Optics Flashcards
What is the spectrum of visible radiation?
400 - 780 nm
Which wavebands are absorbed by:
- Cornea / sclera
- Lens
And which ones pass through the ocular media to fall on the retina?
Cornea/ sclera: UV-C, UV-B (below 315nm) and IR-B, IR-C (above 1400nm)
Lens: UV-A (315-400nm)
Falls on retina:
- Visual spectrum (400-780nm): perceived as light
- Near infrared, IR-A (780-1400nm): heat / burns if severe
Which chromosome is the blue pigment gene carried on?
Chromosome 7
Which chromosome are the red and green pigments carried on?
X chromosome
Which is the most common type of colour blindness?
Red-green colour blindness (8% men, 0.5% of women)
Most commonly a defect in the green cones i.e. deuteranomaly (5% of men, 0.3% of women)
Tritan (blue) defects are very rare
What are the peak spectral sensitivities of the 3 cone pigments?
Blue: 480nm
Green: 535nm
Red: 560nm
In glaucoma, is blue-yellow or red-green colour vision affected first?
Blue-yellow
What are the absorption and emission spectra of fluorescein?
Absorption: 465-490nm (blue)
Emission: 520-530nm (yellow-green)
The RPE absorbs these wavelengths, so the choroidal circulation (hidden beneath the RPE) is not visible using fluorescein
What are the absorption and emission spectra of indocyanine green?
Absorption and emission spectra are outside the visual spectrum in the infrared spectrum:
- Absorption: 805
- Emission: 835
RPE does not absorb these so it is possible to visualise the choroidal circulation using ICG imaging
What are the two sources of error that limit the resolving power of an optical system?
- Aberration - from anomalies in the optical system
- Diffraction - from the finite aperture of the optical elements (cannot be resolved by improving the quality of the optical system)
What is the Rayleigh criterion for determining the limit / angle of resolution of an optical system?
The limit of resolution is reached when two Airy discs are separated just enough that the centre of one falls on the first dark ring of the other.
The minimum angle of resolution can be calculated from this using the formula:
Theta = 1.22 (lambda / D)
- Theta = angle in radians
- Lambda = wavelength of incident light
- D = diameter of aperture
What are visual evoked potentials and what are they a measure of?
Electrical response generated in occipital cortex by visual stimulation
Stimulus is usually a high-contrast chequerboard which reverses pattern at a set frequency
Delayed VEPs may indicate demyelination / abnormal patterns may indicate specific areas of visual disruption
What is the minimum angle of resolution of the eye?
One minute of arc (i.e. one cone photoreceptor)
What is the peak photopic sensitivity of the eye?
555nm (yellow-green)
What is Snell’s Law?
Snell’s Law of Refraction.
n1/n2 = sin theta i / sin theta r
What is the critical angle for total internal reflection?
Angle of incidence at which the angle of refraction is parallel to the interface
Prism dioptre
1 prism dioptre produces a vertical linear displacement of 1cm at 1m
(1 centrad - produces a curvilinear displacement of 1cm)
Roughly = ½ a degree of elevation
Definition of a lens
A portion of refracting medium bordered by two curved surfaces which share a common axis
Definition of a prism
A portion of refracting medium bounded by two plane surfaces which meet each other at a finite angle (the apical angle)
To correct convergence, should prisms be base out or base in?
Base out - this will result in diverging rays, enabling correction of abnormal ocular convergence
What 3 factors determine the total vergence power of a spherical lens?
- Vergence power of each surface
- Thickness of lens
- Medium either side of lens
A lens has a power of 5 dioptres. What is its focal length in cm?
20 cm
Lens power = 1/ (2nd focal length in m)
5 = 1/(0.2)
What is the image produced by a convex lens from an object located inside its focal point?
Virtual, upright, magnified, on the same side as the object
What is the image produced by a concave lens from an object located outside its focal point?
Real, inverted, outside F2, opposite side from object
What sort of image is produced by a convex lens for an object located at any distance?
Virtual, upright, minified, same side as object
What is the near point of the eye?
Shortest distance at which the eye can focus unaided = 25cm in a normal eye
What is the far point of the eye?
Furthest distance at which the eye can focus unaided = infinity in a normal eye
What is the lens power of a x8 loupe?
Magnification of a simple loupe = lens power in dioptres / 4
Hence the lens power is 32D.
What is the formula for calculating prismatic power at the periphery of a lens?
P = F x D
Where P is prismatic power in prism dioptres
F is lens power in dioptres
D is the decentration (distance from centre of lens) in centimetres
What are the principles of the duochrome test and what is it used for?
Chromatic aberration of the optic system of the eye means longer wavelengths (red) are focused more posteriorly than shorter ones (blue).
The emmetropic eye focuses for yellow-green light at 555nm. Red and green foci are equal distances from the yellow-green focus.
Thus, the duochrome test tests the focusing point of the red and green wavelengths. Myopes focus the red wavelength more clearly as it has a longer focal length. At the end of the test, myopes should see the red letter more clearly than the green one so as not to be over-corrected.