Human Eye & Colourful World — 50 Q&A

• Cornea: transparent front surface — refracts light.
• Iris & Pupil: control light amount (pupil size changes).
• Lens: focuses light on retina by changing shape (accommodation).
• Retina: light-sensitive layer (photoreceptors — rods & cones) that forms image and sends signals via optic nerve.
• Aqueous & Vitreous humour: fluids maintaining eye shape and optical properties.

Rods: sensitive to low light, do not detect colour, concentrated at peripheral retina. Cones: less sensitive, responsible for colour vision and high acuity, concentrated in fovea (central retina).

Image on retina is real and inverted. Brain interprets the signals and perceives the object upright — perception corrects inversion during visual processing.

Accommodation is change in focal length of the eye's lens by action of ciliary muscles to focus on near and distant objects (lens becomes thicker for near objects).

Near point: closest distance at which the eye can see clearly (≈25 cm for normal young eye). Far point: farthest distance of clear vision (≈infinity for normal eye).

Myopia: eye focuses distant objects in front of retina due to elongated eyeball or too strong refractive power. Corrected using concave (diverging) lenses to move image onto retina.

Hypermetropia: eye focuses near objects behind the retina due to shortened eyeball or weak refractive power. Corrected with convex (converging) lenses.

Presbyopia: age-related loss of accommodation due to decreased lens elasticity; near point moves away. Managed by corrective reading glasses (convex lenses) or multifocal lenses.

Astigmatism: irregular curvature of cornea/lens causing blurred vision. Corrected by cylindrical lenses that compensate for uneven focusing.

Power P = 1/f (f in meters). Unit: dioptre (D). Convex lens positive power, concave negative. Spectacle powers are given in dioptres on prescriptions.

Polarised sunglasses block horizontally polarised glare reflected from surfaces (roads, water) using a polarising filter — reducing glare and improving contrast.

Objects show colour by absorbing certain wavelengths and reflecting/transmitting others. Cones in retina are sensitive to different wavelength ranges, enabling colour perception by brain.

Dispersion: splitting of white light into colours because refractive index of glass varies with wavelength (shorter wavelengths refract more). Order: red (least bent) → violet (most bent).

Rayleigh scattering: air molecules scatter shorter wavelengths (blue/violet) more than longer (red). Our eyes are more sensitive to blue, so sky looks blue (violet largely absorbed/scattered less to eye).

At sunrise/sunset sunlight travels a longer path through atmosphere — most blue/green light is scattered out, leaving longer wavelengths (red/orange) to reach observer.

Tyndall effect: scattering of light by colloidal particles making a beam visible (e.g., dust/smoke in sunbeam, fog, milk in water). Particle sizes comparable to wavelength cause scattering.

Sunlight refracts into raindrop → disperses into colours → reflects internally → refracts again on exit. Each colour emerges at a specific angle producing circular spectrum — observer sees primary rainbow (single internal reflection).

Cloud droplets are large compared to wavelength; they scatter all visible wavelengths almost equally (Mie scattering), resulting in white appearance. When droplets grow heavy, multiple scattering + shadowing can make clouds appear grey.

Blue eyes are due to structural scattering of light in the stroma of the iris and low melanin; short wavelengths scatter more giving blue appearance (similar to sky scattering), whereas brown eyes have more melanin absorbing more light.

An object's apparent colour depends on which wavelengths are available in the illuminating light and which wavelengths the object reflects. A red object under green light appears black because there is no red wavelength to reflect (additive colour mixing concept).

Chlorophyll absorbs red & blue light and reflects green — leaves appear green. In autumn chlorophyll breaks down, revealing other pigments (carotenoids, anthocyanins) showing yellow/orange/red colours.

Glossy surfaces produce specular (regular) reflection — smooth microstructure reflects light coherently. Matte surfaces scatter light diffusely due to rough microstructure, reducing specular highlights.

Complementary colours are pairs that combine to neutral (white/grey) in additive mixing (e.g., red & cyan, green & magenta, blue & yellow in different contexts). In RGB displays additive; in printing CMYK uses subtractive mixing.

Mirages occur due to refractive index gradients in hot air near ground; light rays bend and may reflect internally producing illusion of water or inverted images (e.g., road appearing wet).

