thin lenses and refraction 2.2

Question 1

what is the thin lens approximation ?

Answer 1

. light doesn’t converge or diverge significantly between the surface of thin lens- thus no significant change in vergence
. the refraction of both surfaces can be added - possible to represent lens as one surface refracting light once

Question 2

why is the thin lens approximation advantageous ?

Answer 2

the refraction at both surfaces can be reduced to refraction at a single surface

Question 3

when does thin lens approximation mostly occur ?

Answer 3

. occurs mostly when lens are thin

Question 4

The anterior and posterior corneal surfaces are separated by 0.55mm. Why would a thin lens approximation be questionable in this case?

Answer 4

the surfaces have high curvatures producing a change in vergence between them

Question 5

what happens as object moves away from lens ?

Answer 5

. the value of vergence becomes smaller , light rays is less diverging , cone angle also decreases

Question 6

what is the vergence of light from an infinitely distant object ?

Answer 6

L=OD

Question 7

what are the focal points ?

Answer 7

F’ = back focal point
point object at infinity forms an image at F’
f’ = back focal length
property of the lens ; its surface curvatures and refractive index
F = front focal point
object placed at F forms image at infinity
f = front focal length

Question 8

what is the equation for power of a thin lens (F) ?

Answer 8

. F= -n/f = n’/f’

Question 9

what does relationship between F and f involve ?

Answer 9

refractive index

Question 10

what is paraxial surface power ( F ) ?

Answer 10

paraxial surface power depends on radius of curvature and difference in refractive index
F ( D )=n’-n / r (mm)

Question 11

why do eyes have powerful optical surfaces ?

Answer 11

due to steep radius of curvature

Question 12

what is the refraction equation ?

Answer 12

relates the vergence of the light striking a surface/lens to that leaving it

Question 13

what is the refraction equation ?

Answer 13

L' = L + F 
L = n/l negative as its measured from surface to left
L' = n'/l'

Question 14

What does the thin lens approximation depend on?

Answer 14

depends on whether the light changes our not in its vergence between its surfaces
so for E.G
-Contact lens- due to the surfaces being steeply curved there could be a slight change in vergence between the surfaces although the surface is very thin