What is total internal reflection and when does it occurs ?
When a ray of light travelling in denser medium enters into a rarer medium, it bends away from the normal. If the angle of incidence ‘i’ increases’ the angle of refraction ‘r’ also increases. For a particular value of the angle of incidence, the angle of refraction becomes 90°. The angle of incidence, that cause the refracted ray in the rarer medium to bend through 90° is called critical angle.
When the angle of incidence becomes larger than the critical angle, no refraction occurs. The entire light is reflected back into the denser medium. This is known as total internal reflection of light.It is shown in figure below:
Applications of total internal Reflection:
Totally Internal Reflecting Prism
Many optical instruments use right -angled prisms to reflect a beam of light through 90° or 180° (By total internal reflection) such as cameras, binoculars, periscope and telescope. One of the angles of right angled prism is 90°. When a ray of light strikes a face of prism perpendicular, it enters the prim without deviation and strikes the hypotenuse at an angle of 45°. Since the angle of incidence 45° is greater than critical angle of the glass which is 42°, the light is totally reflected by the prism through an angle of 90°. Two such prisms are used in periscope. The light is totally reflected by the prism by an angle of 180°. Two such prisms are used in binoculars.
Total internal reflection in Optical Fibre
Total internal reflection is used in fibre optics which has number of advantages in telecommunication field. Fiber optics consists of hair size threads of glass or plastic through which light can be travelled.
The inner part of the fibre optics is called core that carries the light and an outer concentric shell is called cladding. The core is made of glass or plastic or relatively high index of refraction. The cladding is made of glass or plastic, but of relatively low refractive index. Light entering from one end of the core strikes the core-cladding boundary at an angle of incidence greater than critical angle and is refracted back into the core. In this way light travels many kilometers with small loss of energy.
Light pipe is the bundle of thousands of optical fibres bounded together. They are used to illuminate the inaccessible places by the doctors or engineers. For example, doctors view inside the human body. They can also be used to transmit images from one point to another.
An endoscope is a medical instrument used for exploratory diagnostics, and surgical purposes. An endoscope is used to explore the interior organs of the body. Due to its small size, it can be inserted through the mouth and thus eliminates the invasive surgery. The endoscope used to examine the stomach, bladder and throat are called Gastroscope, Cystoscope and Bronchoscope respectively. An endoscope uses two fibre-optic tubes through a pipe. A medical procedure using any type of endoscope is called endoscopy.The light shines on the organ of patient to be examined by entering through one of the fibre tubes of the endoscope. Then light is transmitted back to the physician’s viewing lens through the outer fibre tube by the internal reflection. Flexible endoscopes have a tiny camera attached to the end. Doctor can see the view recorded by the camera on a computer screen.
Applications of total internal reflection (video)