Give an example of natural phenomena of reflection of light in a rectangular glass slab
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Refraction through a rectangular glass slab
To understand the refraction of light through a glass slab consider the figure given below which shows the refraction of light through a rectangular glass slab.
refraction of light through a rectangular glass slab
Here in this figure AO is the light ray travelling in air and incident on glass slab at point O.
Now on entering the glass medium this ray bends towards the normal NN’ that is light ray AO gets refracted on entering the glass medium.
After getting refracted this ray now travels through the glass slab and at point B it comes out of the glass slab as shown in the figure.
Since ray OB goes from glass medium to air it again gets refracted and bends away from normal N1N'1 and goes in direction BC.
Here AO is the incident ray and BC is the emergent ray and they both are parallel to each other and OB is the refracted ray.
Emergent ray is parallel to incident ray because the extent of bending of the ray of light at the opposite parallel faces which are PQ (air-glass interface) and SR (glass-air interface) of the rectangular glass slab is equal and opposite.
In the figure i is the angle of incidence, r is the angle of refraction and e is the angle of emergence.
Angle of incidence and angle of emergence are equal as emergent ray and incident ray are parallel to each other.
When a light ray is incident normally to the interface of two media then there is no bending of light ray and it goes straight through the medium.
Refraction of Light
When a beam of light falls obliquely at the interface of the two optical media, direction of its path changes when it enters into the other medium. This phenomenon exhibited by the light rays is called refraction of light. This is due to the change in speed of light while going from one medium to the other.
During the refraction, when light travels from one medium to another transparent medium, the speed and wavelength of light changes, whereas frequency remains the same.When a light ray travels from rarer to denser medium, its speed gets slow down and the ray bends towards normal (∠i >∠r).When the ray of light travels from denser to rarer medium, its speed goes up and the ray bends away from the normal (∠i < ∠r).
From the above two points, we can conclude that the light bends on undergoing refraction.Laws of Refraction:At the point of incidence the incident ray, the refracted ray and the normal to the interface, all lie in the same plane.The ratio of the sine of angle of incidence to the sine of angle of refraction is constant for the same colour of light and for the same pair of transparent media. Thus,
sin i/sin r = constant = n21
Where n21 is called the refractive index of the second medium with respect to first medium.
This law is also known as Snell’s law of refraction.Refraction through rectangular glass slab with parallel faces:
The refraction takes place at both air-glass interface and glass-air interface has the following characteristics:
When a light ray travels from air to glass, the angle of incidence is greater than angle of refraction as ray bends towards normal.When a light ray travels from glass to air, the angle of refraction (also called angle of emergent in case of glass) is greater than the angle of incidence of glass-air interface as ray of light bends away from the normal.If the angle of incidence is zero, i.e. the incident ray is normal to the interface, the ray of light continues to travel in the same direction after refraction.The angle of emergence and angle of incidence will be equal.
Emergent ray is parallel to the incident ray along with original direction but it will be laterally displaced to the left of the incident ray.For the same angle of incidence, lateral displacement is proportional to the thickness of the glass slab.For the same thickness of glass slab, the lateral displacement is proportional to the angle of incidence.