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Science in Focus: Shedding Light: Highlights

Workshop 4 continued

color deficiency | optical illusions | vision correction | color vision in animals | inverted images in the eye

Color Vision in Animals

Human eyes respond to light only in the visible region of the electromagnetic spectrum, however, many animals have eyes that respond in different ways. Some animals see no color. Others are able to detect infarred or ultraviolet light.

Read on to explore some of the differences:

  • The red squirrel and the guinea pig have only one type of retinal cell (no cones) and are totally color-blind.
  • Bullfighters often use red capes to infuriate the bull. The bull is probably responding to the movement of the cape not its color, since there are no cones in the eyes of cattle.
  • Cats have very poor color vision, but under good conditions can distinguish blue-green colors from orange-red ones.
  • Many birds and fish have excellent color vision. The Australian bower bird decorates its nest with various blue objects such as scraps of paper, juice from blueberries, and feathers from smaller birds.
  • Three types of cone cells have been found in hens and pigeons, who have color vision similar to humans. Owls, however, have no cones and are color-blind.
  • Many fish are able to distinguish colors. Sticklebacks and Siamese fighting fish react vigorously to red and blue in both courtship and defense of territory.
  • Bees are color-blind to red and orange but are able to see blues into the ultra-violet range, and many flowers are able to reflect that end of the spectrum.
  • Ants are unable to see red light. We are able to film their noctural activities using red light, which they perceive it as normal darkness.
  • Mosquitoes and other pesky flying insects don’t see yellow so they are not attracted to yellow bug-lights. However they do see purple so bug-zappers emit blue to ultraviolet lights, attracting the insects to a high voltage source which kills them.


Inverted Images in the Eye

A lens can be made from glass or plastic or even water in a container. Although the lens is transparent to light, light's path is bent (refracted) at the surfaces of the lens since the density of the lens is different from that of air.

Lenses can have a variety of shapes. Two common types are convex and concave lenses. If you take a cross section of a convex lens, it is thicker across the middle and thinner at the edges.

Convex Lens 1Convex Lens 2
Convex lenses
Concave Lens 1 Concave Lens 2
Concave lenses

A concave lens is thinner at the middle and thicker at the edges. The lenses in hand lenses (and in our eyes) are convex lenses, usually double convex lenses -- both sides of the lens are curved outwards.

A convex lens is called a converging lens, since light from a distant object will be focused to a point (see below).When you use the Sun and a magnifying glass to burn a hole in a piece of paper, you are using a convex lens to focus the light from the Sun and placed the paper at the focal point. If the paper is closer to or further from the lens then the focal point, you will see a filled circle of light. The focal length is the distance from the center of the lens to this focal point.

For convex lenses with different sizes and thickness, the focal point can lie at different distances from the lens. However, for the same lens, the focal point is always the same distance from the lens. Unless a change is made to the shape of the lens, the focal length of a particular lens does not change.

Instead of looking at light from an object that's very far away, like our Sun, let's look at how a convex lens can change the path of light for an object, such as a pencil that is closer to the lens.

In this drawing we've put the pencil in front of the lens, and at a distance that's longer than the focal length of the lens. We've also drawn a line, called the principal axis that goes straight through the middle of the lens. (The focal point lies on this line). There are three special paths of light that one can draw from the object through the lens to determine the path of light through the lens.

The first path is a line from the pencil that is parallel to the principal axis and in our example, goes through the upper edge of the lens. Just like any light coming in on a parallel path, the lens will change this path so that the light will go through the focal point.

The second path of light is one from the object through the center of the lens. Although there will be some refraction as the light enters and leaves the lens, we'll make what is called the "thin lens approximation" and draw the path straight through the lens.

Finally, the third path of light will be one that goes through a point on the prinicpal axis that lies in front of the lens at the same distance from the lens as the focal point. Since the lens is symmetrical, photons that follow this path will be changed by the lens so that, as they leave the lens, they follow a path that is again parallel to the principal axis.

Where these lines all cross is where the tip of the pencil would appear to be in the image. We can draw a similar set of special lines from any point on the pencil and see where that point would be in the image.*

Although the real pencil was standing with its eraser on the principal axis and its point up, the image of the pencil through the lens is upside down and below the principal axis.The focal length of the lens doesn't change; if the object is moved closer or further from the lens, the size of the inverted image will change. However, if the object lies closer to the lens than the focal length, the image (now usually referred to as a virtual image) is magnified and appears upright.

In our eye, most of the refraction of the incoming light is done by the cornea, which has the shape of a convex lens. The lens in the eye further focuses the incoming photons. The shape of the lens in the eye can be changed by the ciliary muscles to fine tune the additional refraction and slightly adjust the focal length to properly focus the image from a nearby or a distant object.

The lens of the eye focuses light on the retina producing an inverted image. The rods and cones making up the retina detect the light and send electrical signals in nerve fibers to the visual cortex at the back of the brain. No physical picture if formed in the brain, but rather electrical signals are decoded. The result is that we see the object in its correct orientation. The process of decoding the electrical signals which originate in the retina is not fully understood.

* Since we've put the pencil on the principal axis (a line through the center of the lens), if we draw these three lines from the bottom of the eraser -- one line parallel to the principal axis, one line through the middle of the lens, and one line through a point on the axis that lies a focal length in front of the lens -- these three lines all lie on top of each other, along the principal axis.


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