Light Chapter Notes | Year 4 Science IGCSE (Cambridge) - Class 4 PDF Download

Light in the Solar System

• The Sun is a star and the primary source of light in our solar system.
• It illuminates planets and moons, which reflect its light, making them visible.
• Example: The Earth and Moon appear lit in photographs (e.g., taken by the Galileo spacecraft) because they reflect sunlight.

• Stars: Emit their own light, visible in the night sky as light travels to Earth and enters our eyes.
• Planets: Do not produce light but reflect light from stars, such as the Sun.
• Many stars may have planets orbiting them, similar to our solar system.

• Celestial bodies in the solar system move in defined paths called orbits.
• The Moon orbits the Earth, completing one orbit in approximately 29 days (one month).
• The Earth orbits the Sun, taking 365.25 days (one year) to complete one orbit.
• Planets closer to the Sun have smaller, faster orbits, taking less than an Earth year, while those farther away have larger, slower orbits, taking more than an Earth year.

• The solar system includes the Sun, planets, moons, asteroids, and comets, all orbiting the Sun at its center.

• There are eight planets in our solar system, listed from closest to furthest from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
  

• Rocky masses, smaller than planets, that orbit the Sun.
• Approximately 781,692 asteroids have been identified, mostly located in the asteroid belt between Mars and Jupiter.

• Moons orbit planets or asteroids; for example, Earth has one moon, Mars has two, and Uranus’ largest moon, Titania, is the eighth largest in the solar system.
• Half of a moon may appear dark due to not facing the Sun, thus not reflecting light.

• Lumps of ice and dirt with large orbits around the Sun, often located at the solar system’s edge.
• Comets are characterized by long tails formed by sublimating ice as they approach the Sun.

• Diagrams and models represent the solar system, showing relationships between bodies (Sun, planets, moons, etc.).
• These models use scaled-down sizes and distances to make the vast solar system comprehensible.

Day and Night

• A globe serves as a model of the Earth, shaped like a ball with an axis (a stick from the North Pole to the South Pole).
• The Earth’s axis is tilted, not upright, in space, affecting its orientation relative to the Sun.

• The Earth spins on its axis in an anticlockwise direction (opposite to a clock’s hands) when viewed from the North Pole.
• One complete rotation takes 24 hours, causing the cycle of day and night.
• The spin is rapid, with surface speeds exceeding 120 mph, though imperceptible to humans.

• The Earth’s rotation on its axis results in day and night.
• The half of the Earth facing the Sun experiences daylight, while the opposite half, in shadow, experiences night.
• This rotation causes the Sun to appear to rise in the east and set in the west, though the Sun itself does not move; it is the Earth’s spin that creates this apparent movement.

Investigating Shadow Lengths

• Shadows are formed when an object blocks sunlight, casting a dark area on a surface.
• The length and position of shadows change throughout the day due to the Earth’s rotation on its axis.

• Shadows are longest at sunrise and sunset when the Sun is low in the sky, casting light at a shallow angle.
• Shadows are shortest at midday when the Sun is high, casting light nearly vertically.
• The position of shadows shifts as the Sun’s apparent movement (due to Earth’s rotation) changes its position in the sky.

• The Sun appears to move across the sky from east to west, but this is due to the Earth spinning on its axis.
• This apparent movement affects shadow length and direction, with patterns observable over the course of a day.
  

• Repeating shadow measurements on different days improves data reliability by accounting for variables like cloud cover.
• Consistent patterns in shadow length and position confirm the effect of Earth’s rotation.