Light Flashcards
What is the unit of luminous intensity?
Candela
What is reflection?
Light normally travels in a straight line but will bounce back off shiny surfaces, this is reflection.
The initial light source is called the incident light and the angle in which it hits a surface is known as the angle of incidence.
The angle of reflection is equal to the angle of incidence.
What is refraction and what affects the degree of refraction?
Refraction is when light bends at the junction between to mediums.
It is affected by:
-The angle of incidence
-The types of medium (as light travels at different speeds in different mediums)
As light travels at different speeds in different mediums it gives rise to different index’s of refraction e.g in air 1.0, in water 1.3
What is total internal reflection?
If the angle of incidence is greater than the critical angle when approaching the medium, the light will reflect rather than pass through the 2nd medium.
This occurs when the the refractive index for the 2nd substance (n2) is less than the 1st substance i/e it is less dense.
What is diffraction?
This is the spreading of waves as it travels through a gap.
The extent of diffraction depends on the wavelength of the wave and the width of the gap.
The narrower the gap, the wider the diffraction.
How do fibreoptic laryngoscopes work?
They are constructed with 36,000-85,000 bundles of fine glass fibres 8-10 μm in diameter, which are each coated by a 1 μm thick layer of cladding glass.
As the cladding glass has a lower index of refraction (less dense).
As the cladding glass is less dense as the light passes down the glass bundle and there is total internal reflection, allowing the light to pass to the end of the laryngoscope.
What is Bouger and Lambert’s law?
Each layer of equal thickness absorbs an equal fraction of radiation that passes through it. I.e. increased absorption with increased distance travelled
What is Beer’s law?
The absorption of radiation by a given thickness of solution of a given concentration is the same as that of double the thickness of half the concentration.
I.e. increased absorption with increased concentration of medium.
What is Optical Density?
It is a unitless measure of the absorbance of a substance. It combines the 3 factors of:
-length of path
-wavelength of light
-substance concentration
How is optical density used to determine concentration in an infrared gas analyser?
If in the gas analyser the length of path and wavelength are known. Then the only variable is the substance concentration.
Therefore any change in absorption will be related to the concentration of the substance.
Explain the constituents and workings of an anaesthetic gas analyser?
Infared light is emitted from a source and passes through a filter to only allow a specific wave length through.
It then passes through a crystal (sapphire) window which does not absorb infrared light, into the sample chamber where the sampling gas is present.
It passes through the gas in the sampling chamber and some of the infrared will be absorbed.
The remaining infrared (which has not been absorbed) will pass through another crystal window and lens to focus it onto an infrared detector.
This will calculate how much infrared remains.
This is compared to a known reference, as the wavelength of the infrared light and the length of path are known, any change compared to the reference will be related to concentration.
The whole process is a spectrophotometric technique.
In what conditions does absorption of infrared radiation occur?
Absorption occurs if the substance has more than 1 atom of different elements in a molecule.
CO2 and N2O will absorb infrared radiation, whereas O2 will not.
Volatile anaesthetics have a wide variety of elements and therefore absorb infrared light.
At what wavelength does CO2 maximally absorb EM radiation?
CO2 maximally absorbs at a wavelength of
4.26 μm (infrared)
What are the different types of infrared gas analysers?
Main stream analyser: the infrared analyser is inbuilt to the breathing circuit, as such there is no lag time but they tend to only measure CO2
Side stream analyser: the sample is attached to the side of the breathing circuit, water is removed from a filter and the gas is analysed. There tends to be a lag.
What problems can occur with gas analysers which can give you inaccurate readings?
In the presence of N2O you can get a falsely high CO2 reading, as they have similar IR absorption wavelengths. Analysers usually compensate for this.
O2 can also to a lesser extent broaden the CO2 absorption spectra and hence cause interference but this is by collision with CO2 molecules rather than IR absorption by oxygen.
Water vapour leads to falsely high CO2 readings by absorbing light as well. Reduced through use of Teflon tubing travelling to the spectrometer and a water trap.
