Class 10 Science Human Eye and Colourful World Notes by PW

The human eye is a natural optical instrument. It receives light, forms an image on the retina and sends visual signals to the brain. Questions often test the parts of the eye and their functions, myopia and hypermetropia with their correcting lenses, power of a lens, dispersion and rainbow formation, atmospheric refraction, the Tyndall effect, and the colour of the sky at noon and at sunset.

Physics Wallah offers Human Eye and Colourful World Class 10 notes to help you understand these concepts, remember the key points and solve numerical problems. Use these notes to revise the defects of vision, understand the natural phenomena and prepare effectively for your Class 10 Science board examination.

Structure of the Human Eye

The eye works like a natural camera. Light enters through the front, passes through the lens and forms an image on the retina. The main parts are:

• Cornea: the transparent outer covering. Most of the bending of light takes place here.

• Iris: the coloured muscular part that controls the size of the pupil.

• Pupil: the opening through which light enters the eye.

• Aqueous humour: the fluid in front of the lens that nourishes the eye and helps maintain its shape.

• Crystalline lens: the lens that focuses light on the retina.

• Ciliary muscles: the muscles that change the curvature of the lens, and with it, its focusing power.

• Vitreous humour: the jelly-like fluid behind the lens that keeps the eyeball in shape.

• Retina: the light-sensitive screen where the image forms.

• Optic nerve: the nerve that carries signals from the retina to the brain.

The iris makes the pupil smaller in bright light and larger in dim light. This protects the retina and lets the eye see in different lighting.

The retina has two kinds of photoreceptor cells. Rod cells respond to the intensity of light and work well in dim light. Cone cells help us see colours.

The eye lens can change its shape to focus objects at different distances. This ability is called accommodation. At one point on the retina, called the blind spot, there are no photoreceptor cells. An image falling on it is not detected.

Memory Tip: Follow the path of light as Cornea → Pupil → Lens → Vitreous Humour → Retina → Optic Nerve.

Defects of Vision and Their Correction

Myopia

Myopia is also called short-sightedness. A person with myopia sees nearby objects clearly, but distant objects look blurred. It can happen when the eye lens is too curved or thick, or when the eyeball is elongated. As a result, light from a distant object forms its image in front of the retina.

A concave lens corrects myopia. It spreads the incoming rays slightly, so the eye lens can then form the image on the retina. A concave lens has a negative focal length, so its power is negative. Power is given by:

P = 1/f (with f in metres, power in dioptres)

This follows the sign convention used in light reflection and refraction. A negative spectacle power usually points to myopia.

Hypermetropia

Hypermetropia is also called long-sightedness. A person with hypermetropia sees distant objects clearly, but nearby objects look blurred. It can happen when the eye lens is too flat, or when the eyeball is too short. The image of a nearby object then forms behind the retina.

A convex lens corrects hypermetropia. It converges light before it enters the eye, so the image shifts onto the retina. A convex lens has a positive focal length, so its power is positive.

To compare the two: in myopia, the image forms in front of the retina and a concave lens is used. In hypermetropia, the image forms behind the retina and a convex lens is used.

Numerical questions often ask for the power of a lens. For example, a concave lens of focal length 2 m has f = −2 m, so:

P = 1/(−2) = −0.5 D

Similar questions are solved step by step in the NCERT solutions for the Class 10 Science light chapter.

Memory Tip: Myopia means near is clear, and it needs a concave lens. Hypermetropia means near is blurred, and it needs a convex lens.

Presbyopia, Cataract and Astigmatism

With age, the eye lens becomes less flexible and the ciliary muscles become weaker. The eye then cannot focus on nearby objects easily. This is called presbyopia. It is corrected using bifocal lenses, where the upper part helps with distant vision and the lower part helps with near vision.

A cataract is a cloudy or whitish layer that develops on the eye lens, usually with age or after an injury. It is treated by surgery.

In astigmatism, vision becomes distorted. Straight lines may look curved, tilted or blurred. It can be corrected with a cylindrical lens or by surgery.

Power of Accommodation, Near Point and Far Point

The power of accommodation is the ability of the eye lens to focus on both near and distant objects by changing its focal length.

• Looking at a distant object: The ciliary muscles relax, the lens becomes thinner and its focal length increases.

