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.
The duochrome test is sensitive to changes in refraction of how many dioptres?
0.25D
Is the duochrome test valid in red-green colour blind individuals?
Yes, because it relies on the physical position of the different wavelengths on the retina, not recognition of the wavelength as a colour.
What is the difference between axial and index myopia?
Myopia - 2nd principal focus lies in front of retina
Axial - eye is too long
Refractive - refractive power is too high
What thickness (in terms of fraction of wavelength of incident light) should a lens coating be in order for it to be
(a) anti-reflective
(b) reflective
(a) One-quarter the wavelength of incident light —> reflection from posterior surface emerges out of phase by half a wavelength —> destructive interference —> no reflection.
(b) Half the wavelength of incident light —> as above but now out of phase by one wavelength —> in phase —> constructive interference —> reflection
What is the chromatic aberration of the eye?
2D between red and blue wavelengths
What is an aplanatic lens?
A lens where the peripheral curvature is less than the central curvature.
What is the Stiles-Crawford effect and what is its significance?
Directional sensitivity of cone photoreceptors - much more sensitive to paraxially entering light than peripherally entering light
What are the 4 ways in which ocular spherical aberration is minimised?
- Iris acts as lens stop to block periphery of lens
- Cornea has an aplanatic shape as periphery is thicker (600nm) than middle (500nm)
- Lens nucleus has greater refracting power than lens cortex
- Stiles-Crawford effect makes retina less sensitive to peripherally entering light rays
What is the definition of the principal points of a lens?
Light entering at P1 exits at P2 at the same vertical distance from the principal axis
What is the definition of the nodal points of a lens?
Light entering at N1 exits at N2 at the same angle.
If the medium either side of the lens is the same, then the nodal points are the same as the principal points.
What does a focimeter measure?
Anterior / posterior vertex focal length, i.e. distance of F1/ F2 from principal plane
Using this to calculate lens power will result in the posterior vertex power - not the ‘true’ / equivalent focal power
However, spectacle lenses are graded by their back vertex power as this is what is relevant to refractive correction in the eye.
What is the amplitude of accommodation?
Difference between static refractive power (dioptric power of relaxed eye) and dynamic (dioptric power of eye at full accommodation) refractive power of the eye
What are the near and far points of an emmetropic, myopic and hypermetropic eye?
Emmetropic: 25cm, infinity
Myopic: E.g. 5cm, 30cm
Hypermetropic: E.g. 40cm, -40cm (far point is virtual as only diverging rays can be focused on retina)
What are the 4 Purkinje-Sanson images?
- Formed by anterior corneal surface (+7.7 curvature)
- Formed by posterior corneal surface (+ 6.8 curvature)
- Formed by anterior lens surface (+ 10 curvature)
- Formed by posterior lens surface (-6 curvature)
Images 1,2 and 3 are formed by convex mirrors and are all virtual, erect and diminished. Image 4 is formed by the posterior lens capsule which is concave, so is real and inverted.
Name two causes of index myopia.
- Keratoconus (abnormally steeply curved cornea)
2. Nucleosclerosis (increasing density and refractive power of lens nucleus with age)
State the difference between manifest and latent hypermetropia.
Manifest hypermetropia: Component of hypermetropia that cannot be corrected by accommodation
Latent hypermetropia: Remainder masked by ciliary tone and involuntary accommodation
Why are hypermetropes more likely to develop amblyopia than myopes?
Accommodation is bilateral, i.e. the eyes cannot accommodate to different amounts. Hence near vision will be blurred in the more hypermetropic eye.
In myopia, near vision is clear in both eyes so a greater degree of anisometropia is tolerated.
What amount of anisometropia is sufficient to induce amblyopia in hypermetropes?
1D difference
Is the spectacle image magnified / minified in hypermetropia / myopia?
Hypermetropia (concave lens) - image is minified
Myopia (convex lens) - image is magnified
What is the formula to calculate the new lens power when the corrective lens is moved forward or back from the eye?
F2 = F1 /(1- dF1)
F2: Power of new lens in dioptres
F1: Power of original lens
d: Distance moved from position of original lens (in metres). If moved forwards away from the eye, value is negative; if moved back towards the eye, value is positive.
