Lenses and Defects of vision

1. Its now time to see the light….
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2.  A lens is a curved transparent material that is
smooth and regularly shaped so that when
light strikes it, the light refracts in a predictable
and useful way.
 Made of transparent glass or very hard plastic
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3.  Light undergoes two refractions: the first on
entering the lens and the second on leaving the
lens
 The index of refraction of a lens is greater than
that of air
 When light passes from air into the lens, the
light ray refracts away from the lens surface
and towards the normal
 When the light passes out of the lens at a an
angle, the light rays refract again, this time
bending away from the normal
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5.  Converging Lens
(convex)
 Diverging Lens
(concave)
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6.  Axis of Symmetry: an imaginary vertical line drawn
through the optical centre of a lens
 Principle Axis: an imaginary line drawn through the optical
centre, perpendicular to the axis of symmetry
 F and F’: Both kinds of lenses have two principal focuses.
The focal point where the light either comes to a focus or
appears to diverge from a focus is given the symbol F,
while that on the opposite side is represented by F’
 Focal length (f): the distance from the axis of symmetry to
the principal focus measured along the principal axis.
There are two equal focal lengths since light behaves the
same when travelling in either direction through a lens.Anjan Nepal

7.  Secondary Pricipal focus (F’)
 Optical Centre
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8.  Thicker at the centre of the lens than at the
edges
 Convex lenses are able to form a real image on
a screen
 A common example is the magnifying glass
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9. 1. The first ray of a convex lens ray diagram travels
from the tip of the object parallel to the principal axis.
When it emerges from the lens it passes through the
principal focus.
2. The second ray travels from the tip of the object
through the optical centre of the lens and is not
refracted.
3. Draw the real image where the rays appear to
intersect.
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13. 1. The first ray of a concave lens ray diagram travels from the
tip of the object parallel to the principal axis. When it
emerges from the lens, it appears to come from the principal
focus.
2. The second ray travels from the tip of the object through the
optical centre of the lens, and is not refracted.
3. Draw the virtual image where the rays appear to intersect.
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14.  Thinner in the centre than at the edges
 The image formed is always upright, smaller
than the object, and virtual
 Image is always located on the object side of
the lens
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15.  Ray diagrams may help one determine the
approximate location and size of the image but
will not provide numerical information about
image distance and image size
 To obtain this type of numerical information, it
is necessary to use the Lens Equation and the
Magnification Equation.
 Both equations use
the variables illustrated
in this diagram
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16.  A thin lens is a lens that has a thickness that is
slight compared to its focal length. You can
assume that all refraction takes place at the axis
of symmetry for ray diagrams.
 The thin lens equation expresses the
quantitative relationship between the object
distance (u), the image distance (v), and the
focal length (f). The equation is stated as
follows:
1/f = 1/u + 1/v
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17.  Keep in mind when working with this
equation:
 A concave lens has a negative (-) focal point and a
negative (-) distance to the image
 A convex lens has a positive focal point (+) and
 A positive distance to the image, if the image is real
 A negative distance to the image, if the image is virtual
 Convex and concave lens’ have positive distances to
the object
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18.  The magnification equation relates the ratio of
the image distance (V) and object distance (U)
or the ratio of the image height (I) and object
height (O). The magnification equation is stated
as follows:
M = I / O
M = V/ U
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20. Iris : regulates size of the pupil
Pupil: controls passage of light
Ciliary muscle: controls shape of lens
a/c to the distance objects from lensAnjan Nepal

21.  The ability of eye lens to adjust its focal length
is called accommodation.
 Near point: Nearest point from eye at which an
object can be seen clearly by the eye. Its value
for normal eyes is 25 cm.
 Far point: Farthest point from eye at which an
object can be seen clearly by the eye. Its value
for normal eyes is infinity.
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