(a) Mirage: It is an optical illusion which occurs due to the phenomenon of total internal reflection. When we are travelling on a road on a hot summer day, the road at a distance appears to be covered with water but when we approach the region, we find that there is no water. This phenomenon is called “Mirage”. (mrig trishna)
Formation of mirage:
On hot summer days, the air near the ground may become hotter than air further up. Since the density of hot air is less, so air becomes denser and denser as we go up. When light rays from a tree travel downward they pass from denser to rarer layer of air and so the angle of refraction goes on increasing. At a particular layer total internal reflection occurs and an inverted image of the tree is observed.
(b) Prism: It is a right angled prisms used to turn a beam of light through 90° or 180° without any loss of intensity, used in optical instruments such as binoculars and submarine periscopes. Such a prism is also used to invert images without changing their size.
The total internal reflection take place because the angle of incidence of the beam from glass to air is 45° which is greater than critical angle (41.1°).
(c) Optical fibres: It consists of many thousands of long fine fibres of high quality composite glass/quartz fibre. Each fibre consists of a core and cladding where refractive index of core is higher than cladding
When the light is incident on one end of the fibre at a small angle, the light passes inside, undergoes repeated total internal reflection along the fibre and finally comes out. When a signal in the form of light is directed at one end of the fibre at a suitable angle, it undergoes repeated TIR along the length of the fibre and finally comes out at the other end without any appreciable loss in the intensity of light.
Even if the fibre is bent, light can easily travel along its length. Thus an optical fibre can be used as an optical pipe.
Application of Prism:
1. It is extensively used for transmitting and receiving electrical signals which are converted to light by suitable transducers.
2. Also used for transmission of optical signals. For e.g these are used as light pipe to facilitate visual examination of internal organs like stomach, intestines etc.
The brilliance of many precious stones particularly of diamond is due to the total internal reflection.
Its critical angle is very small (approx 24.4°), so that once light gets into diamond, it is very likely to be totally reflected internally. By cutting the diamond suitably, multiple internal reflections can be made to occur.