Refraction of light

 Physics Optics Refraction Of Light

 Refraction of light class 10th Full concept notes 

What Is Refraction?

We know that light travels along a straight liness. This is true as long as light travels in a one medium, without encountering an obstacle. When light is travelling in a medium such as air enters another medium such as water ,it generally bends at the surface separating the two media. But once it is in the second medium it moves along a straight line. This phenomenon of bending of light at the surface separating two media is commonly known as refraction of light.

Interface

The surface separating the two media is also called the interface between them. 

Refractive index

The ratio of the speed of light in vacuum to the speed of light in a medium is called the refractive index of the medium. 

This ratio is denoted by n or μ. Thus, 

Refractive indice

Q - A ray of light enters a diamond from air. If the referred index of diamond is 2.42 ,by what percent does the speed of light reduce on entering the Diamond? 

Solution:- we have , n=c/v ,where c is the speed of light in vecuum.

so the speed of light in Diamond, v=c/n

= c/2.42

= 0.41c.

The speed of light in diamond is therefore 41% of its speed in air.In other words,in diamond,the speed reduces by 59%

Low of Refraction 

1. The incident ray,the refracted ray and the normal to the refracting surface at the point of incidence are in the same plane.
   
2. The angle of incidence and the angle of refraction satisfy the equation                                            (sin i /sin r) = (n2 /n1)

The second low given above is called snell's law. 

In the situation shown in figure ,if the first medium is air then n1=1.let the refractive index of the second medium be denoted by n. Equation can be written as 

 

Q. A ray of light travelling in air falls on the surface of a transparent slab.The ray makes an angle of 45° with the norma to the surface.find the angle made by the refracted ray with the normal within the slab.Refractive index of the material of the slab =√2.

some important point 

• If a ray falls perpendicularly on the refractin surface,it goes into the second medium without deviation.This is because in this case i=0,and from snell's law sinr=(sin i/n)=0,giving r=0.
•  for bending to take place,a ray has to fall obliquely on a surface separating two media of different refractive indices.
• Even though there is no bending in the case of normal incidence,we say that light has refracted.you can associate refraction with change in the speed of the light as the mediam changes,and that occurs if the rays fall normally and the proceed without bending.

Optically Denser and Rarer media 

• Of a pair of transparent media,the one that has the higher refractive index is called the optically denser medium of the two while the one that has the lower refractive index is called the optical rare medium.
• Thus Water and glass are optically denser than air ,water is optically rare than glass, and so on.
• Remember that optically denser does not mean greater mass density(mass per unit volume).
•  For example ki kerosene is lighter than water(kerosene floats on water,but it has higher refractive index, that is,it is optically denser than water. 
• When light goes from the optically rarer medium to the optical denser medium it slows down and bends toward the normal.
• And when it goes from the denser medium to the rarer medium, it speed up and bends away from the normal. 

Total Internal Reflection

Critical angle 

If the angle of incidence i increases ,the angle of refraction are increased. For a particular value of i called the critical angle(ic), the angle of refraction becomes 90°.

Total internal reflection

The total reflection of light travelling in a medium of higher refractive index when it is incident on the boundary with another medium of lower refractive index at an angle greater than the critical angle, is called the total internal reflection. 

When light Falls obliquely on the interface between two transparent media, a small part of the light is reflected into the first medium. But when the angle of  incidence is greater than the critical angle, the whole of the light guess reflected. Hence, it is called total internal reflection. 

For I=ic, r=90°. From snell's law

 

sparkling diamond

The high refractive index(2.42)of diamond gives it a critical angle of only 24°. so, many rays trying to cross the Diamond-air interference are incident at angles greater than the critical angle. The faces of the Diamond are cut in such a way that most rays inside the Diamond undergo total internal reflection. For example, most rays entering from the top of your properly internal reflections and emerge from the top face. But if we take a similarly cut piece of glass,a large number of incident rays on the other side of the glass. Thus, the Diamond sparkles in comparison ,because the eye receives much more light from the Diamond that it does from the glass. 

Thin Lenses

You have seen when a ray light passes through a transparent slab with parallel faces  the direction of the emergent ray Re remains the same as that of the incident ray. The ray just get shifted sideways. What happens when light passes through a medium bonded by curve surfaces?