CBSE Class 10 Science Notes: The Human Eye and the Colourful World

The Human Eye: A Marvel of Nature

The human eye is a remarkable organ, acting as a natural optical instrument. It allows us to perceive the world around us with incredible detail and vibrancy. This section delves into the structure and function of the human eye.

How the Eye Lens Works: Focusing Light

The eye lens, a convex lens, plays a crucial role in focusing light onto the retina. The process involves refraction, the bending of light as it passes from one medium (air) to another (the lens).

Definitions:

• Cornea: The transparent front part of the eye that covers the iris, pupil, and anterior chamber. It’s the first place where light enters the eye.
• Iris: The colored part of the eye, controlling the size of the pupil and thus the amount of light entering the eye.
• Pupil: The opening in the center of the iris, which appears black. It controls the amount of light entering the eye.
• Ciliary Muscles: Muscles that control the shape of the lens, enabling it to focus on objects at different distances.
• Retina: The light-sensitive layer at the back of the eye, containing photoreceptor cells (rods and cones) that convert light into electrical signals.

Core Principles:

• Refraction: The bending of light as it passes through the cornea and lens.
• Accommodation: The ability of the eye lens to change its focal length to focus on objects at varying distances, achieved by the ciliary muscles changing the lens’s shape.
• Focal Length: The distance between the lens and the point where parallel light rays converge after refraction.

Common Defects of Vision: Myopia, Hypermetropia, and Presbyopia

Sometimes, the eye doesn’t function perfectly, resulting in vision defects. These defects can often be corrected with appropriate lenses.

Definitions and Explanations:

• Myopia (Nearsightedness): The inability to see distant objects clearly. The eye lens focuses the image in front of the retina.
• Hypermetropia (Farsightedness): The inability to see near objects clearly. The eye lens focuses the image behind the retina.
• Presbyopia: A condition associated with aging, where the eye loses its ability to focus on near objects due to the weakening of the ciliary muscles and the loss of elasticity of the lens.

Corrective Lenses:

• Myopia: Corrected using a concave lens (diverging lens), which diverges the incoming light rays before they enter the eye, allowing the image to focus on the retina.
• Hypermetropia: Corrected using a convex lens (converging lens), which converges the incoming light rays before they enter the eye, allowing the image to focus on the retina.
• Presbyopia: Often corrected using bifocal lenses, which have both concave and convex lens components for near and far vision, or progressive lenses.

Formulae:

• Lens Formula: $\frac{1}{f} = \frac{1}{v} – \frac{1}{u}$ where:
  • $f$ = focal length of the lens
  • $v$ = image distance
  • $u$ = object distance
• Power of a Lens: $P = \frac{1}{f}$ (where $f$ is in meters). The unit of power is dioptre (D).

Examples:

A person with myopia might need glasses with a negative dioptre power (e.g., -2.0 D).