Spherical Lenses

Question 1

What do convex lenses do to incident light

Answer 1

convergence of incident light

Question 2

What do concave lenses do to incident light

Answer 2

divergence of incident light

Question 3

How to calculate the total vergence power

Answer 3

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

Question 4

What is the nodal/principal point

Answer 4

The point at which the principal plane and principal axis intersect. Rays of light passing through the nodal point are undeviated.

Question 5

What is the first principal focus F1

Answer 5

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

Question 6

What is the second principal focus F2

Answer 6

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

Question 7

Conventional sign of F2 for a convex lens

Answer 7

positive

Question 8

Conventional sign of F2 for a concave lens

Answer 8

negative

Question 9

Thin lens formula

Answer 9

1/v- 1/u=1/f2

Question 10

What is u and v in the thin lens formula

Answer 10

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

Question 11

How to construct a ray diagram for spherical lenses

Answer 11

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

Question 12

For a convex lens where is the image formed if the Object is located outside the focal point F1

Answer 12

Image real, inverted and outside F2

Question 13

For a convex lens where is the image formed if the object is located at F1

Answer 13

Image virtual, erect and at infinity

Question 14

For a convex lens where is the image formed if the object is located inside F1

Answer 14

Image virtual erect magnified and further from lens than object

Question 15

For a concave lens where is the image formed if the object is at ANY position

Answer 15

Image virtual erect diminished and inside F2

Question 16

What is the formula to calculate the dioptric power of the lens

Answer 16

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)

Question 17

Formula for linear magnification

Answer 17

I/O= v/u
I= image size
O= object size
v=distance of image from principal plane
u=distance of object from principal plane

Question 18

Magnifying power of a magnifying lens

Answer 18

M=F/4 where F is the power of lens in dioptres
eg a x8 louple has a lens power of +32Dioptres

Question 19

What is lens decentration

Answer 19

Use of a non-axial portion of a lens to gain a prismatic effect is called decentration of the lens

Question 20

What does poor lens centration lead to

Answer 20

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

Question 21

Formula to predict prismatic power gained by lens decentration

Answer 21

P=F x D
P=prismatic power in prism dioptres
F= lens power in dioptres
D= decentration in cm

Question 22

Mechanism of spherical aberration

Answer 22

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