What is a thick lens?
A physically large lens with two spherical surfaces separated by a non-negligible distance.
What is the key difference between thin and thick lenses?
hin lens: refraction occurs at one plane.
Thick lens: refraction occurs at both front and back surfaces.
How is an image formed in a thick lens?
Light from the object is refracted at the front surface and again at the back surface to form the image.
What is an equivalent lens?
A hypothetical lens used to simplify analysis, where refraction is assumed to occur at principal planes.
Do equivalent lenses physically exist?
No, they are imaginary.
What are principal planes in a thick lens?
Imaginary planes where all refraction is assumed to occur.
What is the primary principal plane?
H
What happens to refraction in an equivalent lens model?
All refraction is assumed to occur at H and H’.
What is the secondary principal plane?H’
H’
How many focal lengths are there in a thick lens?
4
What is the primary equivalent focal length (fe)?
Distance from H to F.
What is the secondary equivalent focal length (fe’)?
Distance from H’ to F’.
What is the back vertex focal length (fv)?
Distance from the back surface to the secondary focal point.
What is the front vertex focal length (fn)?
Distance from the front surface to the primary focal point.
What is back vertex power?
The power of a lens measured with respect to the back surface.
What is the symbol for back vertex power?
Fv
Which instrument measures back vertex power?
Focimeter
What does a focimeter measure in practice?
The prescription (back vertex power) of a spectacle lens.
What parameters are used in the back vertex power formula?
F1 (front surface power), F2 (back surface power), t (thickness in meters), n (refractive index).
What is front vertex power?
The power of a lens measured with respect to the front surface.
What is the symbol for front vertex power?
Fn
In which procedure is front vertex power determined?
Hand neutralization.
