Week 2 Flashcards
Lens thickness
- separation between the two lens vertex points
- always positive
- always measured in meters
The optical power of a thin lens is…
-the sum of the optical powers of each SSRI
Gullstrand’s relationship
Power of a thick lens= P1+P2-(d/n)(P1P2)
- d is always positive
- n is always positive
The optical power of a thick lens in comparison to the sum of the optical powers of the two SSRI’s is…
-can be larger or smaller
Can a convex lens be negative?
-a VERY thick biconvex lens can be negative in air
Optical Power of a real, thick lens depends on…
- the radii of curvature of the refracting surfaces(R)
- The lens material refractive index in relation to the surrounding medium refractive index
- surface separation(d)
Ball lens
- a biconvex lens with thickness equal to twice the radius of curvature(R) of its surfaces
- easy to handle
- no orientation requirements
For a ball lens of glass(n=1.5), the focal distance in air is…
f=1.5R
-FOCAL DISTANCE IS MULTIPLIED BY R NOT DIVIDED!!
Hoegh meniscus
- flat piece of glass, bent
- refracting surfaces with same radii of curvature, but slightly displaced centers of curvature
Hoegh meniscus power
Material is glass and surrounded by air, has positive optical power
-plus lens
Size lens
- can be plus or minus
- same centers of curvature, but different radii of curvature
- R1>R2
Cornea
-outside is slightly less curvy than inside
If cornea were surrounded by air, what is optical power?
-5.2D
If cornea were surrounded by air on outside and aqueous solution on inside, what is optical power?
POSITIVE
-like +43D
What is most important factor that makes the cornea a strong plus lens?
-the fact that the posterior surface is surrounded by the aqueous with nearly similar refractive index
Thin lens ray diagrams
- pretty much approximation
- we don’t really care where the refraction is, just that is does
Thick lens inflection point
- 2 refractions
- we can pinpoint it fur sure
Principal plane
- H’
- where first refraction happens
Principal point
- P’
- where principal plane and optical axis intersect
Effective focal length
-comes from principal plane
How many principal planes for a thick lens?
- every thick lens has 2 principal planes and 2 principal points
- H and H’
- P and P’
Why stay close to the optical axis?
- stay close to optical axis to make sure you’re not way off.
- more accurate
- paraxial approximation
- if you deviate, your approximations won’t hold in operations
Front focal length
front: to the left, front of lens, actually from lens
- distance along the optical axis that separates the focal point in the object space from the vertex of the first surface
Back focal length
-the distance along the optical axis that separates the back point in the object space from the vertex of the second surface
Nodal points
- preserve angle with optical axis
- parallel displacement
Cardinal points of thick lens
- 2 focal points
- 2 principal points
- 2 nodal points
ALL LOCATED ON OPTICAL AXIS
Focal points
-collimate a beam from the primary focal point or converge a collimated beam to a secondary focal point
Principal points
-preserve ray displacement
Why use Cardinal points?
- image size, location, and orientation are copletely determined by cardinal points
- lens can be treated as if all of the refraction happends at the principal planes
- A ray aimed at one nodal point will be refracted by the lens such that it appears to have come from the other and with the SAME angle with respect to the optical axis
Assumptions of a thin lens
- both sides of the lens are the same medium (would need to check focal distance to be able to tell)
- Shorter focal distance means higher n - thickness is zero
- assume paraxial
Actual rays vs. conventional rays
Actual: subjected to two refractions, one per refracting surface
conventional: change path only when encounter the principal planes or nodal points
Ray rules
- parallel
- focal
- radial/nodal
REAL RAYS DO NOT ALL PASS THROUGH THE CARDINAL POINTS BUT IT IS USEFUL TO SEE FINAL PATH OF THE RAY
Thick lens imaging
- object location is measured from principal plane H, and image location from principal plane H’
- focal length is measured from principal plane H’ when light propagates to the right
- light propagates to the left, focal length is measured from secondary principal plane H
