Unit C: Light and Optical Systems Science 8 Unit C Section 4.0 Science 8

Eyes and cameras capture images using the properties of Light. Science 8 Unit C Section 4.0 Section 4.0

Image Formation in Eyes and Cameras Science 8 Unit C Section 4.0 Topic 4.1

How Light Gets in Light is allowed into the human eye through a hole. Camera Light is allowed into the human eye through a hole. In the human eye, the hole is called a pupil. Light is allowed into a camera through a hole. In a camera, the hole is known as an aperture. Science 8 Unit C Section 4.0

How Light Gets In Science 8 Unit C Section 4.0

What Controls the Size of the Hole in your Eye? Human Eye Camera The pupil is just a hole created by a circular band of muscle called the iris. In a camera, a light sensor directs the diaphragm to change the size of the aperture in the lens to allow in the proper amount of light. Science 8 Unit C Section 4.0

Pupil Hole Like a doughnut hole, the pupil is not really there. The pupil is just a hole created by a circular band of muscle called the iris. When people refer to their eye colour, they are referring to the colour of the iris. The iris controls the size of the pupil, and so regulates the amount of light that enters the eye. In dim light, the iris opens and the pupil dilates (becomes wider) to let in more light. In bright light, the iris closes and the pupil constricts (becomes smaller) to let in less light. Changes in pupil size happen automatically; you don’t have to think about it. Science 8 Unit C Section 4.0

Figure 4.2 The pupil of the eye and the aperture of a camera lens work in the same way. Science 8 Unit C Section 4.0

Another View of Eye Anatomy Science 8 Unit C Section 4.0

Camera Shutter The shutter lies behind the aperture. The shutter acts like a set of doors that open when you press the button to take a picture. The longer the shutter remains open, the more light enters the camera and strikes the film. Science 8 Unit C Section 4.0

When Light Gets Inside In order to see, light rays must strike the sensitive retina at the back of the eye. The retina is a special layer that is filled with photoreceptors, cells that are sensitive to light. Science 8 Unit C Section 4.0

Light-Sensitive Receptors Science 8 Unit C Section 4.0

Light Receptors The retina is a special layer that is filled with photoreceptors, cells that are sensitive to light. There are two types of photoreceptors. Rods are highly sensitive to light, while cones detect colour. Science 8 Unit C Section 4.0

Light Receptors Because rods are sensitive to even small amounts of light, they can function in very low light. Cones can’t function in low light, so all you can see are shades of grey. This is somewhat similar to the difference between colour and black and white film in cameras. Science 8 Unit C Section 4.0

When Light Gets Inside Human Eye Camera When light strikes the retina, photoreceptors are stimulated, and they send messages to the optic nerve (the nerve at the back of the eye), which passes the message to the brain. The brain translates the messages into an image. At the back of a film camera is light- sensitive film. When light strikes the film, the film changes chemically, forming an image. In a digital camera, this is recorded as a digital image. Science 8 Unit C Section 4.0

Sending a Message Science 8 Unit C Section 4.0

Another View of Eye Anatomy Science 8 Unit C Section 4.0

The Retina of a Human Eye Science 8 Unit C Section 4.0

Page 233: Give it a Try Activity: Where’s Your Blind Spot? There is a small spot on the retina of each eye that has no photoreceptors. Because of this, your brain gets no information about a small area of whatever you are looking at. Try to find your blind spot. Close your left eye and stare at the plus sign in Figure 4.3. Slowly move the book toward you and away from you. When the black dot disappears, you’ve found your blind spot! Why does the black dot disappear? Why do you think this spot on the retina has no photoreceptors? Science 8 Unit C Section 4.0

Focussing The Light Human Eye Camera In the eye, muscles attached to the lens relax or contract to change the shape of the lens. Changing the shape of the lens adjusts the focal length so that light forms a focussed image on the retina. In cameras, the lens is moved forward or backward to adjust the focal length. Science 8 Unit C Section 4.0

