What is the role of lenses in microscopy?

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A microscope is used to magnify the image of tiny objects. Objects are clearly seen under the microscope because at least one lens magnifies the image. This lens refracts light so that it enters the eye and magnifies distant objects.

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How do microscopic lenses work?

A microscope is an extra lens placed in front of your eye. The microscope lens works like a magnifying glass, distorting the light to make the object appear larger to achieve 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 specimen and an ocular lens near the observer. The magnification of these two lenses can be the same or different from each other. By multiplying the magnification of each lens, we obtain the overall magnification of the microscope. With a 10x eyepiece and 30x objective, the total magnification of the microscope is 300x.

Types of microscopes

There are different types 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 a beam of electrons to form images.

Microscopic images can also be created using acoustic microscopes to develop high frequency sound waves.

Additionally, the surface morphology of materials can be observed using a scanning tunneling microscope. Such microscopes use an electron beam that can pass 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 convex or concave.

The shape of the lens has a dramatic effect on the refraction of light. In microscopes, convex lenses are generally used because of their ability to focus light on a specific point.

The biological lens of the human eye is also convex because it focuses light on the retina, where the rod and cone cells are located to enable vision.

Materials used to produce microscopic lenses

Optical glass is often used to create microscopic lenses. It is considerably more uniform and has a higher purity than conventional glass.

The main material of optical glass is silicon dioxide (SiO2), which is very pure, i.e. 99.9%.

The optical properties of optical glass are highly dependent on its composition, which includes a mixture of boron oxide, sodium oxide, barium oxide, zinc oxide, potassium oxide, or lead.

Magnesium fluoride is commonly used as an anti-reflective coating on lenses. If a microscope contains a mirror, it is often made of Pyrex glass. Silica (SiO2) is often used as a protective coating for mirrors, while aluminum is used for reflective coatings.

Manufacture of microscopic lenses

The raw materials and the specific optical glass are combined in the right proportions. This cullet (i.e. the optical glass) serves as a flux. A flux helps to lower the temperature at which raw materials normally react. A glass furnace is generally used to melt this mixture around 1400°C. The temperature can fluctuate depending on the type of lens made.

To bring the air bubbles to the surface, the temperature is raised to 1550°C. The mixture is then regularly cooled to 1000° C. with continuous stirring.

The mixture becomes a very thick liquid and is poured into lentil molds at this point. The annealing is carried out at 500°C after cooling the mixture to about 300°C. Annealing eliminates the internal stresses developed during the early cooling phase and weakens the glass. The glass is then gradually cooled to room temperature and the pieces are removed from the moulds. These pieces are called blanks.

The blank is clamped in a vice and held in place under a cylinder-shaped, diamond-tipped cutter that spins at high speed. The surface of the blank is cut with this cutter in the desired curvature.

After the cut, an inspection of the lens is performed and if the required curvature is not achieved, the cut is redone. This process takes a few minutes to well over an hour.

Types of microscopic lenses

A conventional microscope uses many lenses and a light source to greatly enhance the image of the object being examined.

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 ordinary glass.

Objective

When examining a slide or object under a microscope, the closest lens is called the objective lens, which collects light and increases the magnification of the object under examination.

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 offer 4x, 10x, 40x, and 100x magnification, respectively. Generally, shorter lenses have lower magnification power than longer lenses.

Eye lens (eye lens)

The lens through which the observer looks when using a microscope is called an ocular lens. It takes light from the lens and magnifies it to display a large image. In most cases, the ocular lens magnifies 10x or 15x.

Condenser lens

A condenser lens is located between the light source and the sliding platform. It concentrates the beam of light on the object and then transmits it to the lens. The diaphragm controls the amount of light entering the condenser lens. Each time a different lens can be used to view the object, 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 a higher magnification of the object being examined for the observer. At higher magnification, it becomes easy to analyze even the smallest details of the object. By using multiple lenses in a microscope, the image of the object becomes clearer and easier to examine. With the help of multiple lenses, an object image can even be magnified over 1000 times.

References and further reading

Fowler, SA, & Allansmith, MR (1981). The effect of cleaning soft contact lenses: a scanning electron microscope study. Ophthalmology Archives, 99(8), 1382–1386. https://jamanetwork.com/journals/jamaophthalmology/article-abstract/633879

Lorenz, KO, Kakkassery, J., Boree, D. & Pinto, D. (2014). Analysis by atomic force microscopy and scanning electron microscopy 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 Manufacturing 2017 (Freeform, IODC, OFT) (2017), Paper IW4A.1IW4A.1. https://opg.optica.org/

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