What is Refraction of Light?
Refraction of light is defined as:”The process of bending of light as it passes from air into glass and vice versa is called refraction of light.”
If we dip one end of a pencil or some other object into water at an angle to the surface, the submerged part looks bent. Its image is displaced because the light coming from the underwater portion of the object changes direction at it leaves the water. This bending of light as it passed from one transparent medium into another is called refraction.
Refraction of light can be explained with the help of figure given below.A ray of light IO travelling from air falls on the surface of a glass block.
At the air glass interface,the ray of light IO changes direction and bends towards the normal and travels along the path OR inside the glass block.The rays IO and OR are called the incident ray and the refracted ray respectively.The angle ‘i’ made by the incident ray with the normal is called angle of incidence.The angle ‘r’ made by the refractive ray with the normal is called angle of refraction.When refracted ray leaves the glass,it bends away from the normal and travels along the path ME.
Laws of refraction:
- The incident ray,the refracted ray,and the normal at the point of incidence all lie in the same plane.
- The ratio of the sine of the angle of incidence ‘i’ to the sine of the angle of refraction ‘r’ is always equal to a constant i.e.,Sin i/Sin r =constant =n
Where the ratio Sin i/Sin r is known as the refractive index of the sound medium with respect to the first medium.So we have:
Sin i/Sin r =n
It is called Snell’s law.
Refraction of light is caused by the difference in speed of light in different media.For example,the speed of light in air is approximately 3.0 × 108 ms-1 ,however when light travels through a medium,such as water or glass,its speed decreases.The speed of light in water is approximately 2.3 × 108 ms-1,while in glass,it is approximately 2.0 ×108 ms-1. To describe the change in the speed of light in a medium,we use the term refractive index.
The refractive index ‘n’ of a medium is the ratio of the speed of light ‘c’ in air to the speed ‘v’ of light in the medium.
Table of refractive index of some substances is given:
|Substance||Refractive index (n)|
Dispersion of light is due to the variation in refractive index with the colour. Dispersion in drops of water separates the colours of sunlight into rainbow.
Total Internal Refraction:
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 internal refraction 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 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.
Refraction Through Prism:
Prism is the transparent object (made of optical glass) with at least two polished plane faces inclined towards each other from which light is refracted.
In case of triangular prism, the emergent ray is not parallel to the incident ray. It is deviated by the prism from its original path. The incident ray PE makes an angle of incidence ‘i’ at point E and is refracted towards the normal N as EF. The refracted ray EF makes an angle ‘r’ inside the prism and travels to the other face of the prism. This ray emerges out from the prism at point F making an angle ‘e’. Hence the emerging ray FS is not parallel to the incident ray PE but it deviated by an angle D which is called angle of deviation.