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what is a Convex Lens (Convergent Lens)
A convex lens is thick in the middle and thin at the
edges
What is a Concave Lens (Divergent Lens)
A concave lens is thin in the middle and thick at the
edges.
What is Myopia
short sightedness ( difficulty seeing far)
Hyperopia
long sightedness ( difficulty seeing near)
What causes hyperopia
Hyperopia occurs when light focuses behind the retina of the eye
What causes myopia
Myopia occurs when light focuses in front of the retina of the eye
what type of lens is needed to correct myopia
concave lens / diverging lens
what type of lens is needed to correct hyperopia
convex lens / converging lens
What is an optical centre
The centre point of a lens which lies on the principal axis of the lens is called its optical centre.
Which letter represents the optical centre
C
What is a Principle axis
The principal axis of a lens is defined as a straight line passing through the optical centre and the centre of curvature of two surfaces of a lens
What is a principle focus
The principal focus of a lens is a point on its principal axis to which the light rays parallel to the principal axis converge (convex lens) or appear to diverge (concave lens) after passing through it
Which letter represents the principal focus of a lens
F
What is the focal length
The focal length of a lens is the distance between its optical centre and principal focus.
which letter represents the focal length
f
Meniscus lenses
convex front curves and concave back curve
What is the relationship between the front and back surface curves of a lens is referred to as
lens form / lens profile
Which lens combinations can be used for plus lenses
Bi-convex: flat lenses use either convex curves for both the front and back.
Plano-convex: one convex curve and one Plano curve
Which lens combinations can be used for minus lenses
Bi-concave: concave curves for both the front and back
Plano-concave: one concave curve and one Plano curve.
Astigmatism
curvature of the eye , the eye is not spherical
what type of lens is used to treat an astigmatism
cylindrical lens
In cylindrical lenses the scale is always __________
anticlockwise
lens aberration
an imperfection in a lens that prevents light from converging at a single point, resulting in blurred or distorted images
types of lens aberration
oblique astigmatism
spherical aberration
coma
distortion
types of distortion
barrel
pincushion
mustache
chromatic aberration
when the speed of light differs due to colour of light
what are the types of chromatic aberration
longitudinal
lateral
define longitudinal chromatic aberration
when different wavelengths of colour do not converge at the same point after passing through a lens
define lateral chromatic aberration
when different wavelengths of colour coming at an angle focus at different positions along the same focal plane
Define eccentricity
circle eccentricity
e=0
ellipse eccentricity
e= between 0 and 1
parabola eccentricity
e=1
hyperbola eccentricity
e= greater than 1
aspheric lens
a lens which is both good cosmetically and optically makes the eye seem natural and normal
what is a power meridian
where the lens has the maximum or minimum curvature (or refractive power)
what is an axis meridian
The axis meridian is the direction (in degrees) where the cylindrical power of the lens is located
what is a cross cylindrical lens
Two cylindrical lenses are combined with axes 90 degrees apart from one another
what is an oblique astigmatism
an aberration that results when rays of light from an object in the periphery strike the lens obliquely and are refracted differently by the tangential and sagittal meridians of the len
what is a spherical aberration
an optical problem that occurs when all incoming light rays end up focusing at different points after passing through a spherical surface
what is a barrel distortion
Image magnification decreases with distance from the optical axis
what is a pincushion distortion
image magnification increases with distance from the optical axis. Convex (plus) spherical lenses tend to have barrel distortion
what is a mustache distortion
It starts out as barrel distortion close to the image centre and gradually turns into pincushion distortion towards the image periphery, making horizontal lines in the top half of the frame look like a handlebar
what are the benefits of aspheric lenses
High plus lenses steeper base curve worse lenses look
◦Choosing flatter base curve make the lens less bulbous & decrease magnification ◦
Cosmetically look much better
◦ If flattened lens surface is spherical optics suffers
◦ Aspherics mean both good cosmetic & good optics
◦ Eye look like normal or natural
◦ Decrease magnification or minification
◦ Flatter base curve is easier to glaze to keep lens tight in frame ◦ Reduction of oblique astigmatism
◦ Magnification of the eye and surrounding structure as seen by the observer is reduced
◦ Spectacle magnification for the patient is also reduced
◦ It is flatter, thinner and lighter lens
◦ The lens looks flatter which makes the eyes look more natural due to reduced magnification or minification
◦ Flatter lens fits better ensuring the attractive look of the spectacles, thereby, offering the subjects a wider variety of frames to choose from
◦ If the aspheric lens design is coupled with high index material, it will provide thinner, lighter and flatter lens that looks cosmetically the best
