Question

I want the reason also.​

Answer 1

the best observation is 2,3.this is because the angle of incidence is exactly equal to the angle of emergence

Answer 2

The best observations are II and III because when a ray of light enters a glass slab, the first refraction occurs at the first surface of the slab and the second refraction occurs at the second surface of the slab and comes out as emergence Ray this is the second refracted Ray. According to laws of reflection, the angle of incidence must be exactly equal to the angle of refraction in ideal conditions. While during some experiment sone defects may occur and due to these defects the angle of incidence is always same and the angle of emergence is always smaller than the single of incidence....

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