CBSE Notes Class 10 Science Chapter 10 - The Human Eye and The Colourful World

The human eye is a remarkable organ that allows us to see the vibrant world around us. Like a sophisticated camera, it captures light and forms images. Key processes include accommodation for focus adjustment, refraction to bend light, dispersion to split light into colors, and scattering, which affects how light interacts with atmospheric particles.

1.0The Human Eye                                              

Structure of Eye

• Cornea: The eye’s front is covered by a transparent, spherical membrane called the cornea.
• Aqueous Humour: A liquid filling the space behind the cornea.
• Iris: A dark, muscular diaphragm behind the cornea with a central opening called the pupil. It regulates light entry by adjusting the pupil size.
• Pupil: Appears black because it does not reflect light.
• Lens: A convex lens made of transparent, jelly-like protein material. It is firm in the center and softer at the edges, held in place by ciliary muscles.
• Ciliary Muscles: Adjust the lens’s curvature and focal length.
• Retina: The inner back surface of the eye, functioning like a light-sensitive screen. It contains rods and cones which detect light and color.
• Vitreous Humour: A fluid between the retina and lens.
• Vision Process: Light enters through the cornea and pupil, is focused by the lens to form an inverted image on the retina. Rods and cones in the retina convert this light into electrical signals.
• Optic Nerve:Transmits these electrical signals to the brain, which interprets them.
• Color Vision: Cones in the retina enable color vision and are active in bright light. In low light, only rods function, impairing color perception.

2.0Power of Accommodation

The power of accommodation refers to the eye's ability to adjust the lens's focal length to maintain clear focus on objects at varying distances.

• Accommodation of the Eye: The process by which the ciliary muscles modify the focal length of the eye's lens to focus on objects at various distances..
• Power of Accommodation: The eye's capability to focus on objects at various distances by altering the lens's focal length.
• Range of Vision: The distance between the nearest point of clear vision (near point) and the farthest point of clear vision (far point).
• For a typical human eye, the near point is roughly 25 cm away, which is the shortest distance at which the eye can clearly focus on an object without straining.

3.0Defects of Vision and Correction

1. Myopia (Near-Sightedness)-Difficulty seeing distant objects clearly or the eye is not able to see distant objects clearly.
2. Hypermetropia (Far-Sightedness)-Difficulty focusing on nearby objects or an eye defect in which distant vision is clear while near vision is blurred. In the case of a normal eye the rays of light from the object fall on the eye and converge on the retina but in the case of a hypermetropic eye the light rays are focused behind the retina
3. Presbyopia-Age-related loss of near vision or Presbyopia occurs when the center of the eye's lens becomes rigid, impairing its ability to focus on close objects.. This condition generally affects almost everyone over the age of 50 - even those with myopia.

Note-These defects can be corrected with appropriate spherical lenses, which help restore clear vision.

4.0Refraction of Light through a Prism

When a beam of white light enters a prism through one of its faces, it bends or refracts toward the base of the prism. This bending happens because the light slows down as it moves from the less dense air into the denser material of the prism.

5.0Dispersion Through a Prism

• The occurrence where white light is separated into its component colors is known as the dispersion of light.
• The wavelengths of different colours of light are different and the refractive index of glass is different for different wavelengths. Higher the wavelength, lower will be the refractive index and thus, lower will be the deviation and vice-versa. Thus, the deviation of red light is low as its wavelength is large. Deviation of violet light is high as its wavelength is small.

6.0Atmospheric Refraction

• The refraction of light produced by the Earth's atmosphere occurs because the refractive index of air varies across different layers.
• Atmospheric refraction takes place due to gradually changing refractive index of the air.

1. Twinkling of Stars

• Luminous energy reaching our eyes per second from the star increases and decreases.
• Apparent position of a star is slightly higher than its actual position when it is viewed from the earth’s surface with time.
• This apparent position is not stationary but it changes with time because of variable physical condition of the refracting medium

2. Advance Sunrise and Delayed Sunset

• The sun appears visible before the actual sunrise and after the actual sunset due to atmospheric refraction.
• The sun seems to be above the horizon even though it is actually just below it due to atmospheric refraction.
• The apparent shift in the sun's position is approximately ½°, which results in a time difference of about 2 minutes between the actual and apparent sunrise.

7.0Scattering of Light 

The process in which the light rays are deflected by the particles of the medium through which they pass is called scattering of light.

Tyndall Effect

• The scattering of light as it passes through a medium containing small particles is called the Tyndall effect.
• When light passes through smoke, the particles in the smoke scatter the light, making the path of the light visible.

Colour of the Clear Sky - Blue

Air molecules and fine particles in the atmosphere, which are smaller than the wavelength of visible light, scatter shorter wavelengths like blue light more effectively than longer wavelengths such as red light. Since blue light has a shorter wavelength and is scattered more, it is predominantly what we see when looking at the sky.