This document describes an experiment to determine the numerical aperture of an optical fiber. Numerical aperture refers to the maximum angle at which light entering the fiber is internally reflected along its length. The experiment involves connecting an optical fiber to a trainer board and screen. The distance from the fiber end to the screen is varied to make the light spot coincide with circles of different diameters. This allows calculating the numerical aperture using a formula involving the spot diameter and distance. The numerical aperture obtained was 0.4.

1. OBJETIVE OF THE
EXPERIMENT
To determine the numerical
aperture of an optical fibre

4. Numerical aperture of an optical fibre is
defined as the light gathering ability of
the fibre.
Numerical aperture also refers to the
maximum angle at which the light
incident on the fibre end is totally
internally reflected and is properly
transmitted along the fibre .

5. The light ray should strike the fibre end
with in this cone of acceptance else it is
refracted out of the fibre. Numerically, it
is also defined as the sine of the
acceptance angle.

6. An optical fiber (or optical fibre) is a
flexible, transparent fiber made of glass
(silica) or plastic, slightly thicker than a
human hair.
 It functions as a waveguide, or “light
pipe”to transmit light between the two
ends of the fiber.

7. Core–thin glass center of the fiber
where light travels.
Cladding–outer optical material
surrounding the core.
BufferCoating–plastic: Coating
that protects the fibre.

8. It works on the
principle of total
internal reflection.

9. Low Transmission Loss and Wide
Bandwidth
Immunity and Interference
Small Size and Weights
Signal Security
Electrical Isolation
Abundant Raw Material
Less Expensive

10. An optical fibre cable
Optical fibre trainer board
Screen
connecting wires

12. Fiber Optics Trainer
Model is designed
to learn the basics
of fiber Optics.
 A large number of
experiments.

13.  Make all the connections .
 Connect one end of the optical fibre cable to
the optical fibre trainer and other end to the
numerical aperture jig.
 Hold the white screen with four concentric
circles (10, 15, 20 and 25 mm diameter)
vertically at a suitable distance to make the
red spot emitted from the optical fibre
coincide with the 10 cm circle. Note that the
circumference of the spot (outermost) must
coincide with the circle.

14.  Record L, the distance of the screen from the
fibre end and note the diameter (W) of the
spot.
 Compute the numerical aperture (NA) of the
optical fibre by using the formula
 NA = Sinθ=W/(4L2+W2)1/2,
 where θ is called as the acceptance angle
is the maximum angle of incidence at the
input end of the optical fibre so that he
optical ray can just propagate within the
optical fibre.
 Tabulate the reading and repeat the
experiment for 15 mm, 20 mm and 25 mm
diameter too.

15. S.No LENGTH(mm
)
DIAMETER
OF THE
SPOT(mm)
NUMERICAL
APERTUER(
NA)
ACCEPTANC
E ANGLE(
DEGREE)
1 9 10 0.4 23.5
2 14 15 0.4 23.5
3 19 20 0.4 23.5
4 24 25 0.4 23.5

16. Numerical aperture of optical
fibre is 0.4mm.

17. The optical fibre cable should be free
from twists and folds so as to avoid the
power loss.
Connections should be proper and tight
.

18. What do you mean by refractive index?
What is difference between reflection
and refraction?
What is the basic principle of
propagation of light in an optical fibre?

19. Define critical angle?
Numerical aperture is
(a) sine of critical angle
(b) cos of acceptance angle
(c) sine of acceptance angle
(d) acceptance cone

20. Name the two types of optical fibres?
What are the advantages of optical
fibre?
What are the applications of optical
fibres?
Name the two types of optical fibres?

21. Numerical aperture depends upon
(a) Refractive indices of core and
cladding
(b) Material of core
(c) Light source
(d) Length of optical fibre cable.

22.  http://www.labelectronics.com/index.php?o
ption=com_virtuemart&page=shop.browse
&category_id=22&Itemid=53&vmcchk=1&It
emid=53
 http://www.trainer-kits.com/fibre-optical-trainer.
html
 http://en.wikipedia.org/wiki/Optical_fiber
Engineering physics by H.K MALIK