Astronomy Laboratory Exercise 6 About Your Eyes The human eye is the most widely used astronomical instrument; it is, therefore, an appropriate subject for study in an astronomy lab. Many other astronomical instruments require an intelligent eye. Understanding the functions and limitations of the eye will be helpful in using other astronomical instruments. Vision involves three distinct steps: (1) a geometrical optics step, in which light from an object is focused on the retina; (2) a detection step, in which rod and cone cells in the retina detect incident light; and (3) a data processing step, in which electrical signals produced by stimulated detector cells are interpreted. The latter step is accomplished by nervous networks in both the retina and brain, and also involves memory functions. This lab will explore the detection step. Figure 6-1 is an anatomical drawing of the major parts of a human eye ball. The eye focuses light to form images on the retina by refracting (or bending) the incident light with the cornea and the lens. The principal refracting surface of the human eye is the cornea. That is to say, the cornea functions as a lens to focus light on the retina. The part of the eye called the lens serves mainly to adjust the focal length of the cornea-lens system. The lens is said to provide accommodation for distance, meaning that it changes its shape to allow images to be brought into focus on the retina for objects at different distances from the eye. The shape of the lens is controlled by the ciliary muscles in the eye. The shape of the cornea is normally fixed, but can be modified with a contact lens or through surgery. The operation of lenses will be explored in subsequent labs. Cornea Lens Optic nerve Ciliary muscle Figure 6-1: Cross section of the human eye as viewed laterally. The electromagnetic spectrum (the light spectrum) stretches over a wide range, and includes radio, microwave, infrared, visible light, ultraviolet, x-rays, and gamma-rays. These forms of light vary in wavelength. Radio waves have the longest wavelengths, and gamma­ rays have the shortest wavelengths. The amount of energy associated with light is inversely 6 - 1 proportional to its wavelength. So, x-rays, which have short wavelengths, are more energetic than microwaves, which have long wavelengths. Each region of the spectrum can be divided into "colors." For example, radio is divided into VLF (very-low-frequency), AM (amplitude modulation), VHF (very high frequency), FM (frequency modulation), and UHF (ultra high frequency). Although the visible portion of the spectrum is a tiny part of the whole electromagnetic spectrum, it is subdivided too. We call these subdivisions colors. In order, from longest wavelengths to shortest, the colors of visible light are red, orange, yellow, green, blue, indigo, and violet. The human eye is most sensitive to light with a wavelength of about 540 nm (green), but responds to light from about 400 nm (violet) to about 700 nm (red). An exercise in this laboratory allows the student to determine the range of his/her vision. Light detection is accomplished by detector cells located in the retina of the eye. There are two basic types of detector cells: rods (rod-shaped) and cones (cone-shaped). Rod cells come in one variety and provide only gray-scale vision (black and white and shades of gray). These detectors are very sensitive to light and work best in dim lighting. Also, rod cells are slow in delivering their message ("light is detected") to the brain. Running through an obstacle course at night, when rod cells are being used, is dangerous because one may run into an obstacle before the brain receives its images from the eyes. Cone cells come in three varieties: red, green, and blue. The red cone cells detect red light best, but, in order of sensitivity, detect orange, yellow, green, and blue. The green cone cells detect green light best, but also detect other wavelengths of light with varying sensitivity. Similarly, the blue cone cells detect blue best, but also detect some green and violet. Cone cells are not very sensitive to light, so they do not work well in dim lighting. Cone cells provide fast vision by sending their message ("light is detected") to the brain quickly. Cone cells provide humans with "day-vision," while rod cells provide "night­ vision." See Figure 6-2 for a graphical comparison of the sensitivity versus wavelength of the detector cells. 100,000 s e 10,000 n s i t 1,000 i v i 100 t y 10 500 600 700 wavelength (nm) Figure 6-2: The relative sensitivities of the rods and the three sets of cones of the human eye. 6 - 2 Human eyes function in both very bright light and in very dim light. Human eyes perform two adjustments to accommodate to changes in light intensity: changing the pupil size and switching the type of detector cells being used. The size change of the pupil can be seen by placing one's face close to a mirror and making the light alternately dim and bright. In bright lighting, the pupil has a diameter of about 2 mm, but, in dim lighting, the pupil 2 2 expands to a diameter of about 6 mm. This change in area (6 /2 = 36/4 = 9) is roughly ten times. Thus, part of the eyes' increased sensitivity in dim lighting is accomplished by opening the aperture to allow in more light. A second and much more significant change occurs in the detector cells used. Cone cells are used as light detectors in daylight conditions. These cone cells provide fast, color vision. In dim lighting, however, cone cells effectively stop working for the lack of light. This is why colors seem to wane as twilight descends. If one waits in the dark from 5 to 20 minutes, depending somewhat on the individual, rod cells begin to operate. The graph of Figure 6-2 shows that rod cells are most sensitive to blue light and almost insensitive to red light. Consequently, one finds dim, red "safe-lights" in use at astronomical observing sites. This allows people to see dim sky objects using rod-vision and avoid tripping over equipment and each other using red cone-vision. An exercise in this laboratory allows the student to measure the time required for his/her dark accommodation (or adaptation) to occur. A single flash of bright light is enough to destroy one's night vision for several minutes. Rod cells contain a photopigment (a molecule that is sensitive to light) called rhodopsin or visual purple. When light encounters a rhodopsin molecule, the molecule photoisomerizes into a form that is insensitive to light. It takes some time for the molecule to return to its light sensitive form. It is a common experience to see an after-image, resulting from, say, a photographic flash. When such a flash occurs, a bright image is formed on the retina, photoisomerizing all or most of the photopigments of the detector cells on which the image falls. It takes some minutes for the detector cells to fully regenerate the photopigments and regain their original sensitivity to light. The after-image is said to be negative if it appears dark where the original image was bright and bright where the original image was dark; colors may also be switched to their complements. A positive after-image occurs if regions of brightness and darkness in the after-image correspond to the same regions in the original image and if the colors are the same. Eyelids transmit roughly 30 percent of incident light, depending somewhat on blood volume in the eyelid tissue, skin pigments, and the spectral composition of the light. Hemoglobin and myoglobin (red proteins) in the eyelid tissues, cause the eyelids to function as red filters. This means that the eyelids transmit the red end of the visible spectrum best, while most of incident blue light is absorbed. Night-vision can be protected from an expected flash of light by both closing the eyelids and covering the eyes with an opaque object (such as the hands). 6-3 The Random House College Dictionary - Revised Edition, 1980, defines "to see" as "1. to perceive with the eyes; look at; 2. to view; visit or attend as a spectator; 3. to perceive mentally; discern~ understand .. .. " Seeing requires a lot more than letting light enter the eyes. Once the light has been detected (by either rod or cone cells), a signal must be sent to the brain for interpretation. Since there are about 130 million detector cells in each eye and only about one million nerve axons leading from each eye to the brain, some data processing must occur in the retina. Once information about an image reaches the brain, a complex series of analyses and associations with memory is used to relate the image to a concept. How we actually see an object depends strongly on the warehouse of memories from which we draw these associations. 6-4 Procedures Apparatus alpha ray range apparatus, safe-light, and phosphor screen (in box), red, orange, yellow, green and blue color filters, 20 em x 20 em, spectrometer, and incandescent bulb with dimmer lamp. NOTE: Parts A, B and C can be done in any order. Part A should be done by small groups in a room that can be made dark. A. Observing Alpha Ray Scintillations with Dark Adapted Eyes Alpha rays are a type of radiation emitted by some radioactive materials that are a natural part of Earth and other bodies in the Universe.