The Human Eye and the Colourful World - Study Notes
10.1 The Human Eye
The human eye is a highly valuable and sensitive sense organ that enables vision of the colourful world.
When eyes are closed, identification of colours is not possible, highlighting the eye’s role in colour perception.
The eye functions like a camera: a lens system forms an image on a light-sensitive screen called the retina.
Light enters through the cornea, the transparent bulge on the front surface of the eyeball; most refraction occurs at the cornea.
The crystalline lens provides fine adjustment of focal length to focus objects at different distances on the retina.
Behind the cornea lies the iris, a dark muscular diaphragm that controls the size of the pupil; the pupil regulates the amount of light entering the eye.
The eye lens forms an inverted real image on the retina.
The retina contains enormous numbers of light-sensitive cells; these cells activate under illumination and generate electrical signals sent to the brain via the optic nerves.
The brain interprets these signals to perceive objects as they are.
The eyeball is approximately spherical with a diameter of about .
Most refraction occurs at the cornea; the lens mainly adjusts focus for different distances.
The image formed on the retina is inverted relative to the object.
10.1.1 Power of Accommodation
The eye lens is a fibrous, jelly-like material whose curvature can be changed by the ciliary muscles.
Change in curvature changes focal length: finer adjustment enables focusing on objects at varying distances.
When ciliary muscles relax: lens becomes thinner, focal length increases, enabling distant vision.
When looking at near objects: ciliary muscles contract, lens becomes thicker, focal length decreases, enabling near vision.
The ability to adjust focal length is called accommodation.
The minimum focal length reduction is limited; reading very close may blur the image and strain the eye.
For comfortable and distinct viewing of ordinary text, hold the object at about from the eye.
The least distance of distinct vision (near point) is the minimum near-distance where we can see clearly without strain; for a young adult with normal vision, near point ≈ .
The farthest point up to which the eye can see clearly is the far point; for a normal eye, it is (infinity).
A normal eye can see objects clearly between and
Cataract: with ageing, the crystalline lens may become milky and cloudy, causing partial or complete vision loss; cataract surgery can restore vision.
10.2 Defects of Vision and Their Correction
Vision defects arise when accommodation power decreases or refractive errors prevent sharp focus on the retina.
Three main refractive defects:-
(i) Myopia (near-sightedness)
(ii) Hypermetropia (farsightedness)
(iii) Presbyopia (age-related decline in accommodation)
All three can be corrected with appropriate spherical lenses; sometimes contact lenses or surgical interventions are used.
(a) Myopia (Near-sightedness)
A myopic person can see nearby objects clearly but cannot see distant objects distinctly.
The far point is nearer than infinity; distant objects form images in front of the retina, not on it.
Causes: (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball.
Corrected with a concave (diverging) lens of suitable power, which moves the image back onto the retina (Fig. 10.2 (c)).
Notation: the corrective power is negative (in dioptres).
(b) Hypermetropia (Far-sightedness)
A hypermetropic person can see distant objects clearly but struggles with near objects.
Near point is farther away from the eye than the normal near point; comfortable reading requires placing material well beyond 25 cm.
Cause: (i) focal length of the eye lens is too long, or (ii) the eyeball is too small.
Corrected with a convex (converging) lens of appropriate power to help focus light on the retina.
Eye-glasses with converging lenses provide the required extra focussing power.
(c) Presbyopia
Associated with ageing; accommodation power decreases over time.
Near point recedes away, making near vision difficult without corrective lenses.
Often managed with reading spectacles; may require bi-focal lenses for both near and distant vision.
Bi-focal lenses: upper part concave for distant vision, lower part convex for near vision; can be combined with other modern corrective methods (contact lenses, surgical interventions).
10.3 Refraction of Light Through a Prism
Refraction through a triangular glass prism differs from a rectangular slab because the prism has two refracting surfaces inclined to each other.
Angle between the two lateral faces is the angle of the prism (denoted as the angle of the prism, often ).
Key measurements: angle of incidence , angle of refraction , and angle of emergence ; the lines where the rays exit define the angle of deviation .
The line where the two refracting surfaces meet marks the boundary at E (and F for the second boundary); the angle of deviation D is the angle between the incident ray and the emergent ray after passing through the prism.
10.4 Dispersion of White Light by a Glass Prism
White light from the Sun can be dispersed into a spectrum by a prism.
