The human eye is one of the most remarkable and complex organs in our body. It acts like a camera, forming images of the world around us, which are then interpreted by the brain. The study of the human eye and the phenomena of light in nature, such as rainbows, the blue colour of the sky, twinkling of stars and the formation of a spectrum by a prism, comes under this chapter. These phenomena are explained by the concepts of refraction, dispersion and scattering of light.
The eye uses a convex lens to focus light on the retina. Just as a camera adjusts its aperture and focal length, the eye uses the iris to control light and the ciliary muscles to change the focal length of its lens. This ability to adjust is called accommodation. However, the eye has limitations and defects, which we have already studied: myopia, hypermetropia, presbyopia and cataract.
The colourful world around us, including the rainbow, the blue sky and the red sun at sunset, is produced by the interactions of sunlight with air, water droplets and atmospheric particles. In this chapter, we will study the structure of the human eye, the power of accommodation, defects of vision and their correction, the refraction of light through a prism, dispersion of white light, the formation of a rainbow, atmospheric refraction, and the scattering of light and its effects.
The main parts of the human eye are:
The eye lens forms a real, inverted image on the retina. The brain interprets this image as upright. The iris changes the size of the pupil: in bright light the pupil contracts to reduce the amount of light entering, and in dim light it expands to allow more light in.
The ability of the eye lens to change its focal length and focus objects at different distances is called accommodation. The ciliary muscles change the curvature of the lens. When we look at a distant object, the lens becomes thinner and its focal length increases; when we look at a nearby object, the lens becomes thicker and its focal length decreases. The maximum power of accommodation of a normal eye is about 4 dioptres, allowing it to focus objects from infinity to about 25 cm (the least distance of distinct vision).
In myopia, a person can see nearby objects clearly but distant objects appear blurred, because the image is formed in front of the retina. This happens when the eyeball is too long or the eye lens is too convex (focal length too short). Myopia is corrected by using a concave lens of suitable power, which diverges the rays so that they meet on the retina.
In hypermetropia, a person can see distant objects clearly but nearby objects appear blurred, because the image is formed behind the retina. This happens when the eyeball is too short or the lens is too flat (focal length too long). Hypermetropia is corrected by using a convex lens, which converges the rays before they enter the eye.
In presbyopia, which occurs in old age, the power of accommodation of the eye decreases because the ciliary muscles become weak or the lens loses its flexibility. A person with presbyopia has difficulty seeing nearby objects. It is corrected with bifocal lenses, which have an upper part for distance vision and a lower part for near vision.
In cataract, the eye lens becomes cloudy, reducing vision. It is treated by surgical removal of the cloudy lens and its replacement with an artificial lens.
A prism is a transparent refracting medium bounded by two plane surfaces inclined at an angle called the angle of the prism. When white light passes through a prism, it splits into its constituent colours, forming a band called the spectrum. This phenomenon is called dispersion of light.
The band of colours obtained is in the order: violet, indigo, blue, green, yellow, orange and red (VIBGYOR). Violet deviates the most and red deviates the least. Dispersion occurs because different colours of light have different speeds in glass and hence different angles of deviation.
The angle of deviation (D) depends on the angle of incidence and the angle of the prism. For a prism of angle A, the minimum deviation is related by the prism formula. When white light enters the prism, each colour refracts through a slightly different angle, and this produces the spectrum.
A rainbow is a natural spectrum of sunlight formed in the sky after rain. When sunlight falls on raindrops, each drop acts like a tiny prism. The light entering a raindrop undergoes refraction at the surface, total internal reflection inside the drop, and again refraction when it emerges. These three processes together produce dispersion, and the observer sees a circular arc of colours. The rainbow is seen when the sun is behind the observer and the rain is in front.
When light travels from a denser to a rarer medium, it bends away from the normal. When the angle of incidence exceeds the critical angle, the light is reflected back entirely into the denser medium. This is called total internal reflection. It is the basis of optical fibres and explains why a diamond sparkles and why the sun's rays reflect inside raindrops to form a rainbow.
Atmospheric refraction is the refraction of light as it passes through the atmosphere, whose density varies at different heights. The atmosphere becomes less dense with increasing height. Therefore, light entering the atmosphere is gradually refracted.
When light passes through a medium containing particles of size comparable to the wavelength of light, the light is absorbed and re-emitted in different directions. This is called scattering of light. The amount of scattering depends on the size of the particles and the wavelength of light. Shorter wavelengths (blue) are scattered more than longer wavelengths (red).
The scattering of light by colloidal particles is called the Tyndall effect. It is why a beam of light is visible in a dusty room or when headlights pass through fog. The Tyndall effect can be observed when a beam of light passes through a colloid like milk or a chalk solution.
The sky appears blue because the air molecules and fine particles scatter the shorter (blue) wavelengths of sunlight much more than the longer (red) wavelengths. The blue light, being scattered in all directions, reaches our eyes from all parts of the sky.
At sunrise and sunset, the sun is near the horizon, and sunlight has to travel a much longer distance through the atmosphere. Most of the blue light is scattered away from the line of sight, while red light, which is scattered the least, reaches our eyes. Hence the sun and the sky around it appear red at sunrise and sunset.
Red light has the longest wavelength and is scattered the least, so it can travel the farthest through fog and smoke. This is why red is used in danger signals and traffic lights.
| Defect | Cause | Image location | Correcting lens |
|---|---|---|---|
| Myopia | Eyeball too long / lens too convex | In front of retina | Concave lens |
| Hypermetropia | Eyeball too short / lens too flat | Behind retina | Convex lens |
| Presbyopia | Weakening of ciliary muscles with age | Behind retina for near objects | Bifocal lenses |
| Cataract | Clouding of the eye lens | No clear image | Surgery |
| Phenomenon | Cause |
|---|---|
| Rainbow | Dispersion and total internal reflection in raindrops |
| Blue colour of the sky | Scattering of blue light by air molecules |
| Red sun at sunrise/sunset | Longer path of sunlight, scattering of blue, red least scattered |
| Twinkling of stars | Atmospheric refraction |
| Tyndall effect | Scattering of light by colloidal particles |
| Red used in danger signals | Red is scattered the least, travels farthest |
The human eye, with its remarkable ability to accommodate, is a gift of nature that works on the same principles as a camera. Its defects, though common, can all be corrected with appropriately chosen lenses. Beyond the eye, the same phenomena of refraction and dispersion of light create the spectacular colourful world around us. The prism shows that white light is a mixture of seven colours, and raindrops turn this discovery into a rainbow. Atmospheric refraction explains the twinkling of stars and the apparent early sunrise and late sunset, while the scattering of light explains the blue sky, the red sunset and the choice of red for danger signals. Each phenomenon in this chapter connects the physics of light to everyday observation, making it one of the most interesting and rewarding chapters of the science syllabus.