Changing the Shape of the Lens in the Human Eye Science 8 Unit C Section 4.0

Image Formation Science 8 Unit C Section 4.0

Topsy-Turvy! Although the image formed on the retina is upside down, your brain corrects for this and interprets the world right side up. In a camera, it doesn’t matter that the image is upside down. The film is removed and then developed whichever side up you like! Science 8 Unit C Section 4.0

Vision Problems Farsightedness Nearsightedness People who are farsighted cannot see close objects clearly. The eye cannot make the lens fat enough to focus light on the retina, and the image falls behind the retina. People who are near- sighted cannot see distant objects clearly. The eye cannot make the lens thin enough to focus light on the retina, and the image falls in front of the retina. Science 8 Unit C Section 4.0

Vision Problems Science 8 Unit C Section 4.0

Laser Eye Surgery Surgeons can use a laser to reshape the cornea of the eye. The doctor first cuts a thin flap of tissue covering the eye and folds it aside. Then the cornea, which is the clear outer covering of the eye, is reshaped with a laser. After surgery, the newly shaped cornea acts like a corrective lens, bending the light so that the natural lens can focus the light on the retina. Science 8 Unit C Section 4.0

Night Vision Goggles No matter how well your eyes can focus light, you can’t see very well in low-light conditions. In night vision goggles, light is focussed onto an image intensifier. Inside the intensifier, the light energy releases a stream of particles. These particles then hit a phosphor-coated screen. The phosphors glow green when the particles strike them. The person wearing the goggles sees a glowing green image. Science 8 Unit C Section 4.0

Camera vs. Human Eye Science 8 Unit C Section 4.0

Homework! Check and Reflect Page 235 # 1 – 4 # 1 – 4 Science 8 Unit B Section 1.0

Other Eyes in the Animal Kingdom Science 8 Unit C Section 4.0 Topic 4.2

Camera Eyes As you learned previously, the design of the human eye is roughly the same as a camera. For that reason, eyes that have a cornea, a lens, and a retina and are roughly round in shape are referred to as camera eyes. Most vertebrates (animals with backbones) have camera eyes. Depending on how the animal uses the eye, the structure of the camera eye can vary slightly. Science 8 Unit C Section 4.0

Fish Eyes Fish have camera eyes, but instead of an oval- shaped lens, they have a perfectly round lens, that bulges out through the pupil. Because the lenses stick out, a fish can see in practically every direction. This is useful because the fish has no neck and can’t swivel its head to look for danger. Science 8 Unit C Section 4.0

Bird Eyes Birds tend to have much sharper vision than humans. Humans have three types of cones that sense either red, green, or blue light. Birds have five different types of cones, each sensitive to a different wavelength of light. This means that birds can distinguish many more colours and shades than humans can. Having sharp vision is useful for spotting food while in the air. Science 8 Unit C Section 4.0

Nocturnal Vertebrates Animals that are awake at night are nocturnal. No animal can see in complete darkness. The eyes of nocturnal vertebrates allow them to collect as much light as possible. Cats and owls have very large pupils, which allow in as much light as possible. They also have a layer inside their eyes called the tapetum lucidum, which acts as a mirror to reflect light inside their eye. Because they need to make their way about in low-light situations, nocturnal animals have more rods than cones in their retinas. Rods are far more sensitive to low levels of light than cones. Science 8 Unit C Section 4.0

Octopus Eyes Vertebrates aren’t the only animals with camera eyes. Example: An octopus has no backbone, and so is an invertebrate. An octopus eye is very similar to a human eye, having a lens, a cornea, and a retina. However, the lens does not change shape to focus. The lens is moved toward and away from the retina. Science 8 Unit C Section 4.0

Compound Eyes Insects and crustaceans (shrimps, lobsters, and crayfish) have compound eyes. Each eye is made up of many smaller units. Each individual unit is called an ommatidium. An ommatidium looks like a long tube with a lens on the outer surface, a focussing cone below it, and then a light-sensitive cell below that. Science 8 Unit C Section 4.0

