Lens Types: Simple, Compound & Other Types

Instructor: Sarah Phenix
In this lesson we will explore what a compound lens is versus a simple lens as well as what it means for a lens to be concave or convex and how these particular lens shapes can be used to correct certain visual impairments.

What is a Lens?

Have you ever worn prescription glasses, used a microscope, a telescope, or for that matter, your eyes? Well, if you have than you have looked through a lens.

A lens is an instrument that refracts (or bends) light in such a way as to allow the user to see the world around them in a different way. Depending on the type of lens, that 'sight' could vary greatly. Some lenses enable people to see the world more clearly than their own eyes allow (corrective eyeglasses), some can magnify small objects so that they appear larger (microscope lenses and magnifying glasses), while others allow us to see things that are very far away (telescopic lenses). All these instruments have one thing in common - they require lenses.

Let's talk about some of the different types of lenses.

Simple versus Compound lenses

Simple Lens: Hand Magnifying Lens
Magnifying Hand Lens

A simple lens is a basic device that uses a single lens to refract light. A compound lens uses multiple lenses. The most obvious example of a simple lens is a magnifying glass, which uses a single lens to magnify an object, while an example of a compound lens is a compound microscope, which uses multiple lenses to increase the viewer's capacity to magnify an object. So how does a compound microscope work?

Compound Microscopes

Parts of a Compound Micrscope
Compound Microscope

Both simple and compound lenses can magnify an object, but a compound microscope has a greater capacity because it uses multiple magnifying lenses to 'compound' its magnification power. It'd be like if someone was standing very far away from you. If you asked them to move 5 feet closer, that would be like a single lens magnification. If you asked them to move 5 feet closer six times, that would be like a compound lens magnification.

The first level of magnification found in a compound microscope exists in its ocular lenses (ocular meaning the viewer looks through them), which have a magnification of 10x (or ten times magnification). The second device which 'compounds' its ability to magnify an object are the various objective lenses, which sit on a rotating nosepiece so that the magnification may be adjusted.

The objective lenses offer an additional 4x, 10x, 40x, or 100x magnification, on top of the 10x of the ocular lenses. Therefore, to calculate the total magnification you must multiply the ocular lens magnification (10x) by the particular objective lens you are using (4x, 10x, 40x or 100x). In other words, if you were to look at a human hair with the 4x objective lens, you multiply 10 (from the ocular lens) by 4 to get 40 times the normal appearance. If you wanted some serious detail you would switch to the 100x, which (again multiplying by the ocular lens magnification 10) would give you 1000 times the normal size.

Convex & Concave Lenses

Let's take a look at how the shape of the lens affects the direction light is bent.

Light Refracting through a Concave Lense
Light Refracting through a Concave Lens

Concave lenses are thinnest in the middle and widest at the edges, as if the face of the lens were 'caving' in on itself. This shape results in spreading the light into a wider arc. Now, since there are two faces of a lens to consider (a front and a back), there are different ways a lens may be cut.

Planoconcave is where one face is flat while the other is concave, while biconcave has two concave surfaces. Each refracts light outward. Biconcave lenses can enable nearsighted individuals to correct their impairment.

The lens at the front of the eye focuses the light for the retina cells at the back of the eye
eye diagram

In a normal eye, light entering the pupil passes through a biconvex lens and is bent so that the image is focused right on the retina, the light sensitive layer lining the back of the eye.

Nearsightedness is caused when the lens incorrectly focuses the light too early, essentially meaning that the image focuses in front of the retina, rather than on it. In this case the image the brain receives is blurry. Imagine it like standing too far away from the TV screen - you could see general colors and shapes, but couldn't quite make out the details until until moving closer.

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