A microscope is used to magnify the image of tiny objects. The objects are clearly seen with a microscope because at least one lens magnifies the image. This lens refracts the light so that it enters the eye and magnifies distant objects.
Image Credit: Likoper/Shutterstock.com
How Do Microscopic Lenses Work?
A microscope is an additional lens placed in front of your eye. The microscope lens functions like a magnifying glass, bending light to make the object appear wider to get the desired magnification effect. However, a single large lens provides blurry and dark images.
A microscope uses two smaller lenses, i.e., an objective lens near the sample and an ocular lens near the observer. The magnification of both these lenses can be the same or different from one another. Multiplying the magnification of each lens yields the overall magnification of the microscope. With a 10x ocular and a 30x objective, the microscope's total magnification is 300x.
Types of Microscopes
There are various kinds of microscopes used for magnification. An optical microscope is the most common type, creating an image from visible light using lenses. Another commonly used type of microscope is an electron microscope which uses an electron beam to form images.
Microscopic images can also be created using acoustic microscopes to develop high-frequency sound waves.
Furthermore, the surface morphology of materials can be observed with the help of a tunneling microscope. Such microscopes use a beam of electrons that can tunnel through the surface of objects at incredibly small distances and form an image of the surface.
Lenses Used in Microscopes
An optical lens can bend or focus light, and can be either convex or concave.
The shape of the lens has a considerable effect on the refraction of light. In microscopes, convex lenses are usually employed because of their ability to concentrate light on a specific spot.
The human eye's biological lens is also convex because it focuses light onto the retina, where rod and cone cells are located to enable vision.
Materials Used for Producing Microscopic Lenses
Optical glass is often used to create microscopic lenses. It is considerably more uniform and has higher purity than conventional glass.
The primary material of optical glass is silicon dioxide (SiO2), which is highly pure, i.e., 99.9%.
The optical properties of optical glass depend highly upon its composition, which includes a mixture of boron oxide, sodium oxide, barium oxide, zinc oxide, potassium oxide, or lead oxide.
Magnesium fluoride is commonly used as an anti-reflective coating on lenses. If a microscope contains a mirror, it is often made up of Pyrex glass. Silica (SiO2) is often used as a protective coating for mirrors, whereas aluminum is used for reflective coatings.
Fabrication of Microscopic Lenses
The raw materials and specific optical glass are combined in the correct ratios. This cullet (i.e., optical glass) serves as a flux. A flux helps lower the temperature at which raw materials normally react. A glass furnace is typically used for melting this mixture around 1400 °C. The temperature may fluctuate depending on the type of lens being manufactured.
To drive air bubbles to the surface, the temperature is raised to 1550 °C. The mixture is then steadily cooled to 1000 °C with continuous agitation.
The mixture becomes a very thick liquid and is poured into lens molds at this stage. The annealing is carried out at 500 °C after cooling the mixture to approximately 300 °C. Annealing helps eliminate the internal stresses developed during the early cooling phase and weakens the glass. The glass is then gradually cooled to room temperature, and pieces are removed from the molds. These pieces are called blanks.
The blank is clamped into a vice and kept in place beneath a diamond-tipped, cylinder-shaped cutter that spins at high speed. The blank's surface is trimmed with this cutter in the desired curvature.
After cutting, a lens inspection is carried out, and if the required curvature is not achieved, cutting is done again. This process requires a few minutes to well over an hour.
Types of Microscopic Lenses
A conventional microscope employs numerous lenses and a light source to significantly enhance the image of the object under examination.
The compound microscope uses a series of lenses to magnify the image. These lenses are made of optical glass, which is significantly purer and clearer than regular glass.
While examining a slide or an object in a microscope, the lens closest to it is called the objective lens, which collects light and increases the magnification of the object being examined.
Most compound microscopes use four objective lenses, including a low-power lens, a high-power lens, a scanning lens, and an oil-immersion lens.
These lenses provide magnification of 4x, 10x, 40x, and 100x, respectively. Generally, shorter lenses have less magnification power than longer ones.
Ocular lens (eye-piece Lens)
The lens the observer looks through when using a microscope is called an ocular lens. It takes light from the objective lens and re-magnifies it to show a large image. In most cases, the ocular lens magnifies 10x or 15x.
A condenser lens is located between the light source and the slide platform. It focuses the beam of light on the object and further passes it to the objective lens. The diaphragm controls the quantity of light entering the condenser lens. Anytime a different objective lens can be used to view the item, the amount of light entering the lens can be changed. With magnifications of 400x or more, condenser lenses are very useful.
Advantages of Lenses in Microscope
Microscopic lenses result in higher magnification of the object under examination to the observer. At higher magnification, it becomes easy to analyze even minute details of the object. By using multiple lenses in a microscope, object image becomes clearer and easier to examine. With the help of multiple lenses, an object image can even be magnified more than 1000 times.
References and Further Reading
Fowler, S. A., & Allansmith, M. R. (1981). The Effect of Cleaning Soft Contact Lenses: A Scanning Electron Microscopic Study. Archives of Ophthalmology, 99(8), 1382–1386. https://jamanetwork.com/journals/jamaophthalmology/article-abstract/633879
Lorenz, K. O., Kakkassery, J., Boree, D., & Pinto, D. (2014). Atomic force microscopy and scanning electron microscopy analysis of daily disposable limbal ring contact lenses. Clinical and Experimental Optometry, 97(5), 411–417. https://www.tandfonline.com/doi/abs/10.1111/cxo
Zhang, Y., & Gross, H. (2017). Systematic Design of Microscopic Lenses. Optical Design and Fabrication 2017 (Freeform, IODC, OFT) (2017), Paper IW4A.1, IW4A.1. https://opg.optica.org/abstract.cfm?uri=IODC-2017-IW4A.1