2.2 Refraction Flashcards
What is refraction?
Refraction is the change of direction a light ray undergoes when it enters a medium with a different refractive index.
What are the 2 requirements for refraction?
- Ray passes into a material with a different refractive index.
- At an angle of incidence > 0֯.
What is Snell’s law?
At the boundary between two media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction (sin i / sin r) is constant.
What happens when white light enters a prism?
The different colours have different wavelengths. As white light enters the prism and refracts the frequency of each colour stays constant and the wavelength decreases.
What happens during the dispersion of light?
- When the wavelength is smaller (e.g. violet light), the angle of refraction will be smaller (bends most). Therefore, it will refract closer to the normal.
- When the wavelength is larger (e.g. red light), the angle of refraction will be larger, so the ray will be further from the normal (bends the least).
What is meant by the critical angle?
The angle of incidence for which the angle of refraction is 90°
What is total internal reflection?
For any angles of incidence greater than the critical angle, all of the light gets reflected.
What are the 2 conditions for total internal reflection?
- The wave must be travelling from a more optically dense medium to a less optically dense medium (e.g. from glass to air, not the other way around).
(higher refractive index medium to a lower refractive index medium) - Angle of incidence > critical angle.
What is the structure of Monomode fibre optic cables?
- Monomode fibres only use one mode of light (essentially one wavelength/one path), therefore very little dispersion.
- They are expensive and difficult to make.
- Sharp change in refractive index at core cladding boundary.
What is the structure of Multimode/Step-Index fibre optic cables?
- Relatively wide core, thus ‘light’ has various paths.
- As a result, different paths take different lengths of time. This leads to dispersion of the signal.
- Sharp change in refractive index at core cladding boundary.
What are the uses of total internal reflection in industry?
- Fibre optic cables
- Endoscopes
How do flexible endoscopes use fibre optic cables?
- Deliver non-coherent light into the body.
- Take coherent light from the body back to an eyepiece to form a clear image.
What is coherency?
The fibres going from the body / object to eye or computer need to be coherent. It means they have the Same Spatial Relationship.
A coherent bundle means that the force ends at each end must be in the same relative position.
What happens during the use of flexible endoscopes?
- Incoherent light travels down a bundle of optical fibres to illuminate the inside of a patient.
- The light reflects off the surfaces inside the body.
- Coherent bundle is used to produce images by using fibres with the same spatial relationship.
- This bundle is connected to an eyepiece/camera for observation.
- Control cables allow view to be manipulated/tools to be operated (eg cutting samples).
- Often tubes present to facilitate irrigation.
