Internal reflection

• • • Internal reflection is a common phenomenon not only for visible light but for all types of electromagnetic radiation. For internal reflection to occur, the refractive index of the second medium must be lower than that of the first medium. This means internal reflection happens when light moves from a denser medium, such as water or glass, to a less dense medium, like air, but not when light moves from air to glass or water.
    • In everyday situations, light is typically both refracted and reflected at the boundary between water or glass and air, often due to irregularities on the surface. If the angle at which light strikes this boundary is less than the critical angle, the light is refracted as it crosses into the second medium.
    • When light strikes the boundary exactly at the critical angle, it neither fully reflects nor refracts but travels along the boundary between the two media. However, if the angle of incidence exceeds the critical angle, the light will undergo total internal reflection, meaning no light passes through, and all of it is reflected back into the original medium.
    • The critical angle is the specific angle of incidence, measured with respect to the normal (a line perpendicular to the boundary), above which total internal reflection occurs.
    • In ray diagrams, the normal is an imaginary line drawn perpendicular to the boundary between two media, and the angle of refraction is measured between the refracted ray and the normal. If the boundary is curved, the normal is drawn perpendicular to the curve at the point of incidence.
    • Here are the different outcomes that result from different angles of incidence:
      • At a 0⁰ degree angle of incidence, there is no internal reflection; the light passes straight through the boundary without deviation.
      • As the angle of incidence increases, more and more light is internally reflected and less and less light is refracted at the boundary. This means that less is refracted and so progressively less crosses the boundary into the medium with the lower refractive index.
      • At the critical angle, the light grazes the boundary, and all of it is internally reflected, resulting in no refraction into the second medium.
      • Beyond the critical angle, total internal reflection occurs, and the light is entirely reflected into the first medium.

Example

• • Here is an example. If light travels from water to air where the critical angle is about 48.6 degrees.
    • This means that if light reflects off a fish in a fish tank and then strikes the surface of the water at an angle of less than 48.6 degrees, the angle of incidence determines how much light is internally reflected.
    • If light reflects off a fish in a fish tank and then strikes the water’s surface at an angle of 48.6 degrees or greater, it will experience total internal reflection and no light will pass out of the water and into the air.
  • In reality, light is usually partially refracted and partially reflected because of irregularities in the surface at the boundary. This causes differences in the angle of incidence at different points across the boundary.
• Here are the different outcomes that result from different angles of incidence:
  • At a 0⁰ degree angle of incidence, there is no internal reflection; the light passes straight through the boundary without deviation.
  • As the angle of incidence increases, more and more light is internally reflected and less and less light is refracted at the boundary. This means that less is refracted and so progressively less crosses the boundary into the medium with the lower refractive index.
  • At the critical angle, the light grazes the boundary, and all of it is internally reflected, resulting in no refraction into the second medium.
  • Beyond the critical angle, total internal reflection occurs, and the light is entirely reflected into the first medium.

Example

• Here is an example. If light travels from water to air where the critical angle is about 48.6 degrees.
  • This means that if light reflects off a fish in a fish tank and then strikes the surface of the water at an angle of less than 48.6 degrees, the angle of incidence determines how much light is internally reflected.
  • If light reflects off a fish in a fish tank and then strikes the water’s surface at an angle of 48.6 degrees or greater, it will experience total internal reflection and no light will pass out of the water and into the air.
• In reality, light is usually partially refracted and partially reflected because of irregularities in the surface at the boundary. This causes differences in the angle of incidence at different points across the boundary.