Spherical Lenses Flashcards
What do convex lenses do to incident light
convergence of incident light
What do concave lenses do to incident light
divergence of incident light
How to calculate the total vergence power
depends on the vergence power of each surface and the thickness of the lens. Most of the lenses used in ophthalmology are thin lenses, and for a thin lens the thickness factor may be ignored.
Thus the total power of a thin lens is the sum of the two surface powers
What is the nodal/principal point
The point at which the principal plane and principal axis intersect. Rays of light passing through the nodal point are undeviated.
What is the first principal focus F1
The first principal focus, F1, is the point of origin of rays which, after refraction by the lens, are parallel to the principal axis. The distance F1N is the first focal length f1
What is the second principal focus F2
Incident light parallel to the principal axis is converged to or diverged from the second principal focus, F2. The distance F2N is the second focal length
Conventional sign of F2 for a convex lens
positive
Conventional sign of F2 for a concave lens
negative
Thin lens formula
1/v- 1/u=1/f2
What is u and v in the thin lens formula
v is the distance of the image from the principal point; u is the distance of the object from the principal point; and f2 is the second focal length
How to construct a ray diagram for spherical lenses
1) A ray from the top of the object which passes through the principal point undeviated
2) A ray parallel to the principal axis, which after refraction passes through (convex) or away from (concave) the second principal focus
For a convex lens where is the image formed if the Object is located outside the focal point F1
Image real, inverted and outside F2
For a convex lens where is the image formed if the object is located at F1
Image virtual, erect and at infinity
For a convex lens where is the image formed if the object is located inside F1
Image virtual erect magnified and further from lens than object
For a concave lens where is the image formed if the object is at ANY position
Image virtual erect diminished and inside F2
What is the formula to calculate the dioptric power of the lens
the reciprocal of the second focal length. The reciprocal of the second focal length expressed in metres, gives the vergence power of the lens in dioptres (D)
Formula for linear magnification
I/O= v/u
I= image size
O= object size
v=distance of image from principal plane
u=distance of object from principal plane
Magnifying power of a magnifying lens
M=F/4 where F is the power of lens in dioptres
eg a x8 louple has a lens power of +32Dioptres
What is lens decentration
Use of a non-axial portion of a lens to gain a prismatic effect is called decentration of the lens
What does poor lens centration lead to
especially high power lenses, may produce an unwanted prismatic effect. This is a frequent cause of spectacle intolerance, especially in patients with aphakia or high myopia
Formula to predict prismatic power gained by lens decentration
P=F x D
P=prismatic power in prism dioptres
F= lens power in dioptres
D= decentration in cm
Mechanism of spherical aberration
increasing prismatic power of the more peripheral parts of a spherical lens is the underlying mechanism of spherical aberration
Furthermore, it causes the troublesome ring scotoma and jack-in-the-box effect which give rise to great difficulty to those wearing high-power spectacle lenses
