Thick lenses

Question 1

How can you work out the power of lens?

Answer 1

lens maker equation

F= (n-1)(1/r1-1/r2)

Question 2

How does light change vergence in a lens?

Answer 2

when it enters a medium of higher refractive index
light slows down when moving from air to lens
-More material, more light slow down, greater refraction

Question 3

How do we assign powers to lenses?

Answer 3

in terms of their back surface power - F’v

Question 4

Why does the point A2 (which is the F’v) represent the whole power of the lens?

Answer 4

as light is passing through the lens is altered by both surfaces and the material it is passing through
-point of A2, F1, F2, t (thickness) is taken into account.

Question 5

What is F’v?

Answer 5

is the sum of the effective power of the front surface at the back surface and the back surface power.
-back surface power closest to eye as light exits this surface and enters the eye.

Question 6

What is the equivalent thickness of air (d) equation?

Answer 6

d= t/n
t= (when light travels through a medium other than air, the real thickness of the material)(t) can be replaced by an equivalent thickness of air (d)

Question 7

What happens when you request for a lens from a lab?

Answer 7

usually the lens comes with one surface either f1 or f2 already worked

Question 8

How do you create the final power that is requested of yer lens?

Answer 8

the lab will work curves on the opposite surface

Question 9

How can we compensate a surface for the change in prescription ?

Answer 9

due to thickness of lens

Question 10

What can you know if the thickness and one surface power are known?

Answer 10

then to make a lens of a given back vertex power, the other surface power needs to be compensated for the thickness and worked with a different tool.

Question 11

So, how can calculate the other surface needed?

Answer 11

using equations or a modified from step along

-Thick lens equation

Question 12

What does the thick lens equation do?

Answer 12

takes into account that the power of the lens will be changed by its thickness.

Question 13

What is accurate transpositon?

Answer 13

re-calculate one surface power of a lens

Question 14

What is nominal/uncompensated surface power?

Answer 14

surface power that is worked by the lab if they used F1+F2; if they assumed the surfaces to be thin and the thickness of the lens played no part.
F1= F’V-F2

Question 15

What is reduced/compensted surface power?

Answer 15

actual surface power when taking into account the the lens thickness
F1 or F2- what we want to calculatw

Question 16

What is vertex power allowance ?

Answer 16

the difference between the compensated and uncompensated powers
VPA=F1-F1N or VPA= F2-F2N
-change in thickness caused
-Always a negative value

Question 17

What is front surface compensation?

Answer 17

when the back surface of the lens has already been worked to get the correct prescription we need to take into account the lens thickness and calculate what power is required on the front surface to give the correct prescription.
-So in this example we assume cannot make any changes to F2.
So what power needs to be placed on the F1 to get the correct prescription

Question 18

What is the equation for the front surface compensation?

Answer 18

F1= F1N/1+(t/n)F1N

Question 19

What is a back surface compensation?

Answer 19

when the front surface of the lens have been worked already ti gets the correct prescription we need to take into account the lens thickness and calculate what power is required on the back surface to give the correct prescription.

Question 20

What is the equation of the back surface compensation?

Answer 20

F2= F’v - F1/1-(t/n)F1