1. Lenses can be convex or concave, with convex lenses converging light and causing magnification. 2. The power of a lens is determined by its focal length, with shorter focal lengths indicating higher powers. Spectacles and magnifiers use convex lenses of varying powers for low vision needs. 3. Telescopes use combinations of convex and concave lenses to provide angular magnification without changing vergence. Galilean telescopes have a positive objective lens and negative eyepiece, producing an erect image, while Keplerian telescopes have two positive lenses and an inverted image.

1. Optics of Convex Lenses,
Magnifiers and Telescopes.

2. Lenses:
A lens is defined as a portion of a refracting media border
by two curved surfaces, which have a common axis. When
each surface forms part of a sphere, the lens is called
spherical lens.
Spherical lenses are either convex lenses (plus lenses or
converging lenses) or concave lenses (minus lenses or
diverging lenses).

3. Convex lenses:
A convex lens causes convergence of incident of light
Forms of Convex lenses.
Bi-Convex
Plano-Convex
Meniscus

4. Dioptric power of lenses. Vergence:
The reciprocal of the second focal length expressed in meter
gives the vergence power of the lens in diopters (D) thus
F = 1/f‑
Where F is vergence power of the lens in diopter and f is the
second focal length in meters.
A converging lens of second focal length 5cm has a power of
F=1/f =1/0.10 =10D

5. SPECTACLES:
Spectacles are the low vision aids that are most often
prescribed.
They are simply reading glasses with higher powers than
normal, providing a shorter focal distance, resulting in relative
distance magnification.
When selecting spectacles for a patient, it is important to keep
in mind the patient refractive error. If a patient is 4 D myopic,
he already has a “ built-in” 4D reading add. If the dioptric
demand for appropriate magnification is 6D, only the
remaining 2D needs to be provided in spectacles and the
reading distance will be

6. 100/6 = 16.7cm.
Similarly for a patient with 3D hyperopia who requires
+10D reading aid will need +13D spectacles for reading.
Beyond powers of 4D to 6D, single vision glasses
become necessary because reading distance is so closer
and patient have to converge excessively therefore a
base-in prism is included.
Above power of 12D, spectacles are referred as
microscopes. It is better to recommend monocular low
vision microscope for just the preferred eye.

7. When using spectacles reading materials must be hold at
the focusing distance of lens, which is closer to the eyes
than normal.
Touch the reading material to your nose and slowly move
it away until it is in clear focus. Maintain that while
reading.
It may help to move reading material from side to side
keeping the eyes still, instead of scanning material in the
normal ways by moving your eyes from the beginning to
the end of a line print.

8. HAND MAGNIFIERS:
Hand magnifiers consist of a convex lens surrounded by a plastic
or metal carrier attached to a handle.
For maximum magnification with hand magnifier, place the
object at the focal point.
The distance from a hand magnifier to the printed page must be
kept constant .To find this distance start the lens on the page and
slowly pull it away from the print until optimal focus is reached.
Hand magnifier should be used with distance glasses.
If the magnifier is brought closer to your eyes, bring the reading
material closer as well.

10. STAND MAGNIFIERS :
Stand magnifiers are similar to hand magnifiers.
They consist of a convex lens surrounded by a plastic or metal
carrier. Instead of a handle, however, they are attached to legs
or some other support. These enable the magnifier to stand
freely on a page of print.
The legs of the stand magnifier are shorter than the focal length
of the lens so divergent light comes out and the patient needs
accommodation or reading glasses.
Always keep the distance from your eyes to the magnifier
constant. Do not pull the magnifier up to your eyes, the focus
will be lost.

12. Magnifiers Advantages Disadvantages
Spectacle Magnifiers 1.Patients are used to them and
adaptation is easy.
2.Leave hands free & are
comfortable.
3.Comparativeluy light weight
1.Short working distance.
2.With higher power depth of field
becomes narrow
Hand magnifiers 1.No need of reading glasses or
accommodation
2. Light weight & easy to carry
around
3.Illuminated & non illuminated
4.Less expensive
5.Resolution is independent of the
position
1.Require steady hands & not
comfortable for patients with tremors
2.Bussy hands
Stand magnifiers 1.Less expensive
2.Wider range of magnification
3.Fairly light weight
1.Bulky as compare to glasses
2.Needs reading glasses or
accommodation
3.Low visual field
4.Uncomfortable viewing posture
Telemicroscopes 1.Large working distance
2.Leave hands free
3. Binocular up to 4x
1.Bulky
2.Smaller field of view
3.Expensive

13. Telescope:
An optical system that provides angular magnification without
bringing about a change in vergence.
It contains two optical elements. The objective lens and the eyepiece
Objective lens--- positive towards the object
Eye piece--- positive or Negative towards the eye and is of stronger
power than objective
Secondary focal plane of the objective lens coincide with the primary
focal plane of the eye piece lens
Parallel rays incident on the objective lens form an image at secondary
focal plane and this image becomes the object for the eye piece.

14. Types:
1.Galilean
2.Keplerian
o
ω
ω
/

15. Galilean Telescope:
Objective_________ positive
Eye piece_________ negative
The objective lens form an optically real image of height h with the chief ray
passing through the center of objective lens and forming an angle α with the
optical axis which becomes a virtual object for the eye piece lens.
Parallel rays enter the objective lens at angle α to the optic axis and a parallel
bundle of emergence from the eyepiece lens at angle α′ from the optic axis.
Characterisics of G.T
1.Since for a GT F--e is always negative lens and F0 is always a positive lens ,
in the formula
M= -Fe/F0
The magnification has a positive sign, which indicate the image formed is erect.

16. Keplerian Telescope:
It has a positive powered objective lens and a stronger positive powered
eye piece lens separated such that the secondary focal point of the
objective lens coincide with the primary focal point of the eye piece lens.
Parallel light enters a plus objective
The light is focused towards the focal point of the objective
Because the focal point of the objective and the focal point of the
eyepiece coincide, light leaves parallel

17. Magnification by Telescopes:
For either type of telescopes the angular magnification it provides is the negative of
the power of the eyepiece lens (Fe) divided by the of the objective (Fo)
M= -F2/F1
Example: Magnification of a Galilean telescope having +10D objective lens and –
20D-eyepiece lens
M= -Fe/Fo
M= -20/-10
M=2X
Magnification of Keplerian Telescope having+10D objective lens and +20D eyepiece
lens
M=-Fe/Fo
M=-(+20)/10
M=-20/10
M= -2X
The minus sign shows that the image is inverted

18. Comparison of Galilean and Keplerian Telescopes:
Characteristics Galilean Keplerian
Power <4X >4X
Exit pupil Virtual I (not floating because
insideTS)
Real I (floating)
Image Erect Inverted
Field of view Smaller field of view Larger field of view
Magnification Low magnification up to 4x High magnification up to 10x
Image Quality Brighter Much brighter
Design Simple Complicated
Weight Light Heavy
Path length Short Long

19. Bioptic Telescope:
Spectacle mounted; it is drilled into the lens
Called bioptic because it has two optical systems: a carrier lens and a
telescope
Not mounted centrally because you would have a hard time getting
around, unless special occupational needs
Mounted superiorly – tilt head down to see through it.

20. Telemicroscope:
Telemicroscope is a telescope with a microscope or (plus
lens) attached which allows you to see at a certain
distance.
Telemicroscope allow you to see things closer; so it is
like a telescope with a reading cap
Working distance is based on reading cap
It provides high magnification at a large working
distance.