MP Board Class 10 Science Chapter 10: The Human Eye and t…
MP Board Class 10 Science Chapter 10: The Human Eye and the Colourful World — This chapter explores the structure and working of the human eye, common vision defects and their corrections, and the fascinating phenomena of light that create colourful sights in our sky. With 6–8 marks weightage in the MP Board 2027 exam, this chapter combines biology (eye anatomy) with physics (refraction, dispersion, scattering). Mastering the ray diagrams for defects, prism dispersion, and atmospheric refraction is key to scoring full marks.
📑 Table of Contents
👁️ 1. Structure of the Human Eye
The human eye is one of the most valuable sense organs. It is like a natural camera that allows us to see the colourful world around us. The eye has a nearly spherical shape with a diameter of about 2.3 cm.
Key Parts of the Human Eye
Rods and Cones — The Photoreceptors
The retina has two types of light-sensitive cells:
- Rods: Sensitive to light intensity (brightness). Work in dim light. Cannot distinguish colours. About 120 million rods in each eye.
- Cones: Sensitive to colours. Work in bright light. Three types — red-sensitive, green-sensitive, and blue-sensitive. About 6 million cones.
Blind Spot and Yellow Spot
- Blind Spot: Point where the optic nerve leaves the eye — no rods or cones present. No image is formed here.
- Yellow Spot (Fovea): Point of highest visual acuity (sharpest vision) — maximum concentration of cones.
⚙️ 2. Working of the Eye — Accommodation and Power of Accommodation
How the Eye Sees
Light rays from an object enter the eye through the cornea, pass through the pupil (whose size is regulated by the iris), then through the crystalline lens. The lens refracts the light to focus it onto the retina. The retina sends electrical signals via the optic nerve to the brain, which interprets the image.
Power of Accommodation
The ability of the eye lens to adjust its focal length to focus objects at different distances is called the power of accommodation. The ciliary muscles contract or relax to change the curvature (and hence focal length) of the lens.
- Distant object: Ciliary muscles relax → lens becomes thin (less curved) → focal length increases → image falls on retina
- Nearby object: Ciliary muscles contract → lens becomes thick (more curved) → focal length decreases → image falls on retina
Range of Vision
🔬 3. Defects of Vision and Their Correction
With age, the power of accommodation of the eye decreases. Many people develop vision defects. The three most common defects are myopia, hypermetropia, and presbyopia.
(a) Myopia (Nearsightedness)
- Problem: Cannot see distant objects clearly; near objects are clear.
- Cause: Either the eyeball is too long (elongated), or the lens is too curved (high converging power). Image of distant object forms in front of the retina.
- Correction: Concave lens (diverging lens) of appropriate power. The concave lens diverges the incoming rays so that they converge on the retina.
- Far point: Less than infinity. Example: If far point is 1 m, the person can only see clearly up to 1 m away.
(b) Hypermetropia (Farsightedness)
- Problem: Cannot see near objects clearly; distant objects are clear.
- Cause: Either the eyeball is too short, or the lens is too flat (low converging power). Image of near objects forms behind the retina.
- Correction: Convex lens (converging lens) of appropriate power. The convex lens converges the rays before they enter the eye.
- Near point: More than 25 cm. Example: If near point is 1 m, the person cannot read a book held at 25 cm.
(c) Presbyopia
- Problem: Age-related loss of accommodation. Can’t see nearby objects clearly (common in people aged 40+).
- Cause: Ciliary muscles weaken and the lens loses elasticity over time.
- Correction: Bifocal lenses — upper part (concave) for distance, lower part (convex) for near vision. In some cases, progressive lenses are used.
(d) Cataract
- Cloudiness develops in the crystalline lens, causing blurred or dim vision.
- Treatment: Surgical removal of the cloudy lens and replacement with an artificial intraocular lens (IOL).
🔺 4. Refraction of Light Through a Prism
A prism is a transparent optical element with two triangular ends and three rectangular faces. When a light ray passes through a prism, it bends twice — once entering (from air to glass) and once exiting (from glass to air). The net effect is a deviation towards the base of the prism.
