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Revision: The Human Eye and the Colourful World Science English Medium Class 10 CBSE

Definitions [5]

Define the following term:

Adaptation

Adaptation is the process by which the human eye adjusts to changes in light intensity.

Light Adaptation: When a person moves from a dark environment to a brightly lit area (e.g., stepping out of a cinema hall in the afternoon), they initially experience a dazzling effect. After a few seconds, the eyes adjust to the brightness. This process is called light adaptation.
Dark Adaptation: When a person enters a dark area from a brightly lit environment (e.g., entering a cinema hall), they initially struggle to see clearly. Gradually, their vision improves as the eyes adapt to the darkness. This process is called dark adaptation.

Define the term dispersion of light.

The phenomenon of the splitting of white light by a prism into its constituent colours is known as dispersion of light.

When a beam of white light or composite light is refracted through any transparent media such as glass or water, it is split into its component colours. This phenomenon is called ‘dispersion of light’.

Definition: Dispersion

The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion.

The splitting of light into its component colours is called dispersion.

The process of separation of light into its component colours while passing through a medium is called the dispersion of light.

Definition: Spectrum

On passing white light through a prism, the band of colours seen on a screen is called the spectrum.

The band of the coloured components of a light beam is called its spectrum.

Definition: Scattering of Light

Scattering is the process of absorption and then re-emission of light energy by the dust particles and air molecules present in the atmosphere.

Key Points

Key Points: Human Eye

The human eye works like a camera, forming a real and inverted image on the retina, which is light-sensitive.
The cornea allows light to enter the eye and performs most of the refraction, while the lens fine‑tunes the focus.
The iris controls the pupil, regulating the amount of light entering the eye—contracting in bright light and widening in dim light.
The power of accommodation is the ability of the eye lens to change its focal length by altering its curvature using the ciliary muscles.
For a normal eye, the near point is 25 cm and the far point is at infinity.

Key Points: Myopia

Myopia is a vision defect in which distant objects appear blurry, while near objects are seen clearly.
This occurs because the image of distant objects forms on the retina.
The far point is not at infinity but is shifted closer to the eye.
Causes include increased curvature of the cornea/lens or elongation of the eyeball.
Corrected using a concave lens of negative power, which diverges light rays to focus the image on the retina.

Key Points: Hypermetropia

Hypermetropia is a condition in which distant objects are seen clearly, but nearby objects appear blurred.
The near point shifts beyond 25 cm, making close-up tasks like reading difficult.
The image of nearby objects forms behind the retina.
Causes include reduced curvature of the lens or cornea and shortening of the eyeball.
It is corrected using a convex lens of positive power, which converges light rays to focus the image on the retina.

Key Points: Presbyopia

Presbyopia is an age-related vision defect where the eye’s ability to focus on nearby objects decreases.
It is caused by weakened ciliary muscles and reduced flexibility of the eye lens.
The near point shifts farther, making close-up vision difficult.
Bifocal lenses are commonly used for correction—concave at the top (for myopia) and convex at the bottom (for hypermetropia).
It can also be corrected with contact lenses or, in some cases, surgery.

Key Points: Dispersion of Light

Dispersion is the splitting of white light into seven colours (VIBGYOR) when it passes through a prism or similar transparent medium.
Human eyes can detect light with wavelengths ranging from 400 nm (violet) to 700 nm (red).
Different colours travel at different speeds in a medium like glass, so each colour has a different refractive index.
Violet light bends the most, and red light bends the least, as it passes through a prism, producing a spectrum.
A rainbow is formed due to dispersion, refraction, and internal reflection of sunlight by raindrops acting as tiny prisms.

Key Points: Atmosphere Refraction

Stars twinkle due to atmospheric refraction, which bends their light and makes their position and brightness seem to change.
Planets don’t twinkle because they appear larger, and the light from different points cancels out the flickering.
Sunrise is early, and sunset is late because refraction makes the Sun visible even when it's just below the horizon.

key Points: Scattering of Light

Shorter wavelengths (violet and blue) are scattered more than longer wavelengths (red).
The intensity of scattering follows Rayleigh’s law:
$\frac {1}{λ^4}$
Very small particles (smaller than the wavelength of light) scatter light more effectively than larger particles.
The sky appears blue because blue light is scattered more due to its shorter wavelength.
Red light scatters least, causing red/orange sunsets and making sunlight near Earth richer in red light.