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6.1 Human Vision I. Parts of the Eye • The pupil is the dark transparent region in the centre of the eye where light enters. • The iris is the coloured circle of muscle surrounding the pupil. – The iris controls the amount of light entering the eye. • The sclera is the white part of the eye surrounding the iris. b a • The cornea is the transparent tissue covering the iris and the pupil. • Behind the pupil is a flexible convex lens. • The lens focuses light onto the retina located in back of the eye. – The retina is covered with light sensitive cells that convert light energy into electrical energy. • Electrical signals are sent to the brain by the optic nerve. c d II. The Cornea-Lens-Retina System • Light rays first entering the eye are refracted by the cornea so that they converge toward the retina. • Light then passes through the lens which “fine-tunes” the focus. • The image that forms on the retina is inverted. • The area where the optic nerve enters the retina is called the blind spot. – This area has no light-sensing cells. III. Black and White vs. Coloured Vision • The retina contains two types of light-sensitive cells called rods and cones. • Rod cells are cylinder-shaped cells that allow us to see images in shades of light and dark when the light is dim. rod cone • Cone cells are cone-shaped cells that allow us to see colour in bright light. • http://faculty.washington.edu/chudler/after.html Near-Sighted Vision • Can not clearly focus on distant objects. • Occurs because the lens converges the light rays to form an image in front of the retina. • A concave lens is used to correct near-sighted vision. Far-Sighted Vision • Can not clearly focus on nearby objects. • Occurs because the lens converges the light rays to form an image behind the retina. • A convex lens is used to correct far-sighted vision. Astigmatism • Blurred vision due to a irregular shaped cornea. • Causes the image to focus on more than one point on the retina. • Corrected by using eyeglasses, contact lenses, or laser surgery. Blindness • Blindness is any vision impairment that keeps people from carrying out important life functions. • Most people who are legally blind can perceive some light. – May be able to see a tiny part of the middle of the whole scene (tunnel vision). – May be able to only see the edges but not directly ahead. – May be able to see light and dark but not clearly, even with visual aids. Other Types of Blindness • Snow blindness is a temporary blindness caused by overexposure to the glare of sunlight. • Night blindness is a condition in which it is difficult to see in dim light. • Colour blindness is the ability to see only in shades of grey. • Colour vision deficiency is the inability to distinguish certain colours. – The most common is the inability to distinguish between red and green. Section 6.2 Extending Human Vision Extending Human Vision • With light, mirrors, and lenses, we have been able to make microscopes and telescopes • Microscopes and telescopes have allowed us to see and learn new things I. How to Bring an Image into Focus • to focus properly, the screen that is receiving the image must be the correct distance from the lens (where the light rays converge) • if the screen is too close or too far from the lens, the image will appear blurry • this is similar to near-sightedness and farsightedness II. Microscopes • a compound microscope uses two convex lenses • focuses on small objects • light reflects off the mirror, through the object, though the objective lens and eyepiece lens, into the eye • the objective lens has a fixed focal point • if an object is between 1-2 focal lengths away from the lens, the image is enlarged • the eyepiece lens magnifies the image (10X), and the objective lens magnifies it again (usually 4X, 10X, 40X) III. Telescopes • it is difficult to see far away images because further away objects reflect less light into your eye. This makes far away objects appear dim. • telescopes mirrors or lenses that is bigger to gather more of the light from distant objects. This makes far away objects appear much brighter. • the brighter image is then enlarged a) Refracting Telescopes • Uses convex lenses • the objective lens collects light and focuses the image inside the telescope; the image is then magnified by the eyepiece lens Problems with Refracting Telescopes • It needs very large lenses! Sometimes, they are too big: • Heavy • Distort image • Expensive • Difficult to make b) Reflecting Telescopes • most large telescopes today are reflecting telescopes • uses a concave mirror, a plane mirror, and a convex lens to collect and focus light • light enters one end of the telescope and strikes a concave mirror at the opposite end of the telescope. • the light reflects off the concave mirror and strikes a plane mirror at an angle before it converges to its focal point • the light reflects into the eyepiece, which is a convex lens that magnifies object • some telescopes collect light rays using several mirrors and then combine the rays into a single image (e.g. The Keck Telescope) c) The Hubble Space Telescope • Earth’s atmosphere blurs the image of distant objects in space, just like water blurs your vision • to overcome this blurry effect, some telescopes are placed in space • The Hubble Space Telescope was launched into outer space in 1990- it is a reflecting telescope that uses two mirrors. V. Binoculars • made of 2 telescopes mounted side by side • the telescopes are shortened by putting prisms inside that act like plane mirrors • the prisms make it so the light does not have to travel through a long tube • the thumbscrews on binoculars are used to change the focal length in order to focus on the object being viewed VI. Lasers • In laser light, all of the light rays move in the same direction, have the same wavelength and the crests and troughs line up • laser light can travel great distances without spreading out and can contain a lot of energy • laser light contains only one wavelength- it does not make a rainbow • lasers are used to remove cataracts, re-attach retinas, stop bleeding, and reshape corneas VII. Optical Fibres • optical fibres are transparent glass tubes that can transmit light from one place to another • light is reflected from side to side down the optical fibre like water moving through a pipe • TOTAL INTERNAL REFLECTION: a type of reflection in which light strikes a boundary between to materials and is completely reflected • optical fibres are used in medicine to transmit images of the inside of a person’s body from a tiny camera at one end of the cable and a monitor at the other end. • in telecommunications, optical fibres are used with laser light to transmit telephone, video, and internet signals.