5. Spherical Lenses Flashcards
Spherical lenses
part of a sphere
Convex lens
converge light
plus lens
for hypermetropiaco
Concave lens
diverge light
minus lens
for myopia
total vergence power for THIN lenses
sum of each surface
- Principal / nodal point, N
point where the principal plan and axis intersect (light passing through here is undeviated)
First principal focus, F1
point of ORIGIN of rays which AFTER refraction are parallel to the principal plan
- First focal length, f1
F1 to N
- Second principal focus, F2
where incident light parallel to the principal axis BEFORE refraction and brought to after refraction
- Second focal length, f2
N to F2
o Positive for convex
o Negative for concave
o Lenses are designated by the f2
B-first principal focus
F-second principal focus
C-first focal length
D-second focal length
A, E-nodal point
A, E-nodal point
C-first focal length
F-second focal length
B-first principal focus
D-second principal focus
thin lens formula
how to construct a ray diaram
- Ray from top of object passing through the principal point
- Ray parallel to the principal axis through / away from second principal focus
image formed by a thin convex lens outside F1
real, inverted, outside F2
image formed by a thin convex lens at F1
virtual, erect. at infinity
image formed by a thin convex lens inside F1
virtal, erect, magnified, further from the lens than the object
imaged formed by a thin concave lens
virtual, erect, diminished, inside F2
dioptric power
reciprocal of the second focal length in metres
* F = 1/f2
o F: vergence power in dioptres
o f2: second focal length in metres
the shorter the focal length
the more powerful the lens
converging lens with a second focal length of +5cm
F = 1 / 0.05 = +20D
linear magnification
= I / O = v / u
o I: image size
o O: object size
o v: distance of the image from the principal plane
o u: distance of the object from the principal plane.an
angular magnification
o Angle subtended by the eye governs the retinal image size
o Apparent size = size / distance
the loupe
- Bigger angle subtended by the eye = bigger image
- Loupe is a convex lens
- E.g. for the loupe = x8 loupe = 32D lens / 4
magnifiying power
M = F / 4
F = lens power in dioptres
- Peripheral portion of the lens acts as
a prism
o Convex = towards axis
o Concave = away from axis
prismatic effect
P = F X D
o P: prismatic power in prism dioptres
o F: lens power in dioptres
o D: decentration in cms
increasing prismatic power
o Greater spherical aberration
o Ring scotoma
o Jack on the box effect
