How does Human eye work
-A camera functions very much like the human eye. Both see light that is reflected from the object being viewed. It transmits itself to the eye as sight or the camera as a picture in film or digital form. Light rays diffuse and bounce randomly from the myriad of objects around us so in order to see or form a picture; the rays need to be focused into a coherent form. To begin, the light enters the eye through the cornea and, like a camera's aperture, the pupil enlarges or contracts to ensure that the right amount of light enters the eye.  |
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Eye anatomy
-The human eye is a complex anatomical device that remarkably demonstrates the architectural wonders of the human body. Like a camera, the eye is able to refract light and produce a focused image that can stimulate neural responses and enable the ability to see.
-The eye is essentially an opaque eyeball filled with a water-like fluid. In the front of the eyeball is a transparent opening known as the cornea. The cornea is a thin membrane that has an index of refraction of approximately 1.38.
-Like the aperture of a camera, the size of the pupil opening can be adjusted by the dilation of the iris. The iris is the colored part of the eye - being blue for some people and brown for others.
-Light that passes through the pupil opening will enter the crystalline lens. The crystalline lens is made of layers of a fibrous material that has an index of refraction of roughly 1.40. Unlike
-the lens on a camera, the lens of the eye is able to change its shape and thus serves to fine-tune the vision process.
-the lens on a camera, the lens of the eye is able to change its shape and thus serves to fine-tune the vision process.
-The lens is attached to the ciliary muscles. These muscles relax and contract in order to change the shape of the lens.
A human eyeball is like a simple camera!
Sclera: outer walls, hard, like a light-tight box.
Cornea and crystalline lens (eyelens): the two lens system.
Retina: at the back of eyeball, like the film.
Iris: like diaphragms or stop in a camera.
Pupil: camera aperture.
Eyelid: lens cover.
Polarization by reflection
One way to polarize light is by reflection. If a beam of light strikes an interface so that there is a 90° angle between the reflected and refracted beams, the reflected beam will be linearly polarized. The direction of polarization is parallel to the plane of the interface.
The special angle of incidence that satisfies this condition, where the reflected and refracted beams are perpendicular to each other, is known as the Brewster angle. The Brewster angle, the angle of incidence required to produce a linearly-polarized reflected beam.
This expression can be derived using Snell's law, and the law of reflection. The diagram below shows some of the geometry involved.
Using Snell's law:
Lenses of eye
The human eye can be compared to a camera. Like a camera the human eye gathers and focuses light through a lens to make a image. The lens’s job is to bend the light when it enters the eye. The clear covering on the front of your eye is the cornea. The cornea also helps in focusing the light onto the retina.
Optics of the eye
The human eye is a wonderful instrument, relying on refraction and lenses to form images. There are many similarities between the human eye and a camera, including:
· A diaphragm to control the amount of light that gets through to the lens. This is the shutter in a camera, and the pupil, at the center of the iris, in the eye.
· A lens to focus the light and create an image. The image is real and inverted.
· A method of sensing the image. In a camera, film is used to record the image; in the eye, the image is focused on the retina, and a system of rods and cones is the front end of an image-processing system that converts the image to electrical impulses and sends the information along the optic nerve to the brain.
Consider the lens equation:
With a camera, the lens has a fixed focal length. If the object distance is changed, the image distance (the distance between the lens and the film) is adjusted by moving the lens.
Which kinds of lenses are needed to correct the vision defects
Contact lenses are thin transparent plastic discs that sit on the cornea. Just like eyeglasses, they correct refractive errors such as myopia (nearsightedness) and hyperopia (farsightedness). With these conditions, the eye doesn't focus light directly on the retina as it should, leading to blurry vision. Contact lenses are shaped based on the vision problem to help the eye focus light directly on the retina.
Although nearly 36 million Americans wear contact lenses, not everyone wears them to correct vision problems. Celebrities use contact lenses to change the color of their eyes, athletes put them on to give them extra-sharp vision on the field, and others use them to make Halloween costumes more realistic.
Optical aberration of lenses
-Spherical aberration are due to references in focal length for light rays at the Outer edges of the lens.
-In human eye this effect is corrected by small pupil size which cuts outs light from the outer edges of the cornea.
-Chromatically aberrations are due to wavelength dependence of the index of refraction.
Light with deferent wavelength has different focal length.
General structure of light
The human eye is a adjustable lens system consisting out of two focusing elements and light receptor system.
In human eye this effect is corrected by small pupil size which cuts out light from outer edges of the cornea.
Chromatically aberration are due to wavelength dependence of the index of refraction ; light with different wavelength has different focal length.
As a consequence at identical distance blue objects appear to be further away than red objects.
The diffraction of eye
-The wave nature of light diffraction effects occur on small apertures.
-The resulting diffraction pattern limits the resolution of the optical system.
Defects of eye
-Myopia: This is a defect of vision in which far objects appear blurred but near objects are seen clearly.
-Hyperopia: This is a defect of vision in which there is difficulty with near vision but far objects can be seen easily.
-Astigmatism: This defect is when the light rays do not all come to a single focal point on the retina.
Nearsightedness or myopia is the most common cause of blurred vision when viewing objects at a distance.
Farsightedness
Farsightedness or hyperopia is a common vision condition in which you can see distant objects clearly, but objects nearby may be blurry.
astigmatism
An optical aberration caused because a lens has different focal lengths for rays of different orientations, resulting in a distortion of the image. Rays of light from horizontal and vertical lines in a plane on the object are not focused to the same plane on the edges of the image.
Some good videos about human eye
Done by: Abdurrahman Ibrahim
Class: 12-03
Teacher: Denise
