power and lenses L3

Question 1

what happens when light is moving away from a source ?

Answer 1

light is divergent - negative

Question 2

what happens when light is moving towards a source ?

Answer 2

light is convergent - positive

Question 3

what do capital letters represent ?

Answer 3

(L) powers

Question 4

what do small letters represent ?

Answer 4

(l) distances

Question 5

what is the refractive index of air ?

Answer 5

n=1

Question 6

what is the power of lens ?

Answer 6

. the power of a lens is the reciprocal of its focal length
F=1000/f
. the power of lens can also be calculated from the power of front and back surface
F’v= F1 + F2
F1 = front surface which is furthest away from the eye

Question 7

how else can we calculate power of lens ?

Answer 7

. use radius of curvature of a lens surface to work out the power of a lens
. each surface has its own power dependant on the radius of curvature and the refractive index
-U
- power of lens surface

Question 8

how to know if its n-1 or 1-n ?

Answer 8

. refractive index of the medium which the light is about enter minus the refractive index of the medium which it has left

Question 9

what is a thin lens ?

Answer 9

. thin lens is when the two surfaces are in close contact , we can therefore ignore the refractive index of the material between the two surfaces
F=F1 + F2
-the power of a thin lens is the sum of the power of both its surfaces

Question 10

what is the fundamental paraxial equation ?

Answer 10

. allows us to study the effect that lenses have on light using distances to work out vergences
. L’=L+F

L= incident vergence arriving from the object 
L'= vergence leaving the surface

N and N’ = 1 if its in air
We assume the lens is thin which means the vergence leaving the first surface is the same as the vergence leaving the second surface

Question 11

what happens to vergence in a thin lens ?

Answer 11

in a thin lens vergence leaving first surface is the same as vergence leaving second surface as the material is thin that the material it self is not altering light when it travels through the lens

Question 12

how to work out the back and front vertex of a two lens system ?

Answer 12

use step along method

1. find L1= vergence of light incident at first surface
2. find L1’ = vergence of light leaving first surface
3. find l1’
4. find l2 = l1’ - d
5. find L2 = vergence of light incident on second surface
6. find L2’ = vergence of light leaving the second surface ( back vertex power )

Question 13

what is back vertex distance ?

Answer 13

. the back vertex distance in mm is the distance between the back vertex of the lens and the cornea and it effects the power of lens
. if you increase the distance from the lens to the eye then the lens becomes more positive , therefore you have to make prescribed lens more negative

Question 14

how to work out effectivity ?

Answer 14

. find the focal length of lens f’v = 1000/F’v
. alter for BVD you do this by either adding or dividing distance f’ fe = f’v +/- d
. find the power of new lens

Question 15

what happens to light as it travels through a medium other than air ?

Answer 15

. 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)
d=t/n

Question 16

What represents the back vertex power?

Why we most interested in it?

Answer 16

F’V

because after light leaves the back surface that it then enters the eye.

Question 17

How do you find out the power of lenses surfaces?

Answer 17

• By taking into account the reflectivity of the material
• Because when light reaches that curve its bent down due to the way of slowing as it enters the lens
• the light slows down and bends as it enters the material.

Question 18

What are the new eq for the power of lens surface?

Answer 18

F1= 1000(N-1)/r1 (mm)

F2=1000(1-n)/r2

Question 19

How do we re arrange the equation to work out the radius of lens surfaces?

Answer 19

r1= 1000(n-1)/F1
r2= 1000(1-n)/f2

Question 20

What is the lens makers equation?

Answer 20

• For the overall power of the lens
• We can combine the 2 equations and add them up to get the overall power or can do the lens maker equation

F= (n-1) (1/r1 - 1/r2)
the 1 used represents metres

do in 1000 to get in mm

Question 21

What happens if you want to find the overall power of the lens but do not have the radius of curvature?

Answer 21

Use fundamental paraxial equation
to find out F1 and F2

-Can also re arran ge the equation to find out the power of the unknown lens

Question 22

What happens when calculating front vertex?

Answer 22

You swap the focal powers around F1 becomes F2 and F2 becomes F1