Labelling

The human eye is one of the most complex organs in the body. The outermost layer of the eye is the sclera — the white, fibrous tissue that gives the eye its characteristic white appearance and provides structural support. At the front of the eye, the sclera becomes transparent and curves outward to form the cornea, which acts as the eye's primary focusing lens.

Immediately behind the cornea is the anterior chamber, a fluid-filled space containing aqueous humour. Behind the anterior chamber lies the iris — the coloured part of the eye — which is a ring of muscle that controls the size of the pupil, the dark circular opening at its centre.

Behind the iris is the lens, a transparent, biconvex structure that fine-tunes the focusing of light onto the retina. The lens is held in place by a ring of fibres called the zonules, which connect it to the ciliary body — a ring of muscle that surrounds the lens.

The largest chamber of the eye, located behind the lens, is filled with a gel-like substance called vitreous humour, which helps maintain the shape of the eye and transmits light to the retina. The retina is a thin layer of light-sensitive tissue that lines the inner surface of the back of the eye. It contains two types of photoreceptor cells: rods, which enable vision in dim conditions, and cones, which are concentrated in a small central area called the fovea and are responsible for colour vision and fine detail.

The optic nerve, which exits the eye at a point called the optic disc, carries electrical signals from the retina to the brain's visual cortex.

The water cycle describes the continuous movement of water through the Earth's systems. The cycle is driven primarily by solar energy, which causes water to evaporate from the surfaces of oceans, lakes, rivers, and other bodies of water. As water vapour rises into the atmosphere, it cools and undergoes condensation, forming tiny water droplets that cluster around dust particles to create clouds.

When sufficient water droplets accumulate within a cloud, they fall back to the Earth's surface as precipitation. Rain that falls on land may follow several pathways. Some water is absorbed directly into the soil through a process called infiltration, replenishing underground water stores known as aquifers. Some flows across the surface as runoff, collecting in streams and rivers. Some is absorbed by plant roots and later released back into the atmosphere through the leaves in a process called transpiration.

The combination of evaporation from water surfaces and transpiration from plants is collectively referred to as evapotranspiration. This combined process is responsible for returning a significant proportion of precipitation back to the atmosphere, completing the cycle.

Groundwater — water stored in underground aquifers — plays a critical role in the water cycle. Aquifers are replenished by infiltration and can store water for thousands of years. They are tapped by wells and springs, providing drinking water and irrigation for agriculture in many parts of the world.

Human activities have significantly altered the natural water cycle. Deforestation reduces evapotranspiration and increases surface runoff, leading to more frequent and intense flooding. Urbanisation replaces permeable soil with impermeable surfaces such as concrete and asphalt, further increasing runoff and reducing infiltration.

A standard solar panel consists of multiple photovoltaic cells connected together, each typically made from silicon. The cells are encased in a protective glass cover on the front and a polymer backing on the rear, with an aluminium frame providing structural support.

Each photovoltaic cell consists of two layers of silicon with different electrical properties — a positively charged p-type layer and a negatively charged n-type layer — separated by a junction. When photons from sunlight strike the cell, they transfer their energy to electrons in the silicon, causing them to move across the junction from the p-type layer to the n-type layer. This movement of electrons creates a direct current (DC) of electricity. Metal contacts on the front and rear surfaces of the cell collect this current and channel it to external circuits.

The direct current produced by solar panels must be converted to alternating current (AC) before it can be used by most household appliances. This conversion is performed by a device called an inverter, which is typically installed alongside the solar panels.

The efficiency of a solar panel is determined by several factors, including the quality of the silicon, the design of the cell, and the operating temperature. Standard commercial solar panels typically achieve efficiencies of between 15% and 22%.

A complete solar power system for a residential property typically includes solar panels mounted on the roof, an inverter, a monitoring system, and optionally a battery storage system. The battery storage system allows excess electricity generated during the day to be stored and used at night or during periods of low sunlight. The panels are typically mounted at an angle facing the sun — in the Northern Hemisphere, this means facing south — to maximise exposure to sunlight throughout the day.