Refraction

Refraction

Refraction is the phenomenon in which direction of propagation of light at the boundary when it passes from one medium to the other medium to the other. During a refraction, frequency does not change.

Laws of Refraction

• The incident ray refracted ray, and normal always lies in the same plane.
• The product of the refractive index and the sine of the angle of incidence at a point in a medium is constant.

     μ1 Sin 1 = μ2 Sin2

Absolute Refractive Index

• It is defined as the ration of light in free space ‘c’ to that in a given medium v.  It is represented by μ and n. Hence,

      μ or n =c/v

• Denser is the medium lesser will be the speed of light and so greater will be the refractive index.

Relative Refractive Index

When light passes from one medium to other, then the refractive index of the medium 2 relative to 1is written as 1μ2 and is defined as,

Bending of Light Ray

According to Snell’s law:

• If light passes from rarer to a denser medium μ1 = μ2 andμ2 = μo so that,
• In passing from rarer to a denser medium, the ray bends towards the normal.
• In passing from denser to rarer medium, the ray bends away from the normal.

Apparent Depth

Apparent depth in a medium is the depth of an object in a denser medium as seen from the rarer medium. Its value is smaller than the real depth.

Real Depth

Real depth is the actual distance of an object beneath the surface, as would be measured by submerging a perfect ruler along with it.

Formula to Calculate Depth

Cause of Refraction

The refraction of light occurs because the speed of light is different in different mediums.

Physical Significance of Refractive Index

The Refractive index of a medium gives the following information:

• The value of the refractive index gives information about the direction of the bending of the refracted rays.
• It gives the ratio of the speed of light in a vacuum to that medium. For example, the refractive index of glass is 3/2. It means the speed of light is two-third the speed in a vacuum.

Reversibility of Light

When a light ray, after suffering any number of reflections and refractions, has its final path reversed, it travels back along its initial path.

Critical Angle

The critical angle for two given media is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°.

Lateral Shift

The perpendicular distance between the incident and emergent rays is known as the lateral shift.

Some Illustration of Refraction

Bending of an Object

When an elongated object is seen from a rarer medium it appears to bent.

Twinkling of Stars

Due to fluctuations in the refractive index of different layers of the atmosphere, the refraction becomes irregular so that the light sometimes reaches the eye, and sometimes it does not. This gives the effect of the twinkling of stars.

Total Internal Reflection

When a ray of light travelling from a denser medium to a rarer medium is incident at the interface of the two media at an angle greater than the critical angle for the two media, the ray is totally reflected back into the same medium.

Conditions for Total Internal Reflection

• The ray incident on the interface of two media should travel in the denser medium.
• The angle of incidence should be greater than the critical angle for the two media.

Applications of Total Internal Reflection

• Mirage.
• The brilliancy of Diamond.
• Totally reflecting Prisms.
• Optical fibre.