Module 16 Section 1

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Section 2

Module 16:

Section 1:

Physical Optics

Geometric Optics

• Refraction

• Reflection

• Prisms

• Spherical lenses

• Induced prism

Physical Optics

The behavior of light can be described by either the wave theory or the quantum (corpuscular) theory. The quantum theory is useful when explaining how light travels through a vacuum and when explaining optical properties such as reflection. The wave theory is useful when explaining the behavior of light as it is "bent" by various media. You only need to be aware that the behavior of light cannot be explained by one theory, and be able to identify the two theories by name.

The electromagnetic spectrum identifies and orders different wavelengths of energy from the shortest wavelength to the longest. The longest waves are radio waves and the shortest are cosmic rays. We cannot see infrared or ultraviolet rays. The visible spectrum lies between these two.

Infrared light is used in some ophthalmic instruments as a component of aiming and measuring devices. Ultraviolet light is considered to be damaging to the human eye and may play a part in the formation of cataracts and in the progression of macular degeneration.

The wavelength of the visible spectrum is from 400 to 800 nanometers, red being the longest and violet the shortest ( a nanometer is a billionth of a meter). All of the colors combine to create white light. White light can be split into it’s component wavelengths by a prism (such as created by water droplets in a rainbow). You can remember the components by remembering the memory helper "Roy G. Biv".

Knowledge of the visible spectrum is clinically useful. In refractometry we make use of the different focal lengths of red and green light to test for a proper spherical correction. The retina surgeon uses his knowledge of the penetrating power of different wavelengths when using a laser to treat the retina. Light sensitive cells of the retina are tuned to different wavelengths, and color vision deficiencies are identified by the wavelength of light that is not correctly perceived.

For test taking purposes you may need to know that light travels through a vacuum (outer space) at 186,000 miles per second, roughly the speed at which a two year old can be out the door when you’re on the phone.

Geometric Optics

A substance which blocks light is termed opaque (e.g., a solid wall). A translucent substance lets light pass but disturbs it (e.g., frosted glass). Substances that transmit light without significantly disturbing it are called transparent (clear glass, clear air). Transparent substances are also called optical media.

Light coming from a single sourse, in the same wavelength, and in phase, is called coherent light.  The concentrated power of this beam is called a laser beam.  Light can be scattered by particles (e.g. smoke) in the medium through which it is passing.  Light that bounces off of a surface is reflected.  If light changes direction while passing from one medium to another, it is said to be refracted.  Light can be polarized by passing through a medium that blocks light waves that are in phase and transmits light waves that are 90 degrees out of phase.

Light will behave in predictable ways when passing from one optical media to another:

If light enters an optical medium that is more dense than the medium from which it came (e.g., from air to glass), it will be slowed by the new medium.

If light strikes the surface (interface) of the new medium in a perpendicular fashion, it’s direction will not be changed.

If light strikes the interface of a new medium at an angle, the direction of the light will be changed. The new direction of the light beam is affected by the density of the new medium and the angle at which the light beam strikes the interface.

These concepts summarize the basic physics of how glasses work to correct vision. The phenomenon of light changing direction when traveling from one optical medium to another is called refraction.

Optical media have what is called an index of refraction. The more dense a material is, the higher the index of refraction.  The index of refraction equals the speed of light in a vacuum divided by the the speed of light in the particular substance.  The index of refraction of air is 1.0.   The index of refraction of water is 1.33.  A "high index" lens might have an index of refraction of 1.6.  High index lenses supply an equal amount of refractive power compared to lower index lenses, but with less thickness.

A higher index of refraction results in a lower angle of refraction, which means the bent light ray comes closer to the perpendicular (normal) line.

In other words, for a given angle of incidence, the higher the index is, the more the light ray is bent. That is why so called "high index" lenses can be made thinner for a given prescription as compared to lower index lenses. The angles and indexes can be computed using Snell’s law of refraction.

Reflection

When light encounters an opaque medium, the light will either be reflected or absorbed, or some of the light will be reflected and some will be absorbed. If the light is absorbed, it is converted to heat energy. If light is reflected, it bounces off at a predictable angle.

