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There are a variety of types of lenses. Lenses differ from one another in terms of their shape and the materials from which they are made. Our focus will be upon lenses that are symmetrical across their horizontal axis - known as the principal axis. In this unit, we will categorize lenses as converging lenses and diverging lenses. A converging lens is a lens that converges rays of light that are traveling parallel to its principal axis. Converging lenses can be identified by their shape; they are relatively thick across their middle and thin at their upper and lower edges. A diverging lens is a lens that diverges rays of light that are traveling parallel to its principal axis. Diverging lenses can also be identified by their shape; they are relatively thin across their middle and thick at their upper and lower edges.

If a piece of glass or other transparent material takes on the appropriate shape, it is possible that parallel incident rays would either converge to a point or appear to be diverging from a point. A piece of glass that has such a shape is referred to as a lens.

If you have any of these symptoms, call your eye care professional who prescribed the glasses. You may need the frame refit or a new prescription.

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A lens is merely a carefully ground or molded piece of transparent material that refracts light rays in such as way as to form an image. Lenses can be thought of as a series of tiny refracting prisms, each of which refracts light to produce their own image. When these prisms act together, they produce a bright image focused at a point.

Hirschberg test. This basic test helps check for strabismus. A pen light is aimed at the eyes. The light's reflection should appear in the center of the pupil in each eye. If the reflection is off-center in either or both eyes, there may be strabismus.

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Cover tests. There are three types of cover tests. With the cover/uncover test, one eye is covered and one is left uncovered. The doctor looks for any movement in the eye that is uncovered. The prism and alternate cover test alternates the cover over both eyes while a prism is placed in front of one eye. This measures the offset (difference) between the two eyes to find the right prism to fix your double vision. With the Maddox Rod test, each eye sees a vertical or horizontal line. The offset of these images allows the doctor to determine what type of double vision you have. Prisms are also used to measure the distance between the two images and provide the prescription value.

Double vision has many causes, and a prism isn't helpful for all types of double vision. Because it can be a sign of a serious health condition, it's important to find out what's causing your double vision.

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As we discuss the characteristics of images produced by converging and diverging lenses, these vocabulary terms will become increasingly important. Remember that this page is here and refer to it as often as needed.

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Prism correction is used in eyeglasses for some people with diplopia, or double vision. This is when someone sees two separate images of one object. The prism helps align the two images, so that only one image is seen. Why Do People See Double?  Normally, we see one image through two eyes because light falls on the same part of the retina in each eye. With double vision, two images are seen because the light falls on different parts of the retina. How Do Prism Glasses Fix Double Vision? A prism added to eyeglasses bends light before it travels through the eye. The light is redirected to the right place on the retina in each eye. The brain then fuses the two images together to produce one, clear picture. Will Prism Glasses Fix My Double Vision? A prism may fix double vision from eye misalignment when caused by: Eye muscle problems, such as myasthenia gravis, Graves’ disease, or strabismus (crossed eyes or wall eyes) Neurological (brain-related) issues, such as head injuries, stroke, migraine, or tumor. Nerve-related problems, like multiple sclerosis or diabetes mellitus Double vision has many causes, and a prism isn't helpful for all types of double vision. Because it can be a sign of a serious health condition, it's important to find out what's causing your double vision. See a doctor immediately if your double vision: is new is caused by eye misalignment or happens for unknown reasons How Do I Get Prism Put in My Glasses? Just like regular eyeglasses, you need an in-person exam to test your vision. This is the only way to get the right prescription. During the exam, your ophthalmologist may do one or more of these tests to evaluate whether you need prism: Hirschberg test. This basic test helps check for strabismus. A pen light is aimed at the eyes. The light's reflection should appear in the center of the pupil in each eye. If the reflection is off-center in either or both eyes, there may be strabismus. Off-center corneal reflection of light in left eye, showing possible strabismus Krimsky test. This test helps measure the degree of misalignment. This is like the Hirschberg test, but with a prism. While looking at the pen light, prisms of different strengths are placed in front of the eye. When a prism brings the reflection in each eye to the center, the correct prism measurement has been found. Cover tests. There are three types of cover tests. With the cover/uncover test, one eye is covered and one is left uncovered. The doctor looks for any movement in the eye that is uncovered. The prism and alternate cover test alternates the cover over both eyes while a prism is placed in front of one eye. This measures the offset (difference) between the two eyes to find the right prism to fix your double vision. With the Maddox Rod test, each eye sees a vertical or horizontal line. The offset of these images allows the doctor to determine what type of double vision you have. Prisms are also used to measure the distance between the two images and provide the prescription value. What Does a Prescription for Prism Look Like? A prescription for prism typically has two numbers: Prism. Like normal eyeglasses, diopters measure the amount of prism correction (from 0.5PD, 1.0PD, 1.5PD, and so on) and indicate how misaligned your eyes are. Base. Depending on your double vision, the prism is placed vertically, horizontally or diagonally in one or both lenses of your eyeglasses. The “base” is the thickest edge of the prism and tells you where the light is redirected. It may be on the outer edge of the lens (Base Out, BO), the inner edge (Base In, BI), or at the top (Base Up, BU) or bottom edge (Base Down, BD). What Do Prism Eyeglasses Look Like? Sometimes, a temporary Fresnel prism (a thin press-on vinyl sticker) is fitted over the front or back of your eyeglasses. With a Fresnel prism lens, the prism is slightly visible. This isn't ideal for long-term use, but it allows you to test drive a prism and see how it works. Fresnel prisms are also used when the prism prescription isn't stable and may change soon. Eyeglasses with a temporary Fresnel prism on left lens. If prism correction is needed for a longer period, it can be ground into (made part of) the lens of your eyeglasses. Eyeglasses with a prism look like any other glasses without a prism, although the lens on one side may be thicker and more noticeable. If this is a concern to you, consider a thicker frame to hide that portion of the lens. Eyeglasses with prism built into the lens. Do Prism Eyeglasses Cause Side Effects? It may take a few days to get used to wearing new prism eyeglasses or those with an updated prescription. You may have some eye strain during this time. Rare symptoms include: double vision headache nausea eye pain If you have any of these symptoms, call your eye care professional who prescribed the glasses. You may need the frame refit or a new prescription.

