Optical laws: refraction and reflection

Refraction

Light travels in a straight line.
When light passes from one medium to another (for example: water and air o glass and air) it makes a deviation from its original straight path: this phenomenon is called refraction.

In more scientific terms, a medium is identified by its index of refraction that tells us how light travels inside it (the index N is defined in the formula below and its value is given for some materials). Refraction occurs when light passes from a medium with a certain index to a second one, with a different value...


N is the refractive index, c is the speed of light in vacuum and v is the speed of light in the material

material refractive index
air 1.0003
water 1.33
glass 1.52
Refraction is responsible for a variety of familiar phenomena such as the apparent distortion of objects partially submerged in water. Rays that come from inside water are refracted when they pass through the surface giving rise to the illusion that objects in water appear to be closer than they really are. An excellent example is provided in the image on the right using a drinking straw placed in a glass of water.
An important characteristic of refraction is that the angle of this deviation depends on the characteristics of the two means but it also depends on the considered wavelength. In other words, the different wavelengths that compose white light are affected differently by refraction. The red radiations (with longer wavelengths) are less deviated while the blue or violet radiations are more deviated from their initial direction.
For this reason, when light travels through a prism it is decomposed in a rainbow, or when a star is observed by a refractor telescope, we can have chromatic aberration...


Reflection

Reflection happens when a light ray meets a reflecting surface, such as a mirror. For example, when light emitted by the flashlight bulb is directed onto the surface of a mirror at an angle called incident, it will be reflected back into space at another angle (called reflected angle) that must be equal to the incident angle. This concept is called law of reflection.

It is important to note that the light is not separated into its component colors because it is not being "bent" or refracted and all wavelengths are being reflected equally. For this reason, mirrors don't present chromatic aberration.

 

 

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