Observed colors in order: Violet, Indigo, Blue, Green, Yellow, Orange, Red, acronym VIBGYOR.
Spectrum: a band of colours produced when white light splits into its components.
Why dispersion occurs:-
Different colours bend by different amounts when passing through a prism.
Red light bends the least; violet bends the most.
Newton’s experiment: He tried to further split the spectrum with a second prism but found that recombination of the colours produced white light again when the same prism arrangement was used in reverse, confirming that sunlight contains the seven colours.
A rainbow forms due to dispersion and internal reflection within raindrops (Figure 10.7–10.8): sunlight refracts entering a raindrop, reflects internally, and refracts again on exit, separating into colours.
The rainbow appears opposite the Sun.
Key terms
Spectrum: band of distinct colours produced by dispersion.
Dispersion: splitting of white light into its component colours.
10.5 Atmospheric Refraction
Turbulent hot air above a flame or radiator causes apparent wavering or flickering of objects due to atmospheric refraction.
Twinkling of stars is due to atmospheric refraction: starlight bends in a medium with gradually changing refractive index, causing apparent position to shift.
Stars appear slightly higher near the horizon due to refraction; their flickering is due to continuous atmospheric change.
Planets do not twinkle because they are extended sources; the sum of many point-like sources averages out brightness fluctuations.
Advance sunrise and delayed sunset: the Sun’s apparent position is slightly above the horizon before sunrise and after sunset due to refraction, about 2 minutes earlier and later respectively.
10.6 Scattering of Light
Interaction of light with small particles in the environment leads to visible light paths and various natural phenomena.
10.6.1 Tyndall Effect
The Earth’s atmosphere contains fine particles (smoke, dust, droplets, air molecules).
A beam of light becomes visible when it scatters off these particles (diffuse reflection).
Observed in smoke-filled rooms, or sunlight through a forest canopy.
The colour of scattered light depends on particle size: very fine particles scatter blue light more; larger particles scatter longer wavelengths; very large particles may scatter white light.
10.6.2 Why is the colour of the clear sky blue?
Air molecules and fine particles are smaller than visible wavelengths and scatter shorter wavelengths more effectively (Rayleigh scattering).
Blue light (shorter wavelength) is scattered more than red, so the sky appears blue to observers on the ground.
If Earth had no atmosphere, the sky would appear dark.
At high altitudes, scattering is reduced, so the sky can appear darker.
Safety note: red light is less scattered by fog or smoke, making it more visible at distance (hence its use in danger signals).
Ethical, Social, and Practical Implications: Eye Donation
A thought-provoking point raised: eyes can live on after death through donation, lighting the lives of many blind people.
Current statistics: about 35 million people in the developing world are blind; around 4.5 million could be cured by corneal transplantation.
Of these 4.5 million, about 60% are children below age 12.
If vision is a gift, donating eyes can pass that gift to others.
Eye donors can belong to any age group or sex; wearing spectacles or having had cataract surgery does not disqualify donation.
People with diabetes, hypertension, asthma, and those without communicable diseases can also donate eyes.
Eye removal protocol: donors are to have eyes removed within after death; the eye bank team collects, evaluates, and distributes donated eyes using strict medical standards.
Unsuitable donated eyes are used for research and education; donor and recipient identities remain confidential.
One pair of donated eyes can give vision to up to four corneal blind people.
What you have learnt (Key Takeaways)
Accommodation: the eye’s ability to focus on near and distant objects by changing the focal length of the eye lens.
Near point (Least distance of distinct vision): for a young adult with normal vision, ≈ .
Far point: for a normal eye, .
Common refractive defects and corrections:-
Myopia corrected by concave lens; near vision is corrected with appropriate power (negative dioptric value).
Hypermetropia corrected by convex lens; near vision requires additional converging power.
Presbyopia relates to ageing; accommodation decreases; corrective lenses, bi-focal lenses as common remedy.
Dispersion: splitting of white light into a spectrum, with colors ordered as ; dispersion explains how rainbows form and why colors appear.
Atmospheric refraction causes twinkling of stars and affects sunrise/sunset timing.
Scattering of light by atmospheric particles gives the blue color of the sky (Rayleigh scattering) and explains why red light dominates in some foggy conditions or in distant signals.