.
• Absorption in semiconductors is often related to electronic transitions – the absorbed energy of the photon is transferred to an electron and the electron moves from the valence band to the conduction band.
• Light absorbed by a material can also be re-emitted via fluorescence or phosphorescence.
• The absorption spectrum of a material is the fraction of incident radiation absorbed by the material over a range of frequencies.
• Absorption of light is also responsible for the colour of objects. An object appears white when it reflects or scatters all the incident light and appears black when it absorbs all the light.