The Human Eye and the Colourful World
Accommodation, defects of vision, prisms, dispersion, atmospheric refraction and scattering.
Based on NCERT Class 10 Science, Chapter 10
10.1 The human eye
- Light enters through the cornea, passes the pupil (its size is controlled by the iris) and is focused by the eye lens on the light-sensitive retina.
- The retina's rods and cones send signals through the optic nerve to the brain.
- The ciliary muscles change the curvature — and therefore the focal length — of the eye lens. This ability is called the power of accommodation.
- The least distance of distinct vision for a normal young eye is about 25 cm and the far point is infinity.
Near point ≈ 25 cm Far point = infinityQ1.What is the power of accommodation of the eye? What is the near point of a normal eye?
2 marks10.2 Defects of vision and their correction
- Myopia (near-sightedness): the image forms in front of the retina; distant objects look blurred. Corrected with a concave lens of suitable power.
- Hypermetropia (far-sightedness): the image forms behind the retina; near objects look blurred. Corrected with a convex lens.
- Presbyopia: with age the ciliary muscles weaken and the near point recedes; bifocal lenses (concave upper, convex lower) are used.
Correcting power: P = 1/f, with f in metres (negative for myopia)Watch it happen — Eye Defect Corrector
Controls
Live result
A concave lens shifts the image of a distant object to the defective far point.
Q1.A person cannot see objects beyond 1.5 m clearly. Name the defect and find the power of the corrective lens.
3 marks10.3–10.4 Prism, dispersion and the rainbow
- A ray passing through a glass prism bends towards the base and the total bending is measured by the angle of deviation.
- White light splits into its seven colours (VIBGYOR) on passing through a prism because each colour bends by a different amount — red least, violet most. This splitting is dispersion.
- Newton showed that a second inverted prism recombines the spectrum back into white light.
- A rainbow forms when sunlight is refracted, internally reflected and refracted again by raindrops acting as tiny prisms.
Watch it happen — Prism Dispersion Lab
Controls
Live result
Violet has the largest refractive index, so it deviates the most — that's why it lies at the bottom of a spectrum.
Q1.What is dispersion of white light? Why do different colours bend differently in a prism?
2 marks10.5 Atmospheric refraction
- Air layers of different density refract light continuously, which makes stars twinkle — the apparent position of a star keeps changing slightly.
- Planets do not twinkle because they are extended sources: the variations from all their points average out.
- The Sun is visible about 2 minutes before actual sunrise and 2 minutes after sunset because of atmospheric refraction; the Sun also appears flattened near the horizon.
10.6 Scattering of light
- The Tyndall effect is the scattering of light by colloidal particles, which makes the path of a beam visible in smoke or a fine mist.
- Fine particles scatter shorter (blue) wavelengths more strongly, so the clear sky looks blue; on the Moon there is no atmosphere, so the sky looks black.
- At sunrise and sunset light travels through much more air, most of the blue is scattered away and the Sun appears reddish. Danger signals are red because red scatters least and travels furthest.
Watch it happen — Tyndall & Sky Colour
Controls
Live result
Blue light scatters ~5.5× more than red, so the sky looks blue overhead and red at sunset.
Q1.Why is the sky blue and why does the sun appear reddish at sunrise and sunset?
3 marks