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BC Science 8 Chapter 6 6.1 Human Vision Pupil - is an opening that appears dark because light passes through it without reflecting back Iris - is the coloured circle of muscle surrounding the pupil, and controls the amount of light entering the eye (in dim light, the iris dilates, or expands, the pupil to reduce the amount of light entering the eye…in bright light, the iris contracts the pupil to reduce the amount of light entering the eye) Cornea - is a transparent tissue made of cells that are transparent enough to let light pass through, but tough enough to hold the eye together Sclera - the opaque tissue surrounding the cornea; we see it as the white area surrounding the iris. Retina - the inner lining of the back of the eye containing light-sensitive rods & cones that convert light images into electrical signals for interpretation by the brain Optic Nerve - the nerve that connects the eye to the brain Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 1 Figure 6.3 figure 6.4 The Cornea-Lens-Retina System 1. Light rays are refracted by the cornea so that they converge toward the retina. (the cornea does most of the focusing of the light rays) 2. The refracted rays enter the eye through the pupil, where the iris changes the size of the pupil. If light levels are dim; the pupil gets larger to let in more light. If light levels are bright; the pupil gets smaller to let less light in. 3. Light rays then pass through the lens where they converge on the retina. The lens gets thicker to help focus lights rays from objects that are closer, and gets thinner to help focus light rays from objects that are farther away. 4. The image formed on the retina is upside down. Cells in the retina change the image into electrical signals. 5. The electrical signals are sent to the brain along the optic nerve, and as the brain interprets the signals, it changes the image so that it is upright. 6. The blind spot is the area where the optic nerve enters the retina that does not have any light-sensing cells. Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 2 Diagram 6.5 Rod Cells - is called our black–and-white vision system - cylinder-shaped cells, located in the retina, that absorb light and allow us to see images in shades of light and dark and movement when the light is dim Cone Cells - cone shaped cells, located in the retina that absorb light, and allow us to see colour in bright light Correcting Vision Problems Normal Vision - When light rays from a distant object enter the eye, the rays are nearly parallel. The lens, which is convex, causes the rays to converge at the retina, making a sharp image. - When light rays from a nearby object enter the eye, the rays are diverging. The muscles in the eye cause the lens to change shape, making the lens thicker, giving the lens a greater ability to converge the light rays to form a clear image. Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 3 Near-Sighted Vision - when you can see nearby objects clearly but distant objects look blurred - it happens when the light rays converge before they reach the retina. - A concave lens is used to diverge - the parallel rays before they reach the cornea. Then, as the light rays pass into the eye, they converge on the retina to form a clear image. Far-Sighted Vision - when you can see objects far from the eye clearly, but nearby objects look blurred. - it happens when the light rays do not converge by the time they reach the retina. - A concave lens is used to diverge the parallel rays slightly so that the image forms farther back, on the retina to form a clear image. Astigmatism - when the shape of the cornea is irregular, causing an image to focus an image on more than one point on the retina. - can be corrected using glasses, - contact lenses, or with laser surgery to reshape the cornea Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 4 Blindness - any vision impairment that keeps people from carrying out important life activities; ability to detect very little or no light - many people who are legally blind can perceive some light or even have a limited amount of vision - In developing countries, blindness Is most often a result of disease or malnutrition; of the 40 million people who are blind in the world today, about 80% could have some or all of their sight restored through treatment. Other Types of Blindness Snow Blindness - is a painful condition of temporary partial or complete blindness caused by overexposure to the glare of sunlight (on snow fields at high altitudes) - treatment includes resting your eyes in a dark room for several days to allow the inflammation to decrease Night Blindness - is a condition in which it is difficult or impossible to see in dim light - most common cause is the rod cells losing their ability to respond to light Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 5 Colour Blindness - is the ability to see only in shades of grey, and occurs in about 1 person in every 40 000. Colour Vision Deficiency - is quite common, occurring in about 8 % of males, and 1 % of females - is an inability to distinguish certain - colours (1, 2, or all 3 kinds of cone cells can be involved) - the most common involves the inability to tell red and green apart 6.2 Extending Human Vision Pg. 92 diagrams in workbook Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 6 Microscopes – How They Work 1. A compound light microscope uses 2 convex lenses with short focal lengths to magnify small, close objects 2. The object to be viewed is placed on a transparent slide and illuminated from below 3. Light passes by or through the object on the slide and then travels through the objective lens (a convex lens) 4. The convex lens makes an enlarged image inside the microscope tube 5. The light rays from that image then pass through the eyepiece lens, which is another convex lens, and magnifies the image even more 6. This final image can be hundreds of times larger than the actual object, depending on the focal lengths of the 2 lenses. Telescopes – How They Work 1. A telescope uses a lens or a concave mirror that is much larger than your eye to gather more of the light from distant objects. 