# Internal reflection

Internal reflection takes place when light travelling through a denser medium such as water reaches the boundary with a less dense medium such as air and is reflected back into the denser medium.

• Internal reflection is a common phenomenon that occurs with all types of electromagnetic radiation, including visible light.
• Internal reflection takes place when light reaches the boundary between a medium with a higher refractive index and a medium with a lower refractive index.
• So, internal reflection takes place when light travels from glass to air at an angle greater than the critical angle, but not when it travels from air to glass.
• The amount of internal reflection depends upon the angle of incidence as light approaches the boundary. Here are the different outcomes that result from different angles of incidence:
• At 0 degrees 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 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 back into the first medium.
• 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 surface of the water at an angle of 48.6 degrees or greater, it will be totally internally reflected 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.
###### References
• Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
• For internal refraction to occur, the refractive index of the second medium must be lower than the refractive index of the first medium. So internal reflection takes place when light reaches air from glass or water (at an angle greater than the critical angle), but not when light reaches glass from air.
• In most everyday situations light is partially refracted and partially reflected at the boundary between water (or glass) and air because of irregularities in the surface.
• If the angle at which light strikes the boundary between water (or glass) and air is less than a certain critical angle, then the light will be refracted as it crosses the boundary between the two media.
• When light strikes the boundary between two media precisely at the critical angle, then light is neither refracted or reflected but is instead transmitted along the boundary between the two media.
• However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.
• The critical angle is the angle of incidence above which internal reflection occurs. The angle is measured with respect to the normal at the boundary between two media.
• The angle of refraction is measured between a ray of light and an imaginary line called the normal.
• In optics, the normal is an imaginary line drawn perpendicular to the boundary between two media. It is used to measure angles of incidence and refraction.
• In the case of a curved boundary, the normal is drawn perpendicular to the tangent line at the point of incidence.