A white light passes through a prism is separated into all its colours as shown in the picture above.

a. What colours are A and B?

When prism is introduced before one fixating eye (and the contralateral eye is occluded), the eye moves by an amount equal to the angle of deviation of the prism in order to maintain fixation. What would be the eye movement (move nasally, temporally, up or down) if:

b. a BO (base out) prism is placed over the fixating eye?

c. a BU (base up) prism is placed over the fixating eye?

 
Click here for answer A.
 

Answer bThe eye will move nasally in order to re-fixate the target.
 

Answer c
The eye will move downward to re-fixate the target. When a prism is placed in front of a fixating eye the eye will move away from the base of the prism and in the direction of the apex of the prism.