The Human Eye and the Colourful World

The One Thing

Clear vision depends on light being focused precisely on the retina; suitable lenses correct defects in this focusing process. Refraction, dispersion, internal reflection and scattering explain the colourful optical phenomena observed in nature.

Reactions, Processes and Experiments

Key Terms

• Cornea: The transparent, curved front surface of the eye that provides most of its light-bending power.
• Iris: The coloured muscular part of the eye that controls the size of the pupil.
• Pupil: The opening in the iris through which light enters the eye.
• Eye lens: A flexible, transparent lens that changes shape to focus images on the retina.
• Ciliary muscles: Muscles that change the curvature and focal length of the eye lens during accommodation.
• Retina: A light-sensitive layer at the back of the eye where the image is formed.
• Optic nerve: The nerve that carries visual signals from the retina to the brain.
• Accommodation: The ability of the eye lens to change its focal length to focus on objects at different distances.
• Near point: The minimum distance at which a normal eye can see an object clearly; for a normal adult eye it is about 25 cm.
• Far point: The greatest distance at which a normal eye can see clearly; for a normal eye it is effectively infinity.
• Myopia: Short-sightedness, in which nearby objects are clear but distant objects appear blurred; it is corrected using a concave lens.
• Hypermetropia: Long-sightedness, in which distant objects are clear but nearby objects appear blurred; it is corrected using a convex lens.
• Presbyopia: An age-related loss of accommodation caused mainly by weakening ciliary muscles and reduced flexibility of the eye lens.
• Cataract: The clouding of the eye lens, which can reduce vision and may require medical treatment.
• Prism: A transparent refracting medium with two triangular plane surfaces inclined at an angle.
• Refraction: The bending of light when it passes obliquely from one transparent medium to another because its speed changes.
• Angle of deviation: The angle between the original direction of an incident ray and the direction of the emergent ray after passing through a prism.
• Dispersion: The splitting of white light into its constituent colours when it passes through a prism.
• Spectrum: The band of colours produced when white light is dispersed, arranged from red to violet.
• Recombination of colours: The process of combining separated colours to produce white light again.
• Scattering of light: The redirection of light in different directions by small particles in a medium.
• Tyndall effect: The scattering of light by colloidal or very small suspended particles, making the path of a beam visible.
• Atmospheric refraction: The bending of light as it travels through layers of air with different densities.
• Rainbow: A natural spectrum formed when sunlight undergoes refraction, dispersion, and internal reflection inside raindrops.

Easily Confused

• Refraction and dispersion: Refraction changes the direction of light, whereas dispersion separates white light into colours because different wavelengths are refracted by different amounts.
• Myopia and hypermetropia: Myopia makes distant objects appear blurred and is corrected with a concave lens; hypermetropia makes nearby objects appear blurred and is corrected with a convex lens.
• Accommodation and correction of vision defects: Accommodation is the eye’s natural adjustment of focal length, whereas corrective lenses externally alter the path of light to focus the image on the retina.
• Near point and far point: The near point is the minimum clear-viewing distance, approximately 25 cm for a normal adult eye; the far point is the greatest clear-viewing distance, effectively infinity for a normal eye.
• Scattering and the Tyndall effect: Scattering is the general redirection of light by small particles, whereas the Tyndall effect specifically makes a beam’s path visible through scattering by colloidal or very small suspended particles.
• Stars and planets: Stars appear to twinkle because atmospheric refraction varies through moving air layers; planets generally do not twinkle noticeably because they are extended sources.
• Dispersion and recombination: Dispersion separates white light into colours, whereas recombination combines the separated colours to produce white light again.
• Lens power and focal length: Lens power is given by P = 1/f, with focal length measured in metres; a convex lens has positive power and a concave lens has negative power.

What Gets Asked

• Describe image formation by the human eye: state that the image formed on the retina is real, inverted and smaller, and explain that the brain interprets it correctly. Omitting one of these image characteristics costs marks.
• Use lens equations and conventions: apply P = 1/f, P = P1 + P2, 1/f = 1/v - 1/u, or m = h_i/h_o = v/u as appropriate. A common error is failing to use focal length in metres for power or failing to apply the Cartesian sign convention.
• Identify and correct vision defects: distinguish myopia, hypermetropia and presbyopia by the objects that appear blurred and the corrective lens used. Reversing the concave-lens correction for myopia and convex-lens correction for hypermetropia loses marks.
• Explain dispersion through a prism: state that white light separates because different colours have different refractive indices in glass, with violet deviating most and red least. The order VIBGYOR must not be confused or reversed.
• Explain atmospheric colour phenomena: connect the blue sky and reddish sunrise or sunset specifically to wavelength-dependent scattering and atmospheric path length. Atmospheric refraction, rather than scattering, must be used for the apparent advance of sunrise, delay of sunset and twinkling of stars.
• Describe rainbow formation: include refraction, dispersion, internal reflection and refraction again as sunlight enters, reflects inside and leaves raindrops. Listing dispersion alone is incomplete.