The Human Eye & the Colourful World
Explore how the eye forms images, how vision defects are corrected, and how light creates the colours and optical effects we see around us.
What will we explore?
The human eye
Parts of the eye, image formation on the retina and the power of accommodation.
Defects of vision
Myopia, hypermetropia and presbyopia, along with their corrective lenses.
Prism and colours
Refraction through a prism, dispersion, recombination and rainbow formation.
Atmosphere and light
Atmospheric refraction, twinkling of stars, Tyndall effect and the blue sky.
The eye works like a camera
Light rays from an object enter the eye. The eye lens focuses them to form an inverted real image on the retina.
The retina contains light-sensitive cells. They generate electrical signals that travel through the optic nerve to the brain, where the signals are interpreted.
Main parts of the human eye
Cornea
Transparent front membrane; most refraction occurs at its outer surface.
Iris and pupil
The iris controls pupil size; the pupil regulates the amount of light entering.
Crystalline lens
A convex lens that fine-adjusts focus for objects at different distances.
Retina and optic nerve
Light-sensitive cells generate signals; the optic nerve carries them to the brain.
How does the eye focus near and far?
The eye lens is soft and its shape is changed by the ciliary muscles. This ability to change focal length is called power of accommodation.
Lens becomes thinner
Ciliary muscles relax. The focal length increases, allowing distant objects to be seen clearly.
Lens becomes thicker
Ciliary muscles contract. The focal length decreases, allowing nearby objects to be seen clearly.
For a normal adult eye, the range of clear vision is from 25 cm to infinity.
When the eye cannot focus clearly
Refractive defects make it difficult to see objects clearly and comfortably. Spherical lenses are used to correct these defects.
Myopia
Near-sightedness
Nearby objects clear; distant objects blurred.
Hypermetropia
Far-sightedness
Distant objects clear; nearby objects blurred.
Presbyopia
Age-related decrease in accommodation; near point moves farther away.
Myopia — near-sightedness
A person with myopia can see nearby objects clearly but cannot see distant objects clearly.
Image position
For a distant object, the image is formed in front of the retina.
Causes
1. Excessive curvature of the eye lens.
2. Elongation of the eyeball.
Schematic illustration of correction
Two conditions affecting near vision
Hypermetropia — far-sightedness
Nearby objects are not seen clearly; distant objects may be seen clearly. The near point is farther than the usual 25 cm. For a nearby object, the image forms behind the retina.
Causes: Eye lens focal length too long or eyeball too small.
Presbyopia — age-related
With age, accommodation decreases. Nearby objects become difficult to see clearly and the near point gradually recedes.
Causes: Weakening of ciliary muscles and decreasing flexibility of the eye lens.
Refraction through a glass prism
A triangular glass prism has two triangular bases and three rectangular lateral surfaces.
At the first surface, light passes from air to glass and bends towards the normal. At the second surface, it passes from glass to air and bends away from the normal.
White light splits into seven colours
Dispersion is the splitting of white light into its component colours. The band of colours is called a spectrum.
Red
Deviated the least; longest wavelength among visible colours.
Violet
Deviated the most; shortest wavelength among visible colours.
Seven colours can combine to form white light
Newton’s experiment
Isaac Newton passed sunlight through a glass prism and obtained seven colours. He then used an identical inverted prism to recombine the colours, which emerged as white light.
How is a rainbow formed?
A rainbow is a natural spectrum seen after a rain shower. Tiny water droplets act as prisms: they refract and disperse sunlight, reflect it internally and refract it again as it emerges.
Position: A rainbow appears in the direction opposite to the Sun.
Air layers bend light
Atmospheric refraction is the refraction of light by the different layers of the atmosphere.
Advanced sunrise & delayed sunset
The Sun appears about two minutes before actual sunrise and remains visible for about two minutes after actual sunset. Atmospheric refraction makes the Sun appear higher than its actual position.
Stars near the horizon
Light from a star passes through air layers of increasing optical density and bends towards the normal. The star appears slightly higher than its actual position.
Flickering behind a hot surface
Hot air rises and cooler air replaces it. Changing refractive index makes light appear to come from different directions, so the object seems to flicker.
Twinkling of stars
Continuous refraction through changing atmospheric layers causes apparent changes in a star’s position and brightness.
Why do stars twinkle but planets do not?
Stars twinkle
Stars are very far away and appear as point sources. Small changes in the path of starlight through the atmosphere cause noticeable changes in apparent brightness.
Planets do not twinkle noticeably
Planets are extended sources and can be considered collections of many tiny point sources. Their individual fluctuations tend to average out, so the overall intensity remains relatively stable.
Why is the clear sky blue?
Fine molecules of air and tiny particles scatter shorter wavelengths of visible light, such as blue, more effectively than longer wavelengths, such as red.
As sunlight passes through the atmosphere, blue light is scattered more strongly. This scattered blue light makes the sky appear blue.
Tyndall effect
Scattering of light by colloidal particles.
Fine particles
Mainly scatter shorter wavelengths such as blue.
Very large particles
May scatter light of longer wavelengths; scattered light may appear white.
Chapter at a glance
| Concept | Key point | Remember |
|---|---|---|
| Accommodation | Eye lens changes focal length | Near point: 25 cm; far point: infinity |
| Myopia | Image forms in front of retina | Concave lens |
| Hypermetropia | Image forms behind retina | Convex lens |
| Presbyopia | Accommodation decreases with age | Convex or bifocal lens |
| Dispersion | White light splits into colours | VIBGYOR; red least, violet most deviated |
| Rainbow | Refraction, dispersion and internal reflection in droplets | Opposite direction to the Sun |
| Twinkling | Continuous atmospheric refraction | Stars twinkle; planets do not noticeably twinkle |
| Blue sky | Shorter wavelengths scatter more strongly | Blue light is scattered more than red |
Quick self-check: What corrects myopia?
Answer: A concave lens of suitable power.
Quick self-check: What is the Tyndall effect?
Answer: Scattering of light by colloidal particles.
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