Refraction

• Refraction occurs when a wave travels from one medium to another, when light is refracted its direction of travel is changed
• Snells law states: for light traveling from one medium to another the ratio sin I/ sin r is a constant. this constant is known as the refractive index
• N = sin I / sin r
• When a ray of light consisting of one wavelength passes from one material into another, both its speed and wavelength change, its frequency does not. The speed and wavelength are related to the refractive index
• The refractive index can be calculated using the formula N= c/ v where n is the refractive index, c is the velocity of light in vacuum and v is the velocity of light in the substance being measured.
• Refractive index = speed of light in vacuum/ speed of light in material
• Speed of light in vacuum = 3x10^8 m/s
• The largest angle of refraction is 90 degree. the angle of incidence in the glass that produces this angle of refraction is called the critical angle C. the value of the critical angle depends on the refractive index of the material
• Sin C = 1/airNmaterial
• When the angle of incidence in the material with the higher refractive index is greater than the critical angl, a phenomenon known as total internal reflection occurs. total internal reflection involves the reflection of all the incident light at the boundary between the two materials.
• For total internal reflection to occur: the direction of the light is from one material to one of a lower refractive index, the angle of incidence in the material of higher refractive index is greater than the critical angle
• Total Internal reflection is possible for light traveling from water towards air but not from air towards water
• Optical fibers are the use of a long strand of glass to send light from one end of the medium to the other is the basis for modern day use of optical fibers
• Optical fibers are used in communication systems and micro surgery
• Light meets the glass/air boundary the angle of incidence exceeds the critical angle and total internal reflection takes place. none of the incident energy is lost due to transmission of light across the boundary. The intensity of the signal remains constant.
• the step index fiber consists of a glass core, typically 100 micrometers in diameter, surrounded by a glass cladding, typically 150 micrometers in diameter
• The refractive index of the core is slightly greater than that of the cladding. this type of optical fiber is called the step index because the refractive index does not gradually change at the core cladding boundary, rather it changes like a step
• Provided that the angle at the core cladding boundary is greater than the critical angle the light signal propagates in the core by repeated total internal reflections
• The path of light along the center of the core is called the axial mode. The path of light that repeatedly meets the core cladding boundary at the critical angle is called the highest order mode. there is a clear difference between the time taken for light traveling by the central axis and that traveling by repeated total internal reflections at the critical angle. This is called modal dispersion and it is very undesirable.
• In an endoscope modal dispersion would result in a blurred image of the target organ
• Modal dispersion can be prevented by using very thin cores so that only axial mode is possible.
• The endoscope has two bundles of optical fibers. One is called the illumination bundle and carries light towards the object being viewed. The other bundle, the image bundle carries back the reflected light. the optical fibers inside the image bundle are carefully arranged parallel to each other to create what is termed a coherent bundle
• Forceps attached at the viewing end of an endoscope allows a surgeon remove a sample tissue (biopsy)
• Optical fiber