Refraction Of Light

Key Notes:

Definition

  • Refraction: The bending of light as it passes from one medium to another due to a change in its speed.

Cause of Refraction

  • When light moves between two transparent media (e.g., air to water), its speed changes because of the different optical densities of the media.

Key Terms

  • Incident Ray: The ray of light approaching the boundary of two media.
  • Refracted Ray: The ray of light that bends as it enters the second medium.
  • Normal: An imaginary line perpendicular to the surface at the point of incidence.
  • Angle of Incidence (i): The angle between the incident ray and the normal.
  • Angle of Refraction (r): The angle between the refracted ray and the normal.

Laws of Refraction

  • The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.
  • Snell’s Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media.

sin⁡i / sin⁡r = n

where n is the refractive index.

  • Absolute Refractive Index (n): The ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v).

n = c/v

  • Relative Refractive Index: The ratio of the refractive indices of two media.

Effects of Refraction

  • Apparent Depth: Objects under water appear shallower than their actual depth.
  • Bent Appearance of Objects: A stick partially immersed in water appears bent at the water surface.
  • Dispersion: Refraction causes white light to split into its constituent colors (e.g., rainbow formation).

Critical Angle and Total Internal Reflection

  • Critical Angle (C): The angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90∘.
  • Total Internal Reflection: When the angle of incidence exceeds the critical angle, light reflects entirely back into the denser medium.

Applications of Refraction

  • Lenses: Used in eyeglasses, cameras, and microscopes.
  • Prisms: Separate light into colors (spectra).
  • Mirages: Optical illusion due to atmospheric refraction.

Real-Life Examples

  • Formation of rainbows.
  • Twinkling of stars (caused by atmospheric refraction).
  • Bending of light in optical fibers for communication.

Experiments to Understand Refraction

  • Refraction of light through a glass slab: Demonstrates lateral shift.
  • Refraction through a prism: Demonstrates dispersion.

Important Formulas

Snell’s Law:

n1sin⁡i = n2sin⁡r

Refractive Index:

n = Speed of light in vacuum / Speed of light in medium

Let’s practice!