The Human Eye and the Colourful World

The eye works like a camera: the cornea (transparent front bulge) does most of the bending of light; the eye lens fine-tunes the focus to form a real, inverted image on the retina.

The iris (coloured part) controls the size of the pupil, which controls how much light enters. The retina's light-sensitive cells send signals to the brain through the optic nerve.

Power of accommodation: ciliary muscles change the eye lens's thickness — relaxed muscles → thin lens (long focal length) for distant objects; contracted muscles → thick lens (short focal length) for nearby objects.

Normal eye: near point (least distance of distinct vision) = 25 cm; far point = infinity.

Myopia (near-sightedness): distant objects blurred; image forms IN FRONT of the retina (eyeball too long or lens too curved). Corrected with a CONCAVE lens whose focal length equals the person's far point distance.

Hypermetropia (far-sightedness): nearby objects blurred; image would form BEHIND the retina (eyeball too short or lens focal length too long). Corrected with a CONVEX lens of suitable power.

Presbyopia: with age, ciliary muscles weaken and the lens loses flexibility — the near point moves away. Corrected with a convex (reading) lens. Bifocal lenses are for people with BOTH myopia and hypermetropia: upper concave part for distance, lower convex part for reading.

Cataract: the eye lens becomes cloudy/milky, causing loss of vision. It is treated by surgery (replacing the lens).

A prism deviates light towards its base at both refracting surfaces. The angle between the incident ray's original direction and the emergent ray is the angle of deviation.

Dispersion: white light splits into its seven colours (VIBGYOR) in a prism because each colour bends by a different amount — violet deviates the most, red the least. The coloured band is the spectrum.

Newton showed the reverse: a second, inverted prism recombines the spectrum into white light — proving white light is a mixture of seven colours.

Rainbow: raindrops act as tiny prisms — sunlight is refracted and dispersed on entering, internally reflected, and refracted again on leaving each drop. Always seen opposite the sun.

Atmospheric refraction: light bends continuously through air layers of changing density. Effects: stars twinkle (their apparent position keeps shifting); a star near the horizon appears slightly HIGHER than its actual position (light bends towards the normal in denser air); the sun is visible about 2 minutes before actual sunrise and after actual sunset (day appears ~4 minutes longer); and the sun's disc looks slightly flattened at sunrise and sunset.

Planets do NOT twinkle: they are much closer, so they act as extended sources — the tiny shifts from many points average out.

Scattering: air molecules scatter shorter wavelengths (blue) much more than red → the sky is blue. Without an atmosphere there is no scattering — so the sky appears DARK to astronauts and at very high altitudes. Danger signals are red because red scatters least and stays visible from far away or in fog. (The reddish sun at sunrise/sunset is the same physics, but it is excluded from the current CBSE syllabus — enjoy it as enrichment.)

Power of a lens

P = 1/f (f in metres)

Unit: dioptre (D). Convex/converging: positive. Concave/diverging: negative.

Myopia correction

f = -(far point distance)

The concave lens forms a virtual image of a distant object at the person's far point. P = -1/x, with the far point x in metres.

Hypermetropia correction

1/f = 1/v - 1/u

Take u = -25 cm (normal near point) and v = -(defective near point); the convex lens creates a virtual image at the defective near point.

Normal vision range

25 cm (near point) to infinity (far point)

25 cm is the least distance of distinct vision for a normal young adult eye.

Prism deviation

angle of deviation = angle between incident direction and emergent ray

No numerical formula at Class 10 level. Violet deviates most, red least — this difference causes dispersion.

Scattering rule of thumb

shorter wavelength → scattered more

Blue scatters far more than red — blue sky by day; red sun when the light path is long (sunrise/sunset).