The Eye

5.5. SEEING LIGHT - THE EYESEEING LIGHT - THE EYE

Cornea - does most of the focusing

Iris -

Pupil -

has the eye color and controls light intensity

Lens -

the hole in the eye

does remainder of focusing

Retina - location of light sensors, has rods and cones

Blind spot -

Fovea - center of vision, predominantly cones

optic nerve exit, no light sensors

light enters through the cornea

the iris controls the amount of light that gets in, a muscle can close it or open it, the iris is the colored part

the lens is filled with a jelly-like substance; the ciliary muscle can change the shape of the lens and thus change its focal length

The EyeThe Eye

by changing the focal length, (accommodation) the lensis able to focus light onto theretina for objects located at various distances

the physics of the human eyethe physics of the human eye

The relaxed eye can easily focus on distantobjects. To focus on close objects the lens is squeezed to shorten it’s focal length, making it possible to converge the rays onto the retina. The near point is the distance at which the closest object can be seen clearly. It recedes with age.

Depth of FieldDepth of Field

The depth of field will depend on the pupil size.

If the aperture is NARROW, a greater DEPTH OF FIELD is produced.

If the aperture is WIDE, a smaller DEPTH OF FIELD is produced.

Optical IllusionsOptical Illusions

Show Pinwheel

Myopia (Near-Sightedness)Myopia (Near-Sightedness)

People with near-sightedness, also known as myopia, cannot see clearly at distance. Myopia occurs when the eyeball is too long or when the cornea (the clear part on the front of the eye) is too steep.

Someone with short sight can see near objects clearly but cannot focus properly on distant objects. This is because the lens focuses the sharpest image in front of the retina instead of on it.

It is usually an inherited defect, caused by the eyeball being elongated, so that the distance between the lens and the retina is too great. It can be corrected by placing a concave lens in front of the eye. This ensures that the light is diverging more as it enters the eye.

SHORT SIGHT

A person with normal vision will be able to comfortably focus on objects as far away as “infinity” and as close as approximately 18cm.

These points are called the FAR POINT and NEAR POINT respectively.

NORMAL VISION

Hyperopia (Farsightedness)Hyperopia (Farsightedness)or presbyopiaor presbyopia

Someone with long sight can see distant objects clearly but cannot focus properly on near objects.The lens focuses the sharpest image behind the retina instead of on it.

This can be caused either by a loss of elasticity in the lens – the lens is weaker than it needs to be for close vision. This defect is often age-related.

It is corrected by putting a convex lens in front of the eye to refract the light so that it rays are already converging before entering the eye.

LONG SIGHT

Calculations - MyopiaCalculations - Myopia

Far point

Far point of unaided eye

Farthest point that can be seen in focus

Objects at infinity brought to a focus in front of retina

REAL OBJECT AT INFINITY

Focal point of concave lens is at far point of unaided eye.

f = farpoint

P = 1/ (near point in m)

Calculations -HypermetropiaCalculations -Hypermetropia

Near point

Near point of unaided eye, v

Nearest point that can be seen in focus

Close objects brought to a focus behind retina

REAL OBJECT AT A SENSIBLE NEAR

POINT

Convex lens forms virtual image at near point of unaided eye.

1/f = 1/u - 1/vNear object, u

AstigmatismAstigmatismAstigmatism is a very common visual condition where the cornea is slightly oval in shape, rather than round (i.e. more like a rugby ball than a football). This causes objects to appear unclear at any distance, as light is not focussed to a point onto the retina. People with myopia or hypermetropia can also have astigmatism. With an astigmatic eye, the light focuses at differing distances from the retina, and this can cause the image to appear stretched in the direction of the astigmatism. This causes vision to become blurred and in some cases distorted, at any distance.

Astigmatisms can be corrected with cylindrical lenses that vary in power along different axes.

converging lensconverging lens

focalpoint F

a converging lens focuses parallel rays toa point called the focal point. a thicker lens has a shorter focal length

Diverging lensDiverging lens

F

A diverging lens causesparallel rays to divergeas if they came from afocal point F

Image formation by aImage formation by a converging lens converging lens

object

image

F2F

If the object is located at a distance ofat least 2F from thelens, the image is inverted and smaller than the object.The image is called a REAL image since light rays actually converge at the image location

A converging lens is used toA converging lens is used tofocus rays from the sun to a pointfocus rays from the sun to a point

since the sun is veryfar from the lens, therays are nearly parallel

converging lens is used in a converging lens is used in a camera to focus light onto the camera to focus light onto the

filmfilm

when you focus a camera,you adjust the distance between the lens and thefilm depending on the object location.

Image formation by a Image formation by a diverging lensdiverging lens

Object image

The diverging lens produces an image that is uprightand diminished in size.It is a VIRTUAL image, since light rays do not actually pass through the image point

a magnifying lensa magnifying lens

F

F

Objectvirtual image

By placing the lens close to the objectwe get a magnified virtual image.

When a nearsighted person views a distant object, the lenscannot relax enough to focus at the retina. The rays convergetoo quickly. The remedy is to place a diverging lens in frontof the eye to first diverge the

Weh

When a farsighted person tries to focus on a close objectthe lens cannot be squeezed enough to focus on the retina.The focus point is behind the retina. The remedy is to placea converging lens in front of the eye to converge the raysbefore they enter the eye.

AstigmatismAstigmatism

Astigmatism can be corrected with speciallyshaped lenses or, in extreme cases, with surgery

عدسی استوانه ای همگرا

عدسی استوانه ای همگرا

عدسی استوانه ای واگرا

عدسی های استوانه ای

شکست نور در یک دستگاه آستیگمات منظم

گونه های آستیگماتیسمگونه های آستیگماتیسممنظم

ساده–نزدیک بیندوربین

مرکب–نزدیک بیندوربین

درهم–نامنظم

آستیگماتیسم سادهآستیگماتیسم ساده

آستیگماتیسم مرکبآستیگماتیسم مرکب

آستیگماتیسم درهمآستیگماتیسم درهم

تشخیص آستیگماتیسمتشخیص آستیگماتیسم

تشخیص آستیگماتیسمتشخیص آستیگماتیسم

How does the eye judge distance?How does the eye judge distance?

Our brain interprets the images formed on the retinas of both eyes as a single image this is called binocular vision

Our eyes roll inward slightly to focus on the distant point D. Our brain interprets the distance BD by the muscular effort required to roll the eyes inward.

D

Right eye

B

Left eye

Color DeficiencyColor Deficiency