This amazing scientific phenomenon, even Newton made a mistake about it

Long ago, the ancient Egyptians and Mesopotamians knew how to polish quartz crystals into lenses, which might have been used to magnify images or focus sunlight.

In the middle of the 17th century, the scientific community began a debate on the nature of light - the debate between the wave theory of light and the particle theory of light . This debate continued until the middle of the 19th century.

Dutch physicist Huygens was the founder of the wave theory of light; the great British scientist Newton was the advocate of the particle theory of light.

This debate, which lasted for more than 200 years in the history of the development of optics, led optics onto the road of development and enabled mankind to unveil the layers of veils of optics and recognize its essence in the debate.

Dispersion of light. Copyrighted image, unauthorized reproduction

During the debate, Newton conducted an experiment.

Let sunlight pass through a prism, and on the screen behind the prism, we will find that the sunlight (white light) is refracted into a continuous spectrum of colored light bands such as red, orange, yellow, green, cyan, blue and purple. This is the famous Newton dispersion experiment .

The essence of Newton's dispersion experiment is the refraction of light . The refraction of light is the phenomenon that the propagation direction of light changes when it propagates from one medium to a different medium.

As shown in the figure below, the "broken" pencil in the cup in our daily life and the fishermen aiming at the bottom of the fish they see when fishing with a spear are all caused by the refraction of light. When the refraction of light occurs, the same medium has different deflection capabilities for different colors of light, which will lead to the appearance of colored light bands.

Through the prism dispersion experiment, Newton hastily came to the erroneous conclusion that glass lenses cannot eliminate chromatic aberration. This conclusion now seems to be incorrect.

Refraction of light. Image source: Wikimedia

So what exactly is chromatic aberration?

As shown in the figure below, most optical glasses have a weak ability to deflect red light and a strong ability to deflect blue-violet light, that is, the refractive index for red light is low, while the refractive index for blue-violet light is high. When observing cells with a microscope with chromatic aberration, the observed cells appear red in the outer circle and blue-green in the center, which is what we call chromatic aberration.

Chromatic aberration caused by a single lens. Image source: Wikimedia

If there is chromatic aberration in the transmissive optical system (lens) composed of optical glass, the image quality will be greatly reduced. For example, in the photo of white flowers below, the chromatic aberration of the lens causes an obvious "rainbow band" phenomenon to appear on the edge of the petals.

It took hundreds of years for scientists to discover, understand, and correct chromatic aberration, which is still a research topic in the field of optics today , and has produced some classic stories.

The impact of chromatic aberration on imaging. Image source: Wikimedia

The telescope is one of the earliest optical instruments, and its development runs through the development of the optical field, which of course also includes people's research on chromatic aberration.

Chromatic aberration was common in telescopes of the 17th and early 18th centuries due to the uneven refractive properties of a single lens.

Telescope manufacturers of that era found that objects with long focal lengths had better image quality, so in the early days of telescope development, manufacturers always increased the focal length of the lens as much as possible. However, no one could clearly point out that the reason why long focal length telescopes have better image quality is to reduce chromatic aberration.

An early refracting telescope. Image source: Wikimedia

After the prism dispersion experiment in 1666, Newton discovered that white light is composed of multiple colors of light, which led him to conclude that the different refractive indices of different colors of light are the cause of chromatic aberration . This is not only a fundamental advancement in optical theory, but also provides a correct explanation for chromatic aberration.

However, Newton hastily concluded that the refraction and dispersion of all glass materials are related by the same linear function, and thus came to the erroneous conclusion that lens chromatic aberration cannot be corrected. In contrast, mirrors do not have the problem of different refractive indices for different wavelengths of light, and were considered the only way to avoid chromatic aberration at the time.

Newton's mistake led to the birth of the first reflecting telescope that could rival the performance of the refracting telescopes of the time .

A replica of the 6-inch reflecting telescope used by Newton in 1672. Image source: Wikimedia

Because of Newton's achievements and reputation in the scientific community, his erroneous conclusions hampered the further development of refracting telescopes for the next 50 years until the invention of achromatic objectives in the 18th century .