To double check, remember that if a lens is moved forward from the eye, hyperopes will need a weaker lens and myopes a stronger lens.
What is the apex angle in degrees of a 10 prism dioptre glass prism? (RI = 1.5)
10 degrees
What is the thin lens formula?
1/v + 1/u = 1/f
Where v = distance of object from lens
u = distance of image from lens
f = focal distance of lens
What are the 4 tests for stereoacuity and what are their sensitive ranges?
Titmus (3000 - 40 seconds arc)
Frisby (600 - 15 seconds arc)
TNO (450 - 15 seconds arc)
Lang (1200 - 550 seconds arc)
How should a prism be oriented to correct strabismus?
Apex should face direction of deviation and base should face opposite direction
E.g. hypertropia: Base down prism
esotropia: base out prism
What is the field of view in the direct and in the indirect ophthalmoscope?
6 degrees and 15 degrees respectively
When using the direct ophthalmoscope, what is the difference in field of view / magnification between myopes and hypermetropes?
Myopes: Small field of view, low magnification
Hypermetropes: Large field of view, high magnification
Advantages of indirect ophthalmoscope
Binocular vision
Greater field of view
Better illumination
Can be used for teaching via teaching mirror
What are the 3 major advantages of indirect over direct ophthalmoscopy?
- Better illumination
- Stereopsis
- Greater field of view
What is the area of contact in applanation tonometry?
3.06mm in diameter
What is Scheiner’s principle in relation to automated refraction?
Scheiner double pin-hole refraction - 2 pinholes of light enter eye
Emmetrope - perfectly converged, sees one spot
Myope - crossed diplopia, sees 2 spots
Hypermetrope - uncrossed diplopia, sees 2 spots
When using a condensing lens, what factors affect magnification and field of view?
Magnification: Power of lens. M = (power of pt’s eye) / (power of lens), hence a low powered lens gives greater magnification
Field of view: Power and apeture of lens. High power and wide apeture = wider field of view
What is the relative spectacle magnification at 12-15mm from the eye (normal spectacle distance)? What about with contact lenses?
1.33 for spectacles, 1.1 for contact lenses
What are the 4 types of laser effects on the eye?
- Photocoagulation (532nm)
- Photodisruption (1064nm)
- Photoablation (193nm)
- Photoradiation (193nm)
What wavelength is emitted by a ND-YAG laser?
And what is it used for?
1064nm
Photodisruption
What wavelength is emitted by a frequency-doubled YAG laser?
What is it used for?
532nm
Photocoagulation
What wavelength is emitted by excimer laser?
What is it used for?
193nm
Refractive surgery
Prentice position and position of minimum deviation - which is traditionally used to specify glass and which for plastic prisms?
Prentice - glass
Min deviation - plastic
What is the spherical equivalent of -3.5/+1.5 x 70?
Spherical equivalent = ½ (cylindrical power) + spherical power
= 0.75 + (-3.5)
= -2.75
What is the SRK formula?
IOL power required = A - 2.5L - 0.9K
Where L is axial length of eye
K is keratometry value (i.e. refractive power of cornea)
What is Watzke’s sign for macular holes?
A thin strip of light is projected onto the macula using the slit lamp. The patient is asked to describe the appearance of the light - a broken line may indicate a macular hole.
What is the field of view with a direct and an indirect ophthalmoscope?
Direct: 6 deg
Indirect: 25 deg
When using the direct ophthalmoscope, is the field of view larger in a myope or in a hypermetrope?
Field of view is larger (and magnification less) in a hypermetrope.
In indirect ophthalmoscopy, what 3 factors affect the field of view?
- Size of observer’s pupil
- Strength of condensing lens (Higher power = bigger field)
- Aperture of condensing lens (Bigger aperture = bigger field)
In retinoscopy (plane mirror), does an against movement always indicate myopia?
Yes - an against movement indicates that the observer is located beyond the far point of the eye
A 36D lens used as a loupe will produce a magnification of:
9x
Loupe magnification = lens power / 4
