Answers:

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?

24 metres.

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?

10 degrees.

5. The nodal point of a lens is at the intersection betweenthe principal axisandthe principal plane.

6. The refracting power of a cylindrical lens is at90degrees to the axis.

7. The image produced by a negative lens isvirtual,erectanddiminished.

8. A focimeter measures theback vertex powerof a lens

9. A decentration of 10 mm from the optical centre of a +5 dioptre lens produces a prismatic effect of5prism dioptres

10. A X8 loupe has an equivalent power of32dioptres.

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 is10.7dioptres

The vergence power can be calculated using the formula:Fn = F1 + F2 - dF1F2 / 1 - dF2Fn = new vergence power;F1 = power of the first lens;F2 = power of the second lensd = 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?+9 dioptresTo calculate this you need to use the formula for lens effectivity which isDn = Do/1-dDo where Dn = the new powerDo = the old lens powerd = 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 of1dioptre over the distance correction.

14. The LTF stands forluminance transmission factorwhen 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?+0.50DS/-1.00DC

The two cylinders are 90 degrees to each other and therefore it is not necessary to designate the axis.

Click here for the questionsClick here to return to the main section