Refraction Through a Triangular Prism
- Angle of Prism (A): The angle between the two refractive surfaces.
- Angle of Incidence (i): Angle at which light enters the prism.
- Angle of Emergence (e): Angle at which light leaves the prism.
- Angle of Deviation (D): The angle between the incident ray (extended) and the emergent ray. D = (i + e) − A.
Minimum Deviation
When the angle of incidence is such that the light ray passes symmetrically through the prism (i = e), the angle of deviation is minimum (Dₘ). For this condition, the ray inside the prism is parallel to the base.
📘 Key Fact: The deviation produced by a prism depends on: (1) Angle of prism (A) — larger A → larger deviation, (2) Refractive index of glass (n), and (3) Wavelength of light — shorter wavelengths (violet) deviate more than longer wavelengths (red).
🌈 5. Dispersion of White Light by a Glass Prism
When a narrow beam of white light (sunlight) passes through a glass prism, it splits into seven colours — the colours of the VIBGYOR spectrum. This phenomenon is called dispersion of light.
The Seven Colours of the Spectrum (VIBGYOR)
- V — Violet (deviates the most, shortest wavelength ~400 nm)
- I — Indigo
- B — Blue
- G — Green
- Y — Yellow
- O — Orange
- R — Red (deviates the least, longest wavelength ~700 nm)
Why Dispersion Happens
Different colours of light have different wavelengths. The refractive index of glass is slightly different for each wavelength. Violet light (shorter λ) slows down more in glass, so it bends more. Red light (longer λ) slows down less, so it bends less. This difference in refraction causes the colours to spread out into a spectrum.
Recombination of Spectrum — Newton’s Experiment
Isaac Newton showed that if the dispersed colours are passed through an inverted prism (or a converging lens), they recombine to form white light again. This proves that white light is composed of seven colours — it is polychromatic.
Rainbow — Natural Dispersion
A rainbow is nature’s most beautiful example of dispersion. Sunlight is dispersed by millions of tiny water droplets suspended in the air after rain. Each droplet acts as a tiny prism — light enters, undergoes refraction + dispersion + total internal reflection, and exits as a spectrum. We see a circular arc of colours — red on the outer edge and violet on the inner edge.
🌅 6. Atmospheric Refraction
The Earth’s atmosphere has layers of air with different densities and refractive indices (denser near the surface, rarer at higher altitudes). When light passes through these layers, it bends continuously — this is called atmospheric refraction. It creates several fascinating natural phenomena.
(a) Twinkling of Stars
Starlight enters the Earth’s atmosphere from near-vacuum. Due to atmospheric refraction, the light bends as it travels through layers of different refractive indices. Changing air density (due to temperature variations) causes the apparent position of the star to fluctuate. This fluctuation in the path of light makes the star appear to twinkle (changes in brightness).
(b) Planets Do Not Twinkle
Planets are much closer to Earth than stars, so they appear as extended sources of light (not point sources). Light from different points on the planet’s disk undergoes different amounts of refraction. The net effect averages out, so the overall brightness remains steady — no twinkling.
(c) Advance Sunrise and Delayed Sunset
Due to atmospheric refraction, the Sun appears to rise about 2 minutes earlier than its actual time and set about 2 minutes later. The apparent flattening of the Sun’s disc at sunrise and sunset (oval shape) is also due to atmospheric refraction — light from the lower edge bends more than light from the upper edge.
(d) Apparent Bending of Objects in Desert (Mirage)
On hot days, air near the ground becomes hotter (rarer) than air above. Light from the sky bends upward (towards the denser, cooler air above). The brain interprets this bent light as if it came from the ground — creating the illusion of water (a mirage).
💡 7. Scattering of Light and Tyndall Effect
Tyndall Effect
When a beam of light passes through a colloid (like smoke, fog, or milk), the path of the light becomes visible. This is because colloidal particles scatter light in all directions. This phenomenon is called the Tyndall effect.
