This is to certify that the investigatory Project Report entitled “Total Internal Reflection” submitted by student of Class XII is original and has been completed by him under my supervision and is completed in all respect for AISSCE -2018-2019.
(Signature of Lecturer in Physics)
3. Total Internal Reflection
4. Conditions for Total Internal Reflection
5. Relation between refractive index () and critical angle (C)
6. Applications of Total Internal Reflection
Whenever a ray of incident light travels from one medium to another, its path is changed i.e. it gets refracted.
If the ray travels from denser medium to rarer medium, it suffers deviation away from the normal at the point of incidence. As the angle of incidence is increased, the angle of refraction also keeps on increasing. At a certain angle of incidence, called the critical angle, the refracted ray grazes along the surface of separation i.e. angle of refraction becomes just go. If the angle of incidence is further increased, no refraction takes place. Instead such a ray gets reflection inside the medium itself. This phenomenon is called total internal reflection.
The phenomenon of bending of light when it come from one medium to another medium is called refraction.
The phenomenon in which a ray comes from one medium and bending of that ray in same medium i.e. Total Internal Reflection takes place called reflection.
Total internal reflection is the phenomenon of reflection of light which occurs when a ray of light traveling in a denser medium is incident at the interfere of the two media at an angle greater than the critical angle for that pair of media.
Consider a surface xy separates the rarer medium a from the denser medium b. A ray of light OA from the object O in denser medium incident normally on the surface of separation, gets refracted into the rarer medium as such along AL. Another ray of light incident along the oblique path OA, is refracted away from normal along the path A1B1 in rarer medium. As the angle of incidence is increased, the angle of refraction also goes on increasing, till for a certain angle of incidence C, called the critical angle. When angle of incidence is greater than critical angle then total internal reflection takes place.
1. The ray incident on the interface should travel in optically denser medium.
2. The angle of incidence should be greater than the critical angle for the given pair of media.
Relation between refractive index (m) and critical angle (C)
When refraction takes place at point C, it follows that the refractive index of medium a w.r.t. medium b is given by
if i = c (critical angle)
r = 90o
Refractive index is inversely proportional to sin of critical angle.
Its relation with polarizing angle :
According to Brewster’s Law, when light is incident at polarizing angle at the interface of a refracting medium, the refractive index of the medium is equal to the tengent of the polarizing angle.
If P is polarizing angle and u, the refractive index of the refracting medium, then u = tan P.
1. Totally Reflection Prism :
To deviate a ray of light through 90o and 180o.
A right angled isosceles prism can be used to cause total internal reflection. In such a right angled prism when a ray of light is incident normally to its face AB, it passes into prism as incidence is 45o. But for glass (m = 1.5), the value of critical angle is about 41.8o. Since the value of incident angle is greater than the critical angle for glass, the ray of right suffers total internal refection. As a result, it gets incident on the force AC at 90o and comes out of the prism as such. It follows that the path of the ray of right was been deviated by the prism through 90o.
The path of a ray undergoing a deviation of 90o due to one internal reflection but in same way and by same process due to two internal reflection, a ray can be deviated through 180o. This arrangement is used in prism-binoculars.
Advantages of Totally Reflecting Prism over the Silvered Plane Mirror.
A reflecting prism does not require any silvering.
A silvered plane mirror always absorbs some light. In a good mirror, the reflection of light about 90-95%. However, in a reflecting prism, almost 100% reflection is secured.
Due to imperfect and non-uniform silvering of the mirror, the image obtained with a mirror is oftenly not of a good quality.
2. Mirage :
It is an optical illusion observed in desert in a hot day. The object such as tree is observed inverted and observer gets an impression that there is a pool of water. This phenomena is known as mirage.
Due to intense heat, the surface of earth becomes quite hot and the temperature of air near the surface of earth is max. The temp of the other layers of the air goes on decreasing as one goes up. Dimity as well as refractive index of air increases slightly for higher layers. Thus a ray of light traveling from point O of a tree passes through air of gradually decreasing refractive index and is therefore refracted more and more away from the normal and accordingly the angle of incidence goes on increasing. At a layer, when the angle of incidence becomes greater than the critical angle, total internal reflection takes place. Then the ray of light starts traversing layers of increasing refractive index and goes on bending more and more towards the normal. Ultimately, when the ray reaches the eye of the observer, it appears to be coming from the point I. Hence the inverted image of the tree produces the impression of reflection from a pool of water.
3. Brilliance of Diamonds and Other Precious Stones :
Diamond shines very brightly because it has a very high refractive index and very low critical angle i.e. = 2.47 & C = 230 . Dur to low critical angle, a diamond cut so as to have a large number of faces, such that a ray of light entering it from one face undergoes repeated total internal reflections from other faces. As a result, the faces through which the light emerges, shine very brilliantly.
4. Optical Fibres :
In 1870, John Tyndall, a British physicist demonstrated that light could be made to follow curved path along a thin stream of water coming out of a water tank. This effect was made use of an illuminated fountains. The light follows curved path it suffers a series of total internal reflections.
Optical fibres commits of thousands of strands of a very fine quality glass or quartz of refractive index about 1.7 or so. The thickness of a strand is about 10-6 cm. The strands are coated with a layer of some material of lower refractive index (m=1.5).
When light is incident at a small angle at are end, it gets refracted into strands and gets incident on the interface of the fibres and the coating. The angle of incidence being greater than critical angle, the ray of light undergoes total internal reflections. Each fibre act as a pipe and such a bundle of fibres can be used to convey images along paths of any shape. The optical fibre does not bend light. Instead, light follows the zin-zag path through the fibres.
5. Looming :
It is an optical illusion observed in the cold countries. In which observers get an impression that object is placed in air.
i) Optical fibres are used inn the field of communication and the computers.
ii) The optical fibres are used for making medical investigation.
iii) The optical fibre sensors have been used to measure temperature and pressure.
iv) The optical are used for transmitting the optical signals and the two dimensional pictures.
v) The optical fibre in the form of photometric sensors are used for measuring the blood flow in the heart.
vi) The optical fibre in the form of refract meters are used to determine the refractive indices of liquids.
vii) Optical fibre are used in telephone and other transmitting cables.
viii) Optical fibre are used in transmission and reception of electrical signals by converting them first into light signal.
1. Modern’s abc of Physics.
2. Pradeep’s Fundamental Physics.
3. Dinesh a to z in Physics.