Normal human vision involves light entering the eye, refracting through the cornea and lens, forming a focused image on the retina. Signals are sent to the brain via the optic nerve, enabling clear sight.

Vision defects include myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and presbyopia. These conditions result from abnormalities in the eye’s shape or lens flexibility, affecting the ability to focus clearly.

Hormones are chemical messengers secreted by glands, regulating various physiological processes. Examples include insulin, estrogen, and adrenaline. They travel through the bloodstream, influencing growth, metabolism, and other bodily functions.

Blood sugar level, or blood glucose, measures the amount of glucose in the bloodstream. It is regulated by insulin and glucagon. Maintaining a stable level is crucial for overall health.

A food chain illustrates the transfer of energy in an ecosystem, showing the sequential flow of organisms consuming one another. Producers, consumers, and decomposers play vital roles in sustaining this dynamic process.

A convex lens is a transparent optical device with curved surfaces that converges light rays, causing them to meet at a focal point. It is used in magnifying glasses, cameras, and eyeglasses.

The human heart is a vital organ that pumps blood through the circulatory system. It consists of four chambers—two atria and two ventricles. Its rhythmic contractions maintain blood circulation and oxygenate tissues.