Red sky indicates scattering of blue light; at evening red sky often means clear air to west (good weather), morning red may indicate dust/aerosols and approaching storms — a traditional weather heuristic linked to sunlight direction & atmospheric conditions.

Moonlight is reflected sunlight, but space lacks atmosphere to scatter light; the surface reflects broadly so Moon appears pale/greyish; atmospheric effects near horizon can redden moonlight similar to sunset effects.

Lasers are coherent, monochromatic and can be focused precisely (used in LASIK, retinal photocoagulation). They can damage retina if misused — strict safety & eye protection required.

Yes. Many animals have different photoreceptor ranges (e.g., bees can see ultraviolet patterns on flowers guiding pollination). Colour perception depends on receptor types and neural processing.

Metals have free electrons that respond to incident electromagnetic waves and re-emit them coherently, producing strong specular reflection and shiny appearance.

Prolonged stimulation bleaches certain cone photopigments. When looking away, the less-fatigued cones respond relatively more, producing an afterimage of complementary colour.

Red has long wavelength, is visible from far and penetrates fog/haze relatively well; combined with cultural convention and high contrast, it's effective for attention and safety.

Find far point (closest point seen clearly without glasses). Required power ≈ −1/(far point in meters). E.g., far point 0.5 m → power ≈ −2.0 D.

UV can damage skin (sunburn, cancer) and eyes (photokeratitis, cataract). Protection: sunscreen, sunglasses blocking UV, protective clothing, limiting direct sun exposure.

Microscopes: objective forms enlarged real image of tiny object; eyepiece magnifies it further. Telescopes: objective gathers light and forms image of distant object; eyepiece provides angular magnification to the eye.

Resolution limited by diffraction (≈ wavelength) and numerical aperture of lenses; shorter wavelengths & higher NA give better resolving power.

Twinkling (scintillation) is due to atmospheric turbulence causing refractive index variations. Stars are point sources — small angular size so light fluctuates. Planets have a finite angular size averaging out fluctuations, so less twinkle.

Black surfaces absorb most incident wavelengths (low reflectance) and convert energy to heat; lighter surfaces reflect more and absorb less, heating less under same illumination.

Illusions exploit how visual system interprets cues (perspective, shading, colour contrast). Brain applies assumptions and prior knowledge that can be led astray by ambiguous/contradictory signals.

Often genetic (X-linked) defects in cone photopigments (red/green), leading to difficulty distinguishing certain colours. Severity and type vary by which cones are affected.

During twilight, long path through atmosphere causes scattering of shorter wavelengths and multiple scattering; ozone absorption and aerosols can add purplish/red hues — complex interplay of scattering & absorption.

Many animals have a tapetum lucidum — a reflective layer behind retina that reflects unabsorbed light back through photoreceptors, enhancing sensitivity in low light and causing visible eye-shine when illuminated.

Rod photoreceptors (scotopic vision) are less sensitive to red light; using red illumination preserves dark adaptation because it stimulates cones more than rods, reducing loss of night sensitivity.

Colour temperature (in Kelvins) describes spectrum: lower (≈2700K) appears warm (yellowish), higher (≈5000–6500K) appears cool/blueish. Affects mood, colour rendering and visibility in different tasks.

• Follow 20-20-20 rule (every 20 min look 20 ft away for 20 sec).
• Proper lighting to reduce glare, adjust screen brightness, use proper ergonomics and regular breaks.
• Use anti-reflective coatings, appropriate font sizes and consult eye doctor if persistent.

Mach bands: perception of bands of increased/decreased brightness at edges due to lateral inhibition in retina — enhances edge detection but creates illusory bands.

White balance adjusts camera processing to the colour temperature of light so that white objects appear neutral; it compensates for warm/cool lighting to render colours accurately.

• Power of lens: P = 1/f (f in meters, unit dioptre).
• Near point ≈ 25 cm (normal young eye).
• Rayleigh scattering ∝ 1/λ^4 — shorter wavelengths scattered more.
• Dispersion: wavelength-dependent refractive index → colours split.

• Eye: optics + photoreceptors — lens focuses image on retina; rods for dim light, cones for colour.
• Common defects: myopia, hypermetropia, presbyopia — corrected by lenses.
• Colour world arises from wavelength-dependent interactions — dispersion (prism, rainbows), scattering (blue sky), Tyndall effect (colloids).
• Practical: sunglasses, spectacles, optical instruments and safety around lasers/UV.