The Eye

5.SEEING 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 Eye  by changing the focal length, (accommodation) the lens is able to focus light onto the retina for objects located at various distances

the physics of the human eye The relaxed eye can easily focus on distant objects. 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 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 Illusions Show Pinwheel

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) or 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 - 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 -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/v Near object, u

Astigmatism Astigmatism 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 lens focal point F  a converging lens focuses parallel rays to a point called the focal point.  a thicker lens has a shorter focal length

Diverging lens F A diverging lens causes parallel rays to diverge as if they came from a focal point F

Image formation by a converging lens object image F 2F  If the object is located at a distance ofat least 2F from the lens, 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 to focus rays from the sun to a point since the sun is very far from the lens, the rays are nearly parallel

converging lens is used in a camera to focus light onto the film converging lens is used in a camera to focus light onto the film when you focus a camera, you adjust the distance between the lens and the film depending on the object location.

Image formation by a diverging lens Object image  The diverging lens produces an image that is upright and diminished in size.  It is a VIRTUAL image, since light rays do not actually pass through the image point

a magnifying lens F F Object virtual image By placing the lens close to the object we get a magnified virtual image.

When a nearsighted person views a distant object, the lens cannot relax enough to focus at the retina. The rays converge too quickly. The remedy is to place a diverging lens in front of the eye to first diverge the

WehWeh When a farsighted person tries to focus on a close object the lens cannot be squeezed enough to focus on the retina. The focus point is behind the retina. The remedy is to place a converging lens in front of the eye to converge the rays before they enter the eye.

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

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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 Deficiency