OL 7

Question 1

What does the focimeter measure?

Answer 1

measures the front or back vertex power of a lens or system of lenses.

Question 2

what can the foci meter derive? (2)

Answer 2

derive the sphere-cylinder prescription

to locate the optical centre of a lens and any prismatic effect it may have

Question 3

what is the collimator and the telescope?

Answer 3

collimator is the focussing system

telescope is the observation system

Question 4

The _______ system produces an image of the target projected to _______

Answer 4

focussing

infinity

Question 5

what power does the standard lens have?

Answer 5

+22.00D and +27.00D

Question 6

where does the lens rest R lie?

Answer 6

second principal focus of the collimating lens.

Question 7

what happens to the image through the telescope if additional lenses are added?

Answer 7

the image through the telescope is always the same size irrespective of the power of any additional lens introduced into the system

Question 8

why must the light rays entering the telescope be parallel? And how can they enter parallel?

Answer 8

for the target to be imaged by the telescope, the light rays entering it must be parallel.
In order to meet this condition, the target T must be moved by a distance of x to T’

Question 9

what does newton’s relationship state?

Answer 9

xx’ = ff’

For a thin lens in air the first and second focal lengths are equal Thus: f = -f’
or
xx’ = -(f’)^2 (f’)^2 = -xx’

Question 10

why is the relationship between the dioptric power and the scale linear?

Answer 10

The vertex power of an unknown lens is directly proportional to the movement of the target.

Question 11

how do electrons focimeters find the lens foci?

Answer 11

automatically, using monochromatic laser light

Question 12

what replaces the target and the human eye in electronic focimeters?

Answer 12

series of lasers replace the target

electronic sensor replaces the human eye

Question 13

forcimeter errors

Answer 13

1. Wavelength of light used for measurement- Electronic focimeters use red laser light and therefore the refractive index must be extrapolated from the position of focus
2. Spherical aberration
3. Eyepiece focus
4. Saggital height error
5. Calibration error
6. Rounding error: just because the focimeter displays to the nearest 0.01 D, this is not an indication of accuracy
7. Wavelength calculation for high index lenses

Question 14

what are the 4 requirements for the design of an instrument?

Answer 14

1. permitted wavelengths
2. range of measurement
3. effective aperture
4. in the range -5.00 D to +5.00 D the allowable tolerance is ±0.06D