Aim: To determine the Acceptance angle and Numerical aperture of the given optical fiber.
Apparatus: Laser source, Optical fiber, Screen, Scale
Formula: (i) the acceptance angle, 0 = tan-1 (D/2L)
(ii) Numerical aperture, NA = Sin 0
where, D – the diameter of the bright circle formed on screen,
L – the distance between the optical fiber end

Procedure :
1. Switch on the laser source and adjust the distance between output end of the optical fiber and the screen ‘L’ (say 5 cm).
2. Place a graph sheet on the screen and observe the circle formed on the graph sheet.
3. Mark the points ‘a’,’b’,’c’ & ‘d’ on the inner bright circle as shown in the diagram. Note down the horizontal diameter D1 and vertical diameter D2 of the inner bright circle in the tabular column.
4. Repeat the above steps for different values of L (for 4cm, 3cm, … ).
5. Find the Acceptance angle from the tabular column and hence the Numerical aperture.