Normally, we see one image through two eyes because light falls on the same part of the retina in each eye. With double vision, two images are seen because the light falls on different parts of the retina.

As we begin to discuss the refraction of light rays and the formation of images by these two types of lenses, we will need to use a variety of terms. Many of these terms should be familiar to you because they have already been discussed during Unit 13. If you are uncertain of the meaning of the terms, spend some time reviewing them so that their meaning is firmly internalized in your mind. They will be essential as we proceed through Lesson 5. These terms describe the various parts of a lens and include such words as

A double convex lens is symmetrical across both its horizontal and vertical axis. Each of the lens' two faces can be thought of as originally being part of a sphere. The fact that a double convex lens is thicker across its middle is an indicator that it will converge rays of light that travel parallel to its principal axis. A double convex lens is a converging lens. A double concave lens is also symmetrical across both its horizontal and vertical axis. The two faces of a double concave lens can be thought of as originally being part of a sphere. The fact that a double concave lens is thinner across its middle is an indicator that it will diverge rays of light that travel parallel to its principal axis. A double concave lens is a diverging lens. These two types of lenses - a double convex and a double concave lens will be the only types of lenses that will be discussed in this unit of The Physics Classroom Tutorial.

Sometimes, a temporary Fresnel prism (a thin press-on vinyl sticker) is fitted over the front or back of your eyeglasses. With a Fresnel prism lens, the prism is slightly visible. This isn't ideal for long-term use, but it allows you to test drive a prism and see how it works. Fresnel prisms are also used when the prism prescription isn't stable and may change soon.

If a symmetrical lens were thought of as being a slice of a sphere, then there would be a line passing through the center of the sphere and attaching to the mirror in the exact center of the lens. This imaginary line is known as the principal axis. A lens also has an imaginary vertical axis that bisects the symmetrical lens into halves. As mentioned above, light rays incident towards either face of the lens and traveling parallel to the principal axis will either converge or diverge. If the light rays converge (as in a converging lens), then they will converge to a point. This point is known as the focal point of the converging lens. If the light rays diverge (as in a diverging lens), then the diverging rays can be traced backwards until they intersect at a point. This intersection point is known as the focal point of a diverging lens. The focal point is denoted by the letter F on the diagrams below. Note that each lens has two focal points - one on each side of the lens. Unlike mirrors, lenses can allow light to pass through either face, depending on where the incident rays are coming from. Subsequently, every lens has two possible focal points. The distance from the mirror to the focal point is known as the focal length (abbreviated by f). Technically, a lens does not have a center of curvature (at least not one that has any importance to our discussion). However a lens does have an imaginary point that we refer to as the 2F point. This is the point on the principal axis that is twice as far from the vertical axis as the focal point is.

If prism correction is needed for a longer period, it can be ground into (made part of) the lens of your eyeglasses. Eyeglasses with a prism look like any other glasses without a prism, although the lens on one side may be thicker and more noticeable. If this is a concern to you, consider a thicker frame to hide that portion of the lens.

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A prism added to eyeglasses bends light before it travels through the eye. The light is redirected to the right place on the retina in each eye. The brain then fuses the two images together to produce one, clear picture.

It may take a few days to get used to wearing new prism eyeglasses or those with an updated prescription. You may have some eye strain during this time.

Krimsky test. This test helps measure the degree of misalignment. This is like the Hirschberg test, but with a prism. While looking at the pen light, prisms of different strengths are placed in front of the eye. When a prism brings the reflection in each eye to the center, the correct prism measurement has been found.

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Just like regular eyeglasses, you need an in-person exam to test your vision. This is the only way to get the right prescription.

Prism correction is used in eyeglasses for some people with diplopia, or double vision. This is when someone sees two separate images of one object. The prism helps align the two images, so that only one image is seen.