In order to see objects, light pass through four layers:
cornea, aqueous humor, lens, & vitreous humor then it
hits the retina in the rods and cones.
Although speed of light changes accordingly depending on
the medium it pass through, highest speed of light is in air
which equals 300km s, when light passes from air to
cornea it refracts, in which it has less speed, and so on ..
Refraction depends on speed, gives rise to the refraction
index which equals the speed of light in air over the speed
of light in medium (RI=speed of light in air speed of lite in
that media). The speed of light in air always more than its
speed in any media & thus the refractive index 1 always.
Light passes through four layers to reach retina, each layer
has its own refractory index. If these layers were replaced
with one lens, this lens would have a power of +60 diopter
Provided that, power of lens in diopter = ,
focal length in meter. (the distance between thr retina &
lens is fixed)
The power of this lens = 60 diopter , although cornea
÷
contributes the most of the refractory power almost 42
diopters .
1
, Types of lens:
- Convex lens: it is a conversion
lens collects the light in the
focus; it is positive lens,
because it collects the light
into a point (focus).
- Concave lens: diverges light
rays (diverging lens), it is a
negative lens.
Requirement to make a picture of an object on the retina:
I. Refraction of light (4 refractive medias), most
refractive media is cornea.
II. Accommodation: the main need of accommodation is
that the distance between the retina and lens is fixed =70mm.
- Simply accommodation occurs by changing the
=
convexity of the lens depending on the distance
between objects & lens, as lens
gets more convex for near objects
& less convex for distant objects.
Thus, less convex = less power =
high focal length
More convex = more power = less
focal length.
2
cornea, aqueous humor, lens, & vitreous humor then it
hits the retina in the rods and cones.
Although speed of light changes accordingly depending on
the medium it pass through, highest speed of light is in air
which equals 300km s, when light passes from air to
cornea it refracts, in which it has less speed, and so on ..
Refraction depends on speed, gives rise to the refraction
index which equals the speed of light in air over the speed
of light in medium (RI=speed of light in air speed of lite in
that media). The speed of light in air always more than its
speed in any media & thus the refractive index 1 always.
Light passes through four layers to reach retina, each layer
has its own refractory index. If these layers were replaced
with one lens, this lens would have a power of +60 diopter
Provided that, power of lens in diopter = ,
focal length in meter. (the distance between thr retina &
lens is fixed)
The power of this lens = 60 diopter , although cornea
÷
contributes the most of the refractory power almost 42
diopters .
1
, Types of lens:
- Convex lens: it is a conversion
lens collects the light in the
focus; it is positive lens,
because it collects the light
into a point (focus).
- Concave lens: diverges light
rays (diverging lens), it is a
negative lens.
Requirement to make a picture of an object on the retina:
I. Refraction of light (4 refractive medias), most
refractive media is cornea.
II. Accommodation: the main need of accommodation is
that the distance between the retina and lens is fixed =70mm.
- Simply accommodation occurs by changing the
=
convexity of the lens depending on the distance
between objects & lens, as lens
gets more convex for near objects
& less convex for distant objects.
Thus, less convex = less power =
high focal length
More convex = more power = less
focal length.
2
