FORM 4: 5.3 UNDERSTANDING TOTAL INTERNAL REFLECTION

• When light enters from a denser medium (glass) to less dense medium (air), it is bent or refracted away from the normal.
•  A small part of the incident ray is reflected inside the glass.

•  As the angle of incidence, i increases, the angle of refraction, r also increases.
• When the angle of refraction reaches 90^o, the light is refracted along the boundary.
• This angle of incidence in the denser medium is called the critical angle, c.
• The critical angle is defined as the angle of incidence in the denser medium when the angle of refraction in the less dense medium is 90^o.

•  If the angle of incidence is increased further so that it is greater than the critical angle, the light is not refracted anymore, but is internally reflected.
• This phenomenon is called total internal reflection.

THE MEANING OF TOTAL INTERNAL REFLECTION

Total internal reflection occurs when a light ray passes from a denser medium towards a less dense medium with the angle of incidence greater than the critical angle of the medium.

THE CONDITIONS FOR TOTAL INTERNAL REFLECTION TO OCCUR ARE:

1. light ray enters from a denser medium towards a less dense medium
2. the angle of incidence in the denser medium is greater than the critical angle of the medium.

REFRACTIVE INDEX, n

NATURAL PHENOMENA INVOLVING TOTAL INTERNAL REFLECTION 

• The layers of air near the ground are hotter and less dense. The upper layers are cooler and denser.
• When light rays from the sky travel towards the Earth’s surface, the light ray are refracted away from the normal due to decreasing air density.
•  The angle of incidence increases as the light rays enter the next layer of air.
• The light rays are reflected when the angle of incidence exceeds the critical angle of air in that layer.

APPLICATIONS OF TOTAL INTERNAL REFLECTION

• An optical fibre consists of an inner core of high refractive index glass and surrounded by an outer cladding of lower refractive index.
• When light is introduced into the inner core at one end, it will undergo a series of total internal reflection.