When light bounces off a mirrored surface, the law of reflection states that the angle of reflection is always equal to the angle of incidence. This is useful knowledge when lining up a mirror for an acuity projector, or when playing pool. Many optical instruments use mirrors to redirect light.

Convex mirrors create a negative vergence (the direction of the light rays relative to one another) to the light rays which makes the reflected image smaller. An example would be the passenger side outside mirror on an automobile.

A concave mirror creates a positive vergence to the light rays which magnifies the reflected image. An example would be a shaving mirror.

A prism is a piece of optical quality glass or plastic with a triangular face. They are used to redirect light to a predictable degree.

Prisms are used in ophthalmology to measure and treat strabismus (misalignment of the eyes). Prisms are combined (apex to apex, and base to base) in optics to create lenses.  

Characteristics of prisms:

Light is bent toward the base of the prism.  

To the viewer, the image will appear to be displaced toward the apex of the prism.

The power (light bending ability) of a prism is measured in diopters.

A one diopter prism will deviate light 1 cm at a distance of one meter.

The mathematical expression is:

prism diopters = deviation (cm) / distance (meters)

For example: If light is deviated 6mm at a distance of 3 meters, then the prism diopter power is .2 diopter. Remember that the deviation is in centimeters. The 6mm must be changed to .6 cm.

All light rays have some degree of vergence. That is, they are either moving closer to one another (convergence), or they are moving away from one another (divergence), or they are parallel (zero vergence). Vergence is measured in diopters.

A plus lens can be considered to be two prisms aligned base to base. A plus lens converges light to a focal point, creating a "real" image.

A minus lens can be considered to be two prisms aligned apex to apex. A minus lens diverges light and has a "virtual" focal point on the same side of the lens from which the light comes.

Of course, a modern glasses lens does not look much like a couple of prisms aligned together. The lenses may have a flat or curved front surface and the edges are usually polished. Although all plus lenses are thicker in the center than at the edge, and all minus lenses are thicker at the edge than at the center, modern high index lenses make it difficult to tell on the lower prescriptions.

One way to tell at a glance if someone is nearsighted or farsighted is to notice how her eyes appear behind her glasses. Plus lenses will magnify the appearance of the eyes and minus lenses will make the eyes look smaller than normal.

The radius of curvature is a way of expressing how steep a curve is. A small circle is more steeply curved than a large circle. A small circle has a smaller radius than a large circle, thus the smaller the radius of curvature, the steeper the curve of the lens is. The steeper the curve is, the more diopter power the lens has.  As the radius of curvature gets smaller, the diopter power increases.  A 44 diopter cornea has a radius of curvature of 7.67 mm.  A 42 diopter cornea has a radius of curvature of 8.04 mm.

The sum of the front side curve and the back side curve of a lens is equal to the total power of the lens.

This lens has a +6 D front curve, and a -2 D back curve.  The total lens power is 4 D (6 - 2 = 4).

Induced Prism

The point at which the lens has no affect on the direction of the light is called the optical center. This is the thickest part of a plus lens and the thinnest part of a minus lens. It is not necessarily the geometric center of the lens.

Glasses should be fit so that the patient normally looks through the optical centers of the lenses. If the patients visual axis is not lined up with the optical center, then a prismatic effect is induced. In this example a base-down prismatic effect is induced. 

This is normally not desirable, but sometimes a prismatic effect by optical center decentration is created on purpose to correct an eye alignment deviation.

Prentice’s rule is used to calculate the prismatic effect: induced prism (in diopters) = decentration in centimeters multiplied by the lens power in diopters.   When performing calculations, it soon becomes obvious that optical center decentration makes little difference in low powered lenses, and that it becomes more and more significant as lens powers increase.

Induced prism calculations are useful in the following situations:

When evaluating glasses complaints. If the optical centers do not align with the pupillary distance, then calculations can be made to determine if the prismatic effect is significant.

When determining the need for a slab-off bifocal correction. A prismatic effect may be induced when a patient looks through the lower edges of lenses that have powers that do not match. 

Optical center decentration can be used on purpose in higher powered glasses lenses to create a desirable effect.