2. The largest telescopes can gather more than a million times more light than the human eye so objects such as distant galaxies appear much brighter. 3. The image can be magnified to a greater extent to reveal more detail because the image formed by a telescope is so much brighter. Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 7 Refracting Telescopes 1. A telescope, like a microscope, has an objective lens and an eyepiece, but the objective lens in a telescope has a longer focal length because the objects viewed are far from the lens. 2. In a refracting telescope, light from a distant object passes through an objective lens and an eyepiece where an image is formed and never seen directly. 3. Instead, the image is magnified by the eyepiece lens, and directed into the eye of the operator or into a camera. 4. In order to form a detailed image of distant objects, such as planets or galaxies, the objective lens must be as large as possible, which can be a problem, because a large lens is heavy, costly, difficult to make, and can distort the image it forms if the lens sags due to its weight. figure 6.15 Reflecting Microscopes 1. Most large telescopes today are reflecting telescopes, due to the problems with making large lenses. 2. A reflecting telescope uses a concave mirror, a plane mirror, and a convex lens to collect and focus light from distant objects. Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 8 3. Light from a distant object enters one end of the telescope and strikes a concave mirror at the opposite end. 4. The light reflects off this mirror and converges, before it converges at a focal point, the light strikes a plane mirror that is placed at an angle within the telescope tube. 5. The light is reflected from the plane mirror toward the telescope’s eyepiece. 6. The light rays converge at the focal point, creating an image of the distant object (a convex lens in the eyepiece then magnifies this image) Figure 6.18 figure 6.19 The Hubble Space Telescope The Hubble Space Telescope is a type of reflecting telescope that uses two mirrors to collect and focus light to form an image. The primary mirror in the telescope is 2.4 m across and can collect visible light, as well as other types of electromagnetic radiation from planets, stars, and distant galaxies. The image from the Hubble Space Telescope is clear, not blurred by the Earth’s atmosphere producing images much sharper and more detailed than the largest ground-based telescopes. Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 9 The Hubble Space Telescope Diagram pg. 221 Binoculars Binoculars are actually two refracting telescopes mounted side by side. The telescopes are shortened by placing prisms inside that serve as plane mirrors. Instead of traveling down the long tube of a telescope, the prisms reflect the light in binoculars back and forth inside a shorter tube. Figure 6.21 Cameras – Wild–Angle, and Telephoto Lenses A Digital Camera works by gathering and bending light with a convex lens. The lens projects an image onto a light detector to record a digital image of a scene. When you take a photograph, a shutter opens to allow light to enter the camera for a specific length of time. Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 10 The light reflected off your subject enters the camera through an opening called an aperture. The light then passes through the lens, which focuses the image on the light detector. The image is inverted and smaller than the actual object because a convex lens is used. Wide-Angle Lenses Wide-Angle Lenses have short focal lengths that produce a small image Telephoto Lenses Telephoto lenses have longer focal lengths; therefore the image through a telephoto lens seems enlarged and closer than it actually is. Comparing a Camera to the Human Eye Human Eye Camera Similarities Human Eyelid Lens Cap Both reduce the chance of accidental damage Pupil Lens Both have an opening to let light in Iris Diaphragm Both limit the amount of light entering the eye Rods and Cones Pixels Detect light in our eyes Retina CCD (light detector) Changes light into electrical signals to produce an image Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 11 Diagram 6.25 Lasers In Laser Light, all of the light has the same wavelength, all the light rays are moving in the same direction, and all of the crests and troughs of the light are lined up. Laser light travels great distances without spreading out, and can contain a lot of energy, making it useful as a tool for eye surgery. Light does not refract into a rainbow, because lasers only have one wavelength. Laser Surgery is used to remove cataracts, re-attach retinas, stop bleeding, and reshape corneas. Figure 6.26A and Figure 6.26B Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 12 Optical Fibres Optical Fibres -are very thin transparent glass fibres that can transmit light from one place to another - laser light enters one end of the fibre and keeps reflecting off the smooth inside walls until it reaches the other end (total internal reflection) - The light in optical fibres can be used to carry sound, movie, and internet signals. - A tiny camera on one end of an optical fibre can send pictures to a monitor and let surgeons view the inside of the body Figure 6.27 Eric Hamber Secondary BC Science 8 – Chapter 6 Human Vision 13