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Refractive Index
If you have ever half submerged a straight stick into water, you have probably noticed that the stick appears bent at the point it enters the water. This optical effect is due to refraction.
Refraction is the bending of light that takes place at a boundary
between two materials having different indices of refraction. Refraction is due to a change in the speed of light as it passes from one medium to another.
At a boundary, an incident ray can undergo partial reflection or, in certain situations, total internal reflection.
No bending of the incident ray occurs if it strikes the boundary along the normal.
The incident ray is the ray approaching the boundary. It strikes the boundary at the point of incidence. The refracted ray is the ray leaving the boundary through the second medium.
The reflected ray is the ray undergoing partial (or total) reflection at the boundary. The normal is a construction line drawn perpendicular to the boundary at the point of incidence.
The angle of incidence (θi) is the angle between the incident ray and the normal. The angle of reflection (θr
) is the angle between the normal and the reflected ray.
The angle of refraction (θR) is the angle between the normal and the refracted ray.
Both Reflection and Refraction occur when the light is incident on a more refractive medium.
Laws of Refraction:
- The ratio of sines of the angles of incidence and refraction is a constant. (Snell's Law) (The ratio is constant for a particular wavelength and a particular set of materials.)
- The incident and refracted rays are on opposite sides of the normal at the point of incidence.
- The incident ray, the normal, and the refracted ray are coplanar.
Snell's Law: where n
is a constant.
Refraction is responsible for image formation by lenses and the eye
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