1. What is the mean angle of resolution of an eye with a 6/6 acuity?
One degree of arc.
2. How far away can an eye with 6/6 visual acuity read the 6/24 lines?
3. What is Snell's law?Snell's law (law of refraction) = the incident and refracted rays and the normal to the surface at the point of incidence lie in the same plane and the ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is a constant for any two media. This constant is called the relative index of refraction ie.
sine i / sin r = refractive index.
4. What is the apex angle in degrees of a 10 dioptres prism?
5. The nodal point of a lens is at the intersection between the principal axis and the principal plane.
6. The refracting power of a cylindrical lens is at 90 degrees to the axis.
7. The image produced by a negative lens is virtual, erect and diminished.
8. A focimeter measures the back vertex power of a lens
9. A decentration of 10 mm from the optical centre of a +5 dioptre lens produces a prismatic effect of 5 prism dioptres
10. A X8 loupe has an equivalent power of 32 dioptres.
11. Two lens of power +4 dioptres are separated by a distance of 10 cm. The final vergence of a ray of light initially parallel to the principle axis is 10.7 dioptres
The vergence power can be calculated using the formula:
Fn = F1 + F2 - dF1F2 / 1 - dF2
Fn = new vergence power;
F1 = power of the first lens;
F2 = power of the second lens
d = distance between the two lenses (in metres)
12. A lens of +10 dioptres fully correct an hyperopia and now the lens is moved forward 10mm, what is the new lens power needed to correct the hyperopia?
To calculate this you need to use the formula for lens effectivity which is
Dn = Do/1-dDo where Dn = the new power
Do = the old lens power
d = difference in the location (in meters), - if it is moved
forward and + if it is moved backward.
The formula shows that moving a plus lens forward increases its effective power and therefore a weaker plus
lens is needed to maintain the same effectiveness.
13. In trifocals the intermediate lens usually has an addition of 1 dioptre over the distance correction.
14. The LTF stands for luminance transmission factor when considering tints.
15. Relative spectacle magnification =
Actual size corrected with spectacle / size seen by the emmetropic eye
16. Which combination make up 0.50 Jackson cross cylinder?
The two cylinders are 90 degrees to each other and therefore it is not necessary to designate the axis.
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