7.5.1: Illustrations

Illustration 1: The Human Eye

The animation depicts a simplified model of the eye in which the front of the eye is a single converging lens (position is given in arbitrary units and angle is given in degrees). Restart.

Initialize the healthy eye and then add a far source of light. Notice how the parallel rays of light from the faraway source converge at the back of the eye on the retina. The retina is to the eye what film is to a camera. The retina is made up of nerves that convert the light energy into an electrical signal that is sent to the brain. So in order for an object to be "seen," its image must be FOCUSED on the back of the retina.

Now remove the far source and add a near source. Notice that the light from the nearby source is focused behind the retina. In this case the person would see a blurry image. As evolution would have it, our eyes have the ability to accommodate. You can change the focal length of your eye by using the muscles of your eye to change the curvature of the lens. Try looking at a faraway object and then at something close by, such as your finger. You will feel the muscles in your eye respond as you change your focus. In the animation, accommodation is accomplished by using the slider at the bottom to vary the focal length of the lens. Now vary the focal length of the lens, using the slider, until the image of the light source is focused on the retina.

People with normal vision focus on faraway objects with their eyes relaxed. Notice that the far source in the animation was focused when the focal length was at its maximum, one unit. As you use your muscles to accommodate, you shorten the focal length of your eye.

Put your finger in front of your eyes about an arm's length away. You should be able to see a clear image of your finger. Now slowly bring your finger toward you. At some point, you will no longer be able to focus on your finger and it will become blurry. This is your near point. It is the closest distance at which you can focus on an object. If you have not already done so, initialize a healthy eye with a near source of light focused on the retina. Now move the source of light toward the eye. At some point you will no longer be able to accommodate (using the slider) to focus the source. That is the near point for the eye in the animation. Notice that the eye in the animation is not to scale relative to a real eye. If we had made it to scale you would need a much larger computer screen.

The far point is just like the near point, except it is the farthest point an eye can focus on. For people with normal vision, the far point is at infinity.

Initialize the nearsighted eye and add a far source. Notice that the light does not focus on the retina when the eye is relaxed. Instead, it focuses in front of the retina. Use the slider to try to focus the light. Notice that accommodation does not help in this situation. Now remove the far source and add a near source. Notice that the nearsighted person has no trouble focusing on the nearby source. A person who is nearsighted can clearly see near objects but not faraway objects.

Now initialize the farsighted eye and investigate it as you did with the nearsighted eye. Notice that a farsighted person can see faraway objects but has difficulty focusing on nearby objects.

Initialize a nearsighted eye with a far source. Unaided, this eye cannot focus on the far source. Now add an eyeglass lens. Notice that you can change the focal length (power) of the eyeglass lens by clicking on it and then dragging on the hotspots. You can make the lens either converging or diverging.

Since light is focused in front of the retina in a nearsighted eye, nearsightedness is corrected using a diverging lens. Can you find the correct focal length to correct this eye? In the same way, farsightedness is corrected using a converging lens.

Illustration authored by Melissa Dancy and Wolfgang Christian.

Illustration 2: Camera

This animation can be used to demonstrate the basic operation of a camera (position is given in arbitrary units and angle is given in degrees). Various lenses and light sources can be added by clicking on the appropriate links. Restart.

Initialize a normal lens and a near source. The camera is "focused" by dragging the lens to change the lens-to-film distance until the rays from the source all converge on the film. An object at the point of the source will be in focus on the film with this film-to-lens separation. Now add an object source. Notice that when it is focused, the image falls directly on the film.

This Illustration models a camera with one lens. A camera is actually comprised of several lenses that work together as a unit. Multiple lenses are necessary to correct for aberrations. For example, the bending of light by a lens is actually somewhat dependent on the color of the light. This property of nature leads to chromatic aberration (misalignment of the colors in an image), which is corrected by using several carefully chosen lenses.

Illustration authored by Melissa Dancy and Wolfgang Christian.

Illustration 3: Laser Cavity

Under some circumstances, an atom in an excited state can be stimulated to drop to a lower energy state when hit by a photon (particle of light). When the atom drops to the lower energy state, a photon identical to the incident photon is released. If nearby atoms are also in an excited state, a chain reaction will be set off, with released photons going on to stimulate the release of even more photons. All of the photons will be identical, meaning they will have the same wavelength, phase, polarization, and direction of travel. If the chain reaction can be maintained a beam of laser light is obtained. Restart.

It is crucial to the operation of a laser that emitted photons are retained to stimulate more emissions. The purpose of the laser cavity (or resonant cavity) is to confine the emitted photons. The laser cavity consists of two mirrors. One of the mirrors is highly reflective and the other is partly reflective. The one that is partly reflective will allow some of the produced laser light to pass, which is the source of the laser beam, and will reflect the rest to maintain the chain reaction.

A model of a laser cavity is demonstrated in the animation. Light is reflected off of two mirrors. If the circumstances are right, the cavity has stability. That is, light will reflect off the mirrors in such a way that it is confined within the cavity. When the animation is initially loaded, the cavity is stable.

• Click on the mirror on the right and drag it to increase the separation of the mirrors. At what point does the cavity become unstable?
• You can change the focal length of the mirrors by dragging the focal point when the mirror is selected (click on it). Drag the mirrors so that a stable condition exists. What happens to the stability when the focal length of one mirror is increased? What about when the focal length is decreased?

Physlets were developed at Davidson College and converted from Java to JavaScript using the SwingJS system developed at St. Olaf College.