Compound Eyes When the light-sensitive cell fires, it sends a message to the animal’s brain through nerve fibres. Insect eyes tend to have a convex surface, so ommatidia lenses face in almost all directions. This makes the compound eye excellent at detecting any motion. As light moves across a compound eye, there is a corresponding change in the ommatidia detecting light. Have you ever noticed how difficult it is to swat a fly? Science 8 Unit C Section 4.0

Drawbacks to the Compound Eye While the compound eye is great at spotting movement, it has drawbacks. Multiple lenses make it difficult to form a single, coherent image. The image formed by a compound eye is referred to as a “mosaic image.” So the number of ommatidia affects how clear an image a compound eye can produce. The more ommatidia, the more detailed the image. Science 8 Unit C Section 4.0

Insect Vision Science 8 Unit C Section 4.0

Homework! Check and Reflect Page 238 # 1 – 3 # 1 – 3 Science 8 Unit B Section 1.0

Image Storage and Transmission Science 8 Unit C Section 4.0 Topic 4.3

Digital Storage: Binary Code Most information today is stored digitally. What is digital storage? The answer is quite simple. If you take any form of information and convert it into numbers, that’s digital! Think about it: “digits” are numbers, right? This is how a computer stores all of its information, including images. How though, does a computer convert an image into numbers? Science 8 Unit C Section 4.0

Digital Images This process of creating a big picture out of small pieces is similar to the process of digital imaging. When a computer receives an image, it divides the picture up into small elements called pixels (short for picture elements). Science 8 Unit C Section 4.0

Pixels in Digital Images Each pixel is assigned coordinates just like the row and seat numbers for a stadium seat. Now the computer has reduced the picture to a series of numbers. This long series of numbers can be stored and saved in a computer. If someone wants to re-create the image, the computer reads all the coordinates, and reassembles the pixels in the correct order. Science 8 Unit C Section 4.0

Colouring A Digital Image In digital imaging, the computer assigns a value to each pixel. This number corresponds to a certain colour. When the picture gets assembled, the computer reads the value of each pixel and makes the pixel that particular colour. If an image is black and white, the value assigned to each pixel corresponds to a shade of grey instead of a colour. Science 8 Unit C Section 4.0

Digital Image Quality The quality of a digital image depends on the size of the pixels that make up the image. If the pixel size is large, then you will notice that the image is made up of small squares, and you won’t be able to see much detail in an image. If two images are the same size, the one with more pixels will look better. A poorer quality digital image has a low resolution. The resolution of an image refers to the number of pixels per unit area. The greater the number of pixels in an area, the higher the resolution, and so the higher the quality of the image. Science 8 Unit C Section 4.0

Digital Image Quality Science 8 Unit C Section 4.0

Capturing Digital Images Scanners, digital video recorders, and digital cameras all work like a regular camera: light enters through an aperture and falls onto a light-sensitive surface. The major difference between regular cameras and digital cameras is that instead of the light falling on film, it falls onto a charge-coupled device (CCD). The CCD is a grid similar to a piece of graph paper. As light falls on a square of the grid, it creates a small amount of electricity in that square. This electrical charge is converted into digital information. Once the image is converted into numbers, this digital information can be stored in a computer hard drive, a compact disk, or digital tape. Science 8 Unit C Section 4.0

Capturing Digital Images Science 8 Unit C Section 4.0

Advantages of Digital Images Digital cameras capture images, turn them into digital information, and beam the information to computers. Computers decode the digital information to reassemble the pictures very quickly (as opposed to being processes in a lab). Digital cameras can also be created to collect different types of light (i.e. radio waves or UV light). For example, some digital cameras can capture infrared radiation, which allows us to gather huge amounts of information about our planet. Science 8 Unit C Section 4.0

Homework! Check and Reflect Assess Your Learning Page 244 # 1 – 6 # 1 – 6 Assess Your Learning Page 245 # 1 – 6 Science 8 Unit B Section 1.0