• Looking at a nearby object: The ciliary muscles contract, the lens becomes thicker and its focal length decreases.

The far point is the greatest distance up to which the eye can see clearly. For a normal eye, it is at infinity. The near point is the closest distance at which an object can be seen clearly without strain. For a normal eye, it is about 25 cm. Holding objects much closer than this for long can strain the eyes.

Glass Prism and Dispersion of Light

A glass prism has two refracting surfaces and bends light passing through it. These terms are often asked in the exam:

• Angle of incidence: the angle between the incident ray and the normal.

• Angle of refraction: the angle between the refracted ray and the normal.

• Angle of emergence: the angle between the emergent ray and the normal.

• Angle of the prism: the angle between its two refracting surfaces.

• Angle of deviation: the angle between the original path of the ray and the emergent ray.

When white light passes through a prism, it splits into seven colours. This is called dispersion. The order of colours is Violet, Indigo, Blue, Green, Yellow, Orange, Red, remembered as VIBGYOR. The band of colours is called a spectrum. Dispersion happens because each colour bends by a different amount, so the refraction of light you studied in the light chapter is the key idea here.

Violet bends the most and red bends the least. If a second prism is placed in the opposite orientation, the colours recombine into white light. Dispersion is cancelled, but the light is still deviated.

Do not mix up these three terms:

• Dispersion: splitting of white light into its colours.

• Deviation: change in the direction of light.

• Scattering: spreading of light in different directions after it strikes particles.

Rainbow Formation

A rainbow forms when sunlight passes through tiny water droplets in the air. It is seen when the Sun is behind the observer, and the droplets are in front. Inside each droplet, three things happen:

1. Light refracts and disperses as it enters the droplet.

2. It undergoes internal reflection at the back of the droplet.

3. It refracts again as it leaves the droplet.

Memory Tip: Violet bends the most and red the least. A rainbow is refraction + dispersion, then internal reflection, then refraction again.

Atmospheric Refraction

The atmosphere has layers of air with different densities. Air closer to the ground is generally denser and has a higher refractive index, while the upper layers are thinner. Light bends continuously as it passes through these layers. This bending is called atmospheric refraction.

Twinkling of Stars

Stars are very far away, so they behave like point sources of light. As their light passes through layers of air whose density keeps changing, its path changes slightly from moment to moment. The amount of light reaching your eye also changes, so the star seems to twinkle.

Planets are much closer and act like a collection of many point sources. The small changes average out, so planets usually do not twinkle.

Advance Sunrise and Delayed Sunset

Because of atmospheric refraction, we see the Sun about 2 minutes before it actually rises above the horizon, and about 2 minutes after it has actually set. This makes the day slightly longer.

Tyndall Effect and Scattering of Light

Scattering happens when light strikes dust, smoke, water droplets or air molecules and spreads out in different directions.

The Tyndall effect is the scattering of light by small particles that makes the path of a beam visible. You can see it when sunlight enters a dark room through a small opening and the dust particles light up.

Why the Sky Looks Blue

Light of shorter wavelengths, such as blue, is scattered much more than light of longer wavelengths, such as red. Scattered blue light reaches our eyes from all directions, so the sky looks blue.

Why the Sun Looks Red at Sunrise and Sunset

At sunrise and sunset, sunlight has to travel a longer path through the atmosphere. Most of the blue light is scattered away before it reaches us. The colours that are scattered least, mainly red and orange, reach our eyes, so the Sun looks red or orange.

Memory Tip: The Tyndall effect makes a beam visible through scattering. The sky is blue because blue is scattered the most. Sunrise and sunset are red because blue has been scattered away over the long path.

The human eye forms images on the retina, and the ciliary muscles change the shape of the lens to focus objects at different distances. Myopia and hypermetropia occur when the image forms in front of or behind the retina, and they are corrected using concave and convex lenses. Dispersion explains how white light splits into seven colours, while atmospheric refraction and scattering explain twinkling stars, advanced sunrise, the blue sky and red sunsets. Revise the Memory Tips, remember the sign of lens power and practise numerical questions to strengthen your preparation for Class 10 Science. Physics Wallah notes and NCERT questions can help you revise the chapter and prepare for board examinations.