Demonstrations: Optics and the Eye



1.  See a network of blood vessels on the surface of your retina. (a) Take off your glasses if you wear them. (b) Stare at a large, uniform, reasonably dim surface (e.g., a blank wall). (c) Take a small flashlight, and hold it next to your eye, right up near the side of the eyeball (the sclera), and shine it at the eye. (Some light will get through the sclera and choroid, and light the retina from the side.) Observation:  Each time your eye (or the light) moves, you will see the branching network of the blood vessels of your eye.  Thus, the receptors must be behind the blood vessels.  Also, you normally don't see these blood vessels because your eye adapts and thus ignores them, but these viewing conditions cause the image of them to move quickly, defeating the adaptation ability of your visual system.

2.  Ophthalmoscope. If you have access to one (and I can borrow one if you are interested), use an ophthalmoscope to examine the features of your friend's eye (and vice versa).  Turn on the device (push the red button and spin the ring).  Light comes out the curved side (point it at your hand and you'll see), so point that into your friend's pupil while you look in the other side.  Twist the outer ring to bring the retina into focus.  Try to spot the optic disk and fovea, if you can!  Note, this is a skill that requires practice.

3.  Depth of field. (a) Make a pin-hole in a piece of paper. (b) Take off your glasses and look through the pin-hole when it is placed as close as possible to your eye. Observation: The world is in reasonable focus through the hole, even though it may be quite dim, and depth of field is increased.  For example, move your finger up as close to your face as you can while keeping it in focus as viewed through the pinhole.  Remove the pinhole and note that you can no longer accomodate that much (your finger is blurry no matter how hard you try to focus).

4.  Blur circle. Move the pin-hole close enough to one of your eyes so that you can't bring the hole itself into focus.  The image of the hole is enlarged, forming a "blur circle".  Cover your other eye with your hand, and the blur circle should expand.  This is because you have darkened one eye, causing it's pupil to dilate, but your two eyes' pupils are locked, so the pupil of the eye viewing the pin-hole also dilated.  The blur circle's size is limited by the edges of the pupil, so a dilated pupil results in a larger blur circle.

5.  Reflection. Any object that reflects light provides an example of reflection. Examine matte versus shiny services.  Note that the shininess of a shiny surface obeys (approximately) the mirror reflection rules (you see an image of the light source reflected from the object).

6.  Refraction. (a) Fill a glass of water. (b) Insert a portion of an object (say, a pen) into the water. Observation:  The object appears to bend at the boundary between water and air, which is a result of the light rays coming from the submerged portions of the object bending at the air/water boundary.

7.  Dispersion. (a) Put a prism in front of a point source of light. (b) Put a piece of paper in front of the prism. Observation:  See different colors on the paper.  Note which wavelengths of light (colors) bend more or less.  Note the ordering of colors in the rainbow, which is the ordering of the colors of monochromatic (single wavelength) lights.

8.  Diffraction.  Look at the image of a point source of light through apertures of various sizes against any suitable surface.  For very small apertures the diffraction should cause the image to spread out relative to larger apertures.  At even larger apertures, the size of the aperture itself is a problem, and the image quality worsens with larger aperture sizes.

9.  Determine the focal length of a lens. (a) Set up a point source of light as the object at an effectively "infinite" distance away (i.e. a few meters). (b) Install the lens on the yardstick on the table. (c) Move a piece of paper on the side of the lens which is opposite to the point source (or fix the paper and move the lens). Observation:  At one particular position, a well-focused image of the point source should be seen on the paper.   The distance between the lens and the well-focused image is the focal length of the lens.

10.  Relation between focal length and lens strength. (a) Repeat (9) with lenses of different strength (or power). Observation: The stronger the lens, the shorter the focal length. (Focal length is inversely related to lens strength.)

11.  Relation between "the distance between the lens and the object" and "the distance between the lens and the image" for a given lens. (a) Install the lens of known focal length on the yardstick. (b) Fix the object some distance away from the lens.  Record the distance between the object and the lens. (c) Move a piece of paper on the other side of the lens and find out the position where a well-focused image occurs.  Record the distance between the image and the lens. (d) Repeat the pair of (b) and (c) several times, moving the object each time. Observations:  When you move the object farther from the lens, the image becomes smaller and is in focus closer to the lens. Move the object closer to the lens, the image becomes larger and farther from the lens.  If the distance between the object and the lens is equal to or less than the focal length of the lens, the lens will no longer be able to project an image of the object.

12.  Correction for myopic errors. (a) Set up a point source at an "infinite" distance from a convex lens of known focal length. (b) Place a piece of paper at a distance slightly farther than the focal length on the other side of the lens.  The image is blurry in much the same way as it is for a near-sighted (myopic) observer viewing a distant scene. Part 1: (c) Place lenses of different shapes and strengths in front of the original lens, opposite to the side the paper is on, to determine which "corrective lens" results in a focused image on the paper. Part 2: (c) Instead, move the single lens towards or away from the paper in order to obtain a focused image.  (Or, move the paper instead.) Part 3: (c) Instead, replace the lens with lenses of different strengths to obtain a clear image. Observations:  Myopic errors can be corrected by (1) adding a concave lens, (2) decreasing the distance between the lens and the surface, or (3) decreasing the strength of the lens.

13.  Correction for hypermetropic errors. (a) Repeat 10, except that the distance between the paper and the lens should be slightly shorter than the focal length this time. Observations:  Hypermetropic errors can be corrected by (1) adding a convex lens, (2) increasing the distance between the lens and the surface, or (3) increasing the strength of the lens.

14.  Optical corrections.  (a)  Use an optical testkit and eye chart to determine which amount of refraction you require (for one eye) to achieve the best focus possible (I can borrow a testkit for you if you are interested) - this is your optical correction.  (b)  Use the cylindrical lenses and the ray diagram to see the effects of cylindrical aberration (astigmatism).  If you have astigmatism, see if you can determine its principal axes (which rays are in focus and which are not), and how much cylindrical correction solves the problem.  These values (refraction, principal axis, cylindrical correction) are the values your optometrist uses to grind the lenses of your glasses.  For some contact lenses, typically the astigmatism is less important since the front of the contact lens doesn't have the cylindrical aberration, and the seal between the lens and your cornea prevents the cylindrical aberration of your cornea from being a factor.