Penta prisms can deviate an incident beam to 90° without inverting or reversing. The deviation angle of 90° is independent of any rotation of the prism about an axis parallel to the line of intersection of the two reflecting faces. Movement of the prism does not affect the right angle at which rays are reflected. The penta prism is the important part in Plumb Level and Rangefinding. It acts as a turning mirror which is insensitive to alignment and also it is a necessary optical tool.