Class 9 Science Note


Refraction of light: The bending of light when it travels from one medium to another is called refraction of light.


Laws of refraction

First law:

The incident rays, refracted rays and the normal at the point of incident on the refracting surface lie on the same plane.


Second law:

The ratio of the sine of the angle of the incidence   to the   sine angle   of the refraction is constant   for any two medium.


Sini /sinr=$\mu $ which is the refractive index of second medium with respect to the first medium.

 When the ray of light passes through rarer medium to denser medium, the refracted bends towards the normal and when it travels from denser to rare medium it bends away from the normal.


Causes of refraction of light

The refraction of light is due to the change in the speed of light when it travels from one medium to another medium. The speed of light in air medium is 3*108m/s. however it decreases in denser medium .the speed of the light in glass is 2*108m/s.



Total internal reflection

 When the ray of light passes from denser medium with the angle of incidence greater than critical angle, it is reflected back to the original denser medium, is known as total internal reflection.


Conditions for total internal reflection

The ray must pass through denser medium to rare medium.

The angle of incidence should be greater than critical angle .the critical angle for glass and water is 420 and 490


Light pipes: light pipe is bent tube in which light travels in a curved path. It is based on the principle of total internal reflection. It is made up of transparent material like glass or plastic. When light enters in such tube successive total internal reflection occurs at different points and finally comes out other end of tube.




Mirage is an optical illusion due to the total internal reflection. On a hot day, when the ray of light passes tree to the upper layers which are cooler and denser than lower than below and the ray of light passes through denser to rarer medium   gets refracted away from the normal and at every refraction angle of incident increases ultimately as stage comes when the angle of incidence becomes greater than critical angle and total internal reflection is said to have been occurred. Due to this, a person sees the inverted image of the tree as if it is reflected from the pool water. This illusion is called mirage.


Dispersion of light:

The phenomenon of splitting of light into its components color is called dispersion of light. When the ray of light passes through the denser medium such as water or glass, the speed of the light decreases and different color of light moves with the different speed This  is the cause of dispersion of light.


Electromagnetic waves:

Electromagnetic waves are those waves which do not need material medium for propagation and can travel even through vacuum.

Properties of electromagnetic waves

They can cast the shadow when the obstacle comes in their path

Their speed is 3*108m/s

 They are transverse in nature.

 They show the properties of reflection, refraction, polarization and diffraction.

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