2. Class 12th Physics | Optical Instruments | Human Eye | by Ashish Arora


let us now study about human eye. we already
studied about human eye and your earlier classes that, it is the most important sensory organ
of our body. which, the senses light and interacts with light and, majority of cases of vision.
here you can see this is typical internal structure of human eye in which. this is the
eye lens. and, the rear surface of eye, we call retina. this is the screen on which the
eye lens produces an image. so which ever light rays are coming onto the eye lens say
from, far away or distant object parallel light rays are coming. so in general in relaxed
state this lens converges all these light rays onto this retina. and approximately the
distance between lens and retina is equal to focal length of, the eye lens. now in this
situation even when an object is placed close to the eye it is able to create the image
at retina by adjusting, the focal length of this lens. because these are. ciliary mussels.
which compresses the eye lens from top and bottom due to which its curvature changes,
and its focal length automatically adjust such that even for a distant. a close object.
image will be produced at, the retina, the inverted image. and this is the optic nerve
which senses. the signals from this retina. the retina consist of 2 kind of cells, which
are called rods and cones. rod cells are use to. detect the intensity of light and cone
cells are used to, detect the color of the light by its wavelength. various kind of cones
which are sensor to different wavelengths are their in retina which. produces a colorful
image and optic nerve is connected to brain where we are able to sense the intensity,
the brightness and color of the object of which the image is produced on retina. now
two important thing to be understood here is. focal length of eye can be adjusted by,
straining the ciliary mussels, so when object is located at far away distant we see that
eye is relaxed. so, at infinity when object is placed this called. far point of, eye,
for which in relax state eye is able to produce the image at retina. when object is brought
closer there is a distance which is called, least distance of distant vision which is
denoted by d. this d we call. least distance of. distant vision. with strained eye. because
as this distance is equal to the focal length of eye as object gets closer. ciliary mussels,
comprises the lens and its focal length reduces due to which the final image is obtained on
retina. so in that situation here we can say. when d is the closest distance of object for
which clear image is obtained on retina this is called least distance of distant vision.
or this point is also called. near point of eye up-to which eye is able to produce the
image on retina clearly. if an object gets more closer. to the eye lens between the near
point and eye lens in that case the image produced will be blurred. or we can say ciliary
mussels can not get further stressed to strain the lens up to a limit to produce the clear
image. so human eye fails to produce, a clear image, when object get closer, more closer
to the near point, of the eye or it is between near point and lens of the eye. here you can
see that clear image can be obtained whenever the object is placed between near point and
anywhere on the far point where near point is the, most strained condition of eye to
get the clear image and far point is the most relaxed state of eye to get the clear image.
one more thing, all students must keep in mind that for an adult eye the value of. capital
d here that is a distance of near point from the. eye lens is generally taken as 25 centimeter.

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