14-06-2013, 04:43 PM
Metrology and interchangeability
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Principles of optical instruments
Many modern instruments utilize optical system in their construction and it is useful for the metrology to have some knowledge of elementary optical principles.
Reflection
Let us consider reflection at a plane surface Fig. Shows how the angle between a reflected ray and the normal to the surface is equal to the incident ray, both rays and the normal being in the same plane. A beam of light is made up of an infinite number of rays which may be made treated collectively in a similar way.
Now, let the reflection surface be tilted through an angle δθ as in Fig . It will be seen that the normal turns through this angle while the incident ray remains stationary. The angle of incidence therefore, change by the same amount relative to the normal, and so does angle of reflection. The resultant angle between the incident and reflected ray therefore change by 2 δθ.
If the reflected ray is again reflected on to the tilting mirror by a second fixed mirror, the change in angle of the reflected ray is then 4 δθ , as shown in Fig. It is sometimes possible to arrange for the movement which tilts the first mirror to tilt the second as well by the same angle, in which case the angular magnification will be eight times. It is not possible to increase this apparently gratis magnification to such an extent, however, without obeying other stringent conditions involving the size and flatness of mirrors.
Interference
The light is trasmitted by electromagnetic vibrations in either.
Fig. (a) represents two wave of light which are 180 degree.
Fig.(b) shows these waves assumed to be travelling in the same line. It will be seen that for every point on one wave there as an equal and opposite point on the other wave. These two will therefore cancel out, and no light will be seen in the direction of propagation.
Condition for interference of light waves
The two source should continuously emit waves of the same wavelength or frequency.
For obtaining interference fringes, the amplitudes of the two interfering wave trains should be equal or very nearly equal.
The two sets of wavetrains from the two sources should either have the same phase or a constant difference in phase.
The two sources should be very narrow.
The source emitting a set of interference beams should be very close to each other.
Optical prism
Optical prisms consist of solid pieces of highly transparent homogeneous glass with three or more polished plane faces, Fig. 4.35.
The optical prisms are widely used in :
Construction of optical measuring instrument for rectifying images which would otherwise de inverted.
Bending rays of light into new directions.
Normally, in case of an optical prism, the light undergoes several interior reflection before it issues from the prism and these reflections are often total reflections. If rays are incident normally on one of the two perpendicular faces of a glass prism whose principal section is a right triangle, they will enter the prism without deviation and by falling on the hypotenuse face at an angle of 45 degree, which is greater than the critical angle of glass, they will be totally reflected and turned through an angle of 90 degree, so that emerge in a direction normal to the other perpendicular face of the prism.
In a case of an optical prism, none of the light is lost by total reflection. But if the prism is made of good optical glass of high transparency, losses by absorption in the prism, or by reflection on entering and leaving the prism will be comparatively small.