Why the Sky is Blue
According to Rayleigh scattering theory, the intensity of scattered light is inversely proportional to the fourth power of the wavelength (I ∝ 1/λ⁴). Blue light (λ ≈ 450 nm) has a much shorter wavelength than red light (λ ≈ 700 nm). Blue light is scattered about 5.6 times more than red light by air molecules. So when sunlight passes through the atmosphere, blue light scatters in all directions, making the sky appear blue.
Why the Sky Appears Dark in Space
There is no atmosphere in space. Without air molecules to scatter sunlight, the sky appears dark even in the daytime. The Sun still shines, but there is no scattering — this is why photos from space show a black sky with bright stars.
Why the Sun Appears Red at Sunrise and Sunset
During sunrise and sunset, sunlight travels through a much thicker layer of the atmosphere (the path length is maximum). Blue light (shorter wavelength) is scattered away completely by the time it reaches our eyes — it gets scattered multiple times in all directions. Only the red and orange light (longer wavelengths), which scatter the least, reach us directly. That is why the Sun and the surrounding sky appear reddish-orange during sunrise and sunset.
📘 Key Comparison: The Sun at noon appears white because the path length through the atmosphere is shortest — less scattering occurs, so all colours reach our eyes nearly equally.
Why Clouds are White
Clouds consist of water droplets that are much larger than the wavelength of light. These large droplets equally scatter all colours of visible light (Mie scattering — more or less wavelength-independent). The combined effect of all colours being scattered equally makes clouds appear white.
Why Are Danger Signals Red?
Red light is scattered the least by fog, haze, smoke, or dust particles. It can travel the longest distance through these media with minimal attenuation. That’s why traffic stop signals, railway signals, and warning lights use red — it is visible from the farthest distance, especially in poor weather conditions.
❓ 8. Frequently Asked Questions (FAQs)
Q1. Define power of accommodation of the eye.
The ability of the eye lens to adjust its focal length by changing its curvature (via ciliary muscles) to form a sharp image on the retina for objects at varying distances is called the power of accommodation.
Q2. What is the least distance of distinct vision for a normal human eye?
The least distance of distinct vision (near point) for a normal human eye is 25 cm. Objects placed closer than 25 cm cannot be seen clearly.
Q3. A person cannot see objects clearly beyond 2 m. Name and correct the defect.
This is myopia (nearsightedness). The far point is 2 m instead of infinity. Correction: Use a concave lens of power P = −1/f = −1/2 = −0.5 D.
Q4. Why does the sky appear dark from space?
There is no atmosphere in space to scatter sunlight. Without atmospheric scattering, the sky appears dark and the Sun appears as a bright white disc against a black background.
Q5. Why do stars twinkle but planets do not?
Stars are point sources of light at enormous distances. Atmospheric refraction causes fluctuations in their apparent position and brightness — hence they twinkle. Planets are extended sources (closer), and the light from different points on the planet averages out the fluctuations, so they appear steady.
Q6. What is dispersion of light? Give a natural example.
Dispersion is the splitting of white light into its constituent colours (spectrum) when it passes through a prism. The natural example is a rainbow, formed when sunlight is dispersed by water droplets in the atmosphere.
Q7. What is the Tyndall effect?
The Tyndall effect is the scattering of light by colloidal particles (dust, smoke, fog) suspended in a medium. The beam of light becomes visible along its path. Example: headlight beam visible in foggy weather.
Q8. Why is the colour of the Sun red at sunrise?
At sunrise, sunlight travels through the maximum thickness of the atmosphere. Blue light is scattered away completely. Only the red (longer wavelength) component reaches our eyes, making the Sun appear red.
Q9. What is the function of the iris in the human eye?
The iris controls the size of the pupil, thereby regulating the amount of light entering the eye. In bright light, the iris contracts the pupil (small opening, less light). In dim light, the iris expands the pupil (large opening, more light).
Q10. Write the formula for refractive index in terms of prism angle and minimum deviation.
n = sin[(A + Dₘ)/2] / sin(A/2), where n is the refractive index of the prism material, A is the angle of the prism, and Dₘ is the angle of minimum deviation.