From which face does the ray emerge? Find the angle of deviation through the prism.The ray of light passes through an equilateral prism, perpendicularly in one refracting face of the prism of...

From which face does the ray emerge? Find the angle of deviation through the prism.

The ray of light passes through an equilateral prism, perpendicularly in one refracting face of the prism of refractive index 1.45 and the ray finally emerge.

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valentin68 | College Teacher | (Level 3) Associate Educator

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If the incoming ray is entering the prism on the left side, the ray will pass without being deviated because it is normal to the entering face. See the figures below. Depending how high the incoming ray is entering the left side of the prism, there will be two possible cases. In both cases first the ray will undergo a reflection because the angle of incidence

`/_i = 90-30 =60 degree`

is bigger than the angle of total reflection `i_r`

`sin(i_r)=1/n =1/1.45` or equivalent `/_i_r =43.6 degree`

Because first, the ray is reflected, we have from the reflection laws

`/_i =/_r`

and from this and the symmetry of the prism (equilateral prism) the emerging ray will be perpendicular to the exiting face, thus it will not be refracted on its way out. (see the figures attached).

The angle of deviation between the emerging and entering rays is the same in both cases

`delta =i+r =60+60 =120 degree`

The only difference is that depending on how high the incoming ray is entering the prism light will be deviated to the left or to the right.

The angle of deviation through the prism is 120 degree.

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