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why is emergent ray parallel to incident ray ?

Whenever a radiation enters into a different medium from a given medium it bends because its speed changes with the change in medium. But, when it re-enters into previous medium, its speed is restored.

In a glass slab, some changes occur when the ray enters through a face. The opposite faces are parallel to each other, so at the time of emergence, the angle of incidence and refraction remains same as before. You can prove it through geometry, as corresponding angles and alternate angles are equal for parallel lines.

In simple words you can say that everything is restored with a lateral shift (because of bending), due to which the incident and emerging rays are parallel to each other.

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Since the refracting surfaces are parallel to each othe the emergent rays are parallel. 

The angle of incidence on the first surface { from air to glass} be i and the angle of refraction at the first surface be r. 

Since the surfaces are parallel the ray inside the glass falls at the other surface at angle r and therefore the ray emerges at angle i that is the angle of incidence. 

Therefore the incident ray is parallel to the emergent ray.
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the incident ray and the emergent ray are parallel because the surfaces of the rectangular glass slab are parallel to each other
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The light emerges from a parallel sided glass slab in a direction parallel with that in which it enters the glass slab, ie, the original direction of Ray is just parallel to the emergent ray
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the extend of bending of the ray of light at the opposite parallel faces of the rectangular glass slab is equal and opposite therefore the ray emerges parallel to the incident ray.
However the light ray is shifted sideward slightly.
 
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Bahut mar marenge
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This is the perfect answer for this question.

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the refracting surfaces are parallel to each othe the emergent rays are parallel. ... Since the surfaces are parallel the ray inside the glass falls at the other surface at angle r and therefore the ray emerges at angle i that is the angle of incidence. Therefore the incident ray is parallel to the emergent ray.
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As the incident ray fall to normal so the angle of incident is not equal to the angle of reflection as the speed of is change in the glass slab so the angle of reflection bend toward the normal when the ray come out from glass so that ray is called emergent ray and the angle of emergent ray is equal to the angle of incident so the ray of incidence and emergent ray is parallel.
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during refraction of light through the glass slab incident ray and emergent ray are parallel to each other explain
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Since opposite faces of glass slab are exactly parallel, and bending of a light ray while undergoing refraction at two surfaces of glass slab is exactly equal But in opposite direction. hence net deviation is zero and emergent Ray is parallel to the incident ray.
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Nfhjcjyku
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it's a imaginary
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