10.4. What you have learnt

The ability of the eye to focus on both near and distant objects, by adjusting its focal length, is called the accommodation of the eye.

Accommodation is the process by which our eye can adjust its lens to focus on objects at varying distances. When we look at distant objects, the ciliary muscles relax, making the lens thinner and consequently increasing its focal length. For nearby objects, the ciliary muscles contract, which makes the lens thicker and reduces the focal length. This adjustment is what allows us to see clearly at different distances.

Think of the eye like a camera. Just as a camera lens can zoom in and out to focus on close or far subjects, our eye lenses change shape to achieve the same effect. For instance, when you are reading a book, you squint a little, which is your eye's way of adjusting to see the text clearly.

The smallest distance, at which the eye can see objects clearly without strain, is called the near point of the eye or the least distance of distinct vision. For a young adult with normal vision, it is about 25 cm.

The near point is the shortest distance at which our eyes can comfortably focus on an object without experiencing strain or blur. For someone with normal eyesight, this distance is typically around 25 centimeters. If an object is closer than this distance, we may struggle to see it clearly, leading to fatigue or discomfort.

Imagine holding a book too close to your face. If it's less than about 25 cm away, the words may start to look fuzzy, just like if you were trying to read a sign while running. Our eyes need a certain distance to work effectively, similar to how our smartphone screens can appear blurry if we hold them too close.

The common refractive defects of vision include myopia, hypermetropia and presbyopia. Myopia (short-sightedness – the image of distant objects is focussed before the retina) is corrected by using a concave lens of suitable power. Hypermetropia (far-sightedness – the image of nearby objects is focussed beyond the retina) is corrected by using a convex lens of suitable power. The eye loses its power of accommodation at old age.

Myopia, or short-sightedness, occurs when distant objects appear blurred because light rays focus in front of the retina. This can be corrected with a concave lens, which spreads light out and helps focus the image correctly on the retina. Hypermetropia, or far-sightedness, is the opposite condition where nearby objects appear blurry because light focuses behind the retina. This is corrected with a convex lens that converges light rays to focus them properly. Presbyopia is an age-related condition where the lens loses flexibility, affecting nearby vision, thus requiring reading glasses.

Imagine trying to take a picture with a camera that can't focus properly. If you're trying to photograph someone far away, and everything looks blurry, that's like myopia. You need a special lens, like corrective glasses, much like using a zoom lens on a camera to get that clear shot. Now, think of a camera that can't zoom in well anymore as you get older – that's what presbyopia is like.

The splitting of white light into its component colours is called dispersion.

Dispersion occurs when white light passes through a prism, causing the light to spread out into its individual colors, creating a spectrum. This happens because different colors of light bend at different angles due to their varying wavelengths. Violet light bends the most, while red light bends the least, leading to a beautiful range of colors displayed in the spectrum, often remembered by the acronym VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).

You can think of dispersion like a pack of crayons. When you draw with many colors and then spread the pigments on a white piece of paper, you see how each color stands out distinctly. Similarly, when light passes through rain or a prism, each color is revealed, creating a lovely display that reminds us of a rainbow.

Scattering of light causes the blue colour of sky.

The scattering of light occurs when sunlight interacts with the tiny particles in the atmosphere. Shorter wavelengths of light (blue) scatter more than longer wavelengths (red). This is why we perceive the sky as blue during the day; the blue light from the sun scatters in all directions and is what reaches our eyes. In contrast, during sunrise and sunset, the sunlight travels through a thicker layer of atmosphere, scattering the shorter wavelengths more and allowing the longer red wavelengths to dominate.

Picture a glass of water with a slight fog. When you shine a flashlight through it, the light scatters, revealing rainbow colors. The same happens in the sky; the atmosphere acts like that glass of water, scattering the sunlight and revealing beautiful colors, especially when it's low on the horizon during sunrise and sunset.

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