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7 Light in Biology and Medicine
7.29.4 Ophthalmoscope
The ophthalmoscope is a simple illumination device that permits a physician to see
the fundus of the eye, i.e. the surface opposite the lens. Ordinarily, looking at the
eye of a subject shows a dark pupil. The ophthalmoscope sends extra light along the
ophthalmologists viewing direction into the eye of a patient. The light is refracted
by the patient’s eye cornea and lens onto the fundus. Some is reflected, and goes
back through the eye lens, which directs much of the light rays back to where they
originated. However, suppose the source of illumination comes from a reflecting
plate placed at a 45 ◦ angle to the line of sight, and that reflecting plate has a small
hole at the line of sight. Then the ophthalmologist can see reflected light coming
back, thereby allowing examination of the fundus, including the retina, fovea, optic
disc, and small blood vessels. A direct ophthalmoscope uses no lens; typically just
a light source and mirror.
The indirect ophthalmoscope has lenses in the path of light from the retina. Those
lenses can enlarge the field of view and provide for magnification (up to 15×) of
the image of the fundus. With a converging (objective) lens, at a position from the
patient’s eye a little greater than the objective lens’ focal length f o , and assuming
the patient’s eye is relaxed (’emmetropic’, which occurs when focusing on a distant
object), then the physician’s field of view will have an enlargement factor of D/f o ,
where D is the diameter of the cone of returned rays of light at the objective lens’
position.
After light reaches the patient’s fundus, then, for an emmetropic eye 48 , the image
of the retina will be a large distance and in front of the patient’s eye. We can estimate
the magnification of the ophthalmoscope as follows: Suppose the objective lens of
the ophthalmoscope has a focal length of f o and placed at a nearby distance from the
patient’s eye. Applying the thin-lens equation twice, once for the patient’s eye, with
focal length f e , and once for the objective lens in the ophthalmoscope, the retina
will appear to the physician magnified by M ≈ f o /f e .
7.29.5 Otoscope
The otoscope is designed to view the eardrum and affords a diffuse view of the
middle ear. A flexible tube fits into the entrance of the ear canal, and conducts light
into the ear, and also is wide enough to allow viewing through the tube. The image
of the eardrum and shadows of the malleus in the middle ear is made visible to the
physician at a comfortable distance from the physician’s eye.
48 Emmetropic vision requires no correction.
7 Light in Biology and Medicine
7.29.4 Ophthalmoscope
The ophthalmoscope is a simple illumination device that permits a physician to see
the fundus of the eye, i.e. the surface opposite the lens. Ordinarily, looking at the
eye of a subject shows a dark pupil. The ophthalmoscope sends extra light along the
ophthalmologists viewing direction into the eye of a patient. The light is refracted
by the patient’s eye cornea and lens onto the fundus. Some is reflected, and goes
back through the eye lens, which directs much of the light rays back to where they
originated. However, suppose the source of illumination comes from a reflecting
plate placed at a 45 ◦ angle to the line of sight, and that reflecting plate has a small
hole at the line of sight. Then the ophthalmologist can see reflected light coming
back, thereby allowing examination of the fundus, including the retina, fovea, optic
disc, and small blood vessels. A direct ophthalmoscope uses no lens; typically just
a light source and mirror.
The indirect ophthalmoscope has lenses in the path of light from the retina. Those
lenses can enlarge the field of view and provide for magnification (up to 15×) of
the image of the fundus. With a converging (objective) lens, at a position from the
patient’s eye a little greater than the objective lens’ focal length f o , and assuming
the patient’s eye is relaxed (’emmetropic’, which occurs when focusing on a distant
object), then the physician’s field of view will have an enlargement factor of D/f o ,
where D is the diameter of the cone of returned rays of light at the objective lens’
position.
After light reaches the patient’s fundus, then, for an emmetropic eye 48 , the image
of the retina will be a large distance and in front of the patient’s eye. We can estimate
the magnification of the ophthalmoscope as follows: Suppose the objective lens of
the ophthalmoscope has a focal length of f o and placed at a nearby distance from the
patient’s eye. Applying the thin-lens equation twice, once for the patient’s eye, with
focal length f e , and once for the objective lens in the ophthalmoscope, the retina
will appear to the physician magnified by M ≈ f o /f e .
7.29.5 Otoscope
The otoscope is designed to view the eardrum and affords a diffuse view of the
middle ear. A flexible tube fits into the entrance of the ear canal, and conducts light
into the ear, and also is wide enough to allow viewing through the tube. The image
of the eardrum and shadows of the malleus in the middle ear is made visible to the
physician at a comfortable distance from the physician’s eye.
48 Emmetropic vision requires no correction.