Light travels at different speeds depending on the medium it passes through. For example, in a vacuum, it moves at 3.0 × 10^8 m/s in a straight line. But when directed towards glass, air, diamond, water, and other mediums, it moves slower. The refractive index is used to show the speed of light in vacuum compared to the speed of light in a particular medium. The higher the measurement, the slower the light travels through the given medium.
Lenses and refraction of light
If light enters a new medium at right angles to the surface, it changes speed but not direction. But if it moves through different media at different angles, it changes both speed and direction. The lenses work on this basis. These are glass and plastic mediums, which take flat or spherical shapes. Lenses can be convex/convergent or concave/divergent.
Convex lenses pick up parallel light rays from distant objects and bend them to converge at a single point called the focal point. The distance between the focal point and the lens is called the focal length, as shown below:
On the other hand, concave lenses bend parallel rays to spread out in such a way that they appear to come from a point in front of the lenses. This focal length is therefore expressed in negative units.
Most telescopes today, and many since the early days, relied on lenses to gather light inaccessible to the human eye. Telescopes used two lenses, the objective lens (convex) and the eyepiece (concave). Thanks to the structure of the refracting telescope, the lenses could focus light and make distant objects closer, brighter and bigger. The longer the focal length, the larger the image will be. Additionally, the light-gathering ability of the telescope affected the brightness of the image.
The effect of chromatic aberration
Although lenses have some flaws (eg glass absorbs UV light), chromatic aberration seems to take center stage when it comes to their limitations. Why is that? Lenses refract light to meet at a focal point. However, light comprises various wavelengths which refract at different speeds and directions. It is difficult to ensure that the light meets at a focal point, resulting in darker or lighter portions of the resulting image, creating blur.
Chromatic aberration results from the scattering of visible light into different wavelengths and can best be shown by passing white light through a glass prism.
For example, in the diagram above, it is clear that indigo refracts at a much higher level than green. You can expect the same when passing light through the lenses. A lens cannot focus all these wavelengths into a single point, which opens the door to chromatic aberration. As different colors of light pass through the lens, they refract at different speeds and in different directions, and the image will have colored edges. That’s why you need to combine different goals from a trusted provider optical lens supplier. You can also invest in lenses such as plano-convex and biconvex lenses, which minimize spherical aberration.
Types of chromatic aberration
The aberration can be axial (longitudinal) or lateral (transverse). Axial aberration occurs when different wavelengths scatter from the lens at different horizontal points. Stopping the aperture often solves this problem. On the other hand, lateral aberration occurs when different wavelengths focus in various positions on the same focal plane.
Resolution of chromatic aberration
Since no single lens can focus different wavelengths of light into a single point, it is necessary to use multiple convex and concave lenses made from different types of glass. It ensures that the lenses respond to all levels of dispersion and can reduce the amount of chromatic aberration. Those that correct two wavelengths are called achromatic lenses, and those that correct three are called apochromatic lenses.
Achromatic lenses include glasses of different refractive indices to create an achromatic doublet that minimizes chromatic aberration. Some systems also include low dispersion glass to counter light scattering. Apochromatic lenses feature three types of glass and are often used in complex systems where clarity is important.
While it is impossible to eliminate the problem of chromatic aberration in lenses, it is possible to minimize its effects by investing in achromatic or apochromatic lenses. It’s also important to understand how the different lenses work. For example, biconvex lenses keep spherical aberration to a minimum by canceling or minimizing distortion. The asymmetry of plano-convex lenses reduces their spherical aberration, more so when objects are at infinity. It is possible to create a system that has the least aberration to ensure the best clarity.