To introduce the birth of achromatic lenses, we must first introduce two types of optical glass. The earliest optical glass was divided into crown glass and flint glass according to the lead oxide content. The content below 3% was crown glass, and the content above 3% was flint glass.

Later, as the number of glass types increased, the refractive index and dispersion coefficient were used for classification. The refractive index of crown glass is usually less than 1.6, and the dispersion coefficient (also known as Abbe number, the larger the value, the smaller the dispersion) is greater than 50. The opposite is true for flint glass.

Abbe diagram of glass. Image source: Wikimedia

In 1695, David Gregory, the nephew of mathematician James Gregory , questioned Newton's theory based on the fact that there is no chromatic aberration when the human eye observes and that the structure of the human eye can be compared to the structure of a lens.

In 1729, Chester Moor Hall , a British lawyer and inventor, proposed the basic theory of achromatic doublet lens. He found that flint glass used for crafts and crown glass used for lenses have different refractive properties for light. Using crown glass as a convex lens for converging light and flint glass as a concave lens for diverging light can effectively reduce chromatic aberration within a specific wavelength range. The principle is shown in the figure below.

Hall made some of these lenses at his London optician's office, and in 1733, the first achromatic transmission telescope with a diameter of 65 mm and a focal length of 500 mm came out.

In 1750, British optician John Dollond realized the possibility of achromatic lens groups, conducted a series of experiments, and won the Copley Medal from the Royal Society of London in 1758.

Achromatic lens. Image source: Wikimedia

The use of achromatic lenses was an important advancement in the development of optical microscopes and telescopes .

Nowadays, in the lens design process of various commonly used photographic equipment, the perfection of chromatic aberration correction is an important evaluation indicator, and the chromatic aberration performance also determines the price of the lens to a certain extent.

The most common example is that some lenses say in their product introduction that they use a fluorite (CaF₂) lens design and have good chromatic aberration correction quality. This is because fluorite has a relatively low dispersion, and its other physical parameters also determine that lenses made of this material are more conducive to chromatic aberration correction. Fluorite is relatively expensive, so the price of this achromatic lens is also more expensive.

Comparison of an un-achromatic image and an image obtained using an achromatic camera lens. Image credit: Wikimedia

With the development and improvement of aberration theory, the enrichment of optical glass types and the popularization of computer-aided design technology, the design and implementation of achromatic optical systems have made great progress. Achromatic lenses can now be seen everywhere, from mobile phone lenses, camera lenses, projectors, portable telescopes to astronomical telescopes.

Currently, achromatic technology is no longer limited to achromatic lens groups. New technologies such as binary optical elements and super lenses are also showing their strengths in chromatic aberration correction.

The binary optical element works on the principle of diffraction of light. It uses computer-aided design and ultra-large-scale integrated circuit manufacturing technology to etch relief structures with different step depths on the surface of the optical element, forming a diffractive optical element with extremely high diffraction efficiency.

Different from ordinary lenses, the focal length of binary optical lenses is inversely proportional to the wavelength, and the order of the color bands obtained by dispersion is also opposite to that of lenses made of the same material. Therefore, the optical system can eliminate chromatic aberration by introducing binary optical elements.

Schematic diagram of Fresnel lens. Image source: Wikimedia

A superlens is a two-dimensional planar lens structure composed of a large number of micro units arranged in a certain way on a two-dimensional plane. It is extremely small, light, and easy to integrate. It can flexibly control the various properties of the incident light beam, thereby achieving the purpose of achromatism.

Figure Schematic diagram of superlens (Opt. Express 28, 26041-26055 (2020))

Although the development of chromatic aberration theory has experienced many twists and turns, thanks to its development, today’s images for general needs are no longer troubled by chromatic aberration, and the images we capture are also richer and more realistic.

After reading the above content, do you understand color difference?

Author: Meng Qingyu, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences; Qi Yunsheng, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Master's degree student

Reviewer: Jiao Shuming Pengcheng Laboratory

Source: Light Science Forum/"China Optics" WeChat Official Account