Light Chapter Notes | Year 8 Science IGCSE (Cambridge) PDF Download

Reflections

Cosmic Prism

Reflection usually brings to mind flat mirrors, but when light hits non-flat surfaces like a spoon, it creates a distorted image. Light travels in straight lines known as rays, which should be drawn with a ruler and an arrowhead to indicate direction.

• When a light ray strikes a mirror, it is called the incident ray and forms an angle with the mirror's surface.
• This angle is measured against a perpendicular line to the mirror's surface called the normal.
• In physics and mathematics, normal refers to a line that is at right angles to the surface.

Ray Diagrams

• To create a ray diagram for reflection, follow these steps:
• Draw the incident ray and the mirror, ensuring they intersect using a ruler.
• Use a protractor to draw the normal line at the intersection point, usually represented as a dashed line.
• Measure the angle between the incident ray and the normal, known as the angle of incidence.
• Measure the angle of reflection on the opposite side of the normal and draw the reflected ray at this angle with an arrowhead.

Ray diagrams illustrate how light behaves during reflection. The law of reflection states that the angle of reflection is equal to the angle of incidence, a principle that is always true. Mirrors are commonly used in daily life, such as in cars, where drivers can see cyclists behind them through reflected light.

• Light from the Sun reflects off a cyclist (incident ray), hits the mirror, and the reflected ray reaches the driver's eyes.

Refraction

Refraction causes distortion when looking through a glass of water or a wet window, making objects appear unclear.

• The material that light passes through is called a medium, which can be air, glass, or water.
• When light enters a glass of water, it bends, changing its path.
• Light travels at different speeds in different media: approximately 300,000 km/s in a vacuum, slower in air, and even slower in water and glass.

Change of Direction

• Refraction refers to the change in the direction of light when it moves between different media due to a change in its speed.
• When light travels from air into glass or water:
• It slows down and bends towards the normal.
• The angle of incidence (measured from the normal) is greater than the angle of refraction.
• When light moves from glass or water into air:
• It speeds up and bends away from the normal.
• The angle of refraction is greater than the angle of incidence.

Refraction can cause issues in everyday situations, such as when water droplets on a wet window refract light in various directions, making the view unclear. Vehicles use windscreen wipers to clear water droplets, improving visibility by maintaining consistent refraction from the glass.

• Refraction is also beneficial in lenses, which are curved pieces of glass that refract light.
• Lenses are used in human eyes, cameras, and eyeglasses.

Making Rainbows

Isaac Newton, a scientist known for his studies on forces, demonstrated in 1666 that white light could be separated into various colours.

• Newton conducted an experiment using sunlight, a small hole, and a triangular glass block (prism) to divide light into its constituent colours, which were then projected onto a screen.

The Spectrum

• The collection of colours present in white light is referred to as a spectrum.
• These colours blend seamlessly into one another rather than appearing as distinct segments.
• The spectrum comprises seven colours: red, orange, yellow, green, blue, indigo, and violet.
• A helpful way to remember these colours is by using the acronym “ROY G. BIV.”

Dispersion

• Dispersion is the process through which white light is divided into its various colours.
• This phenomenon occurs because each colour of light is refracted at slightly different angles when passing through a medium.
• A triangular prism effectively demonstrates dispersion, as violet light is refracted at the largest angle, while red light is refracted at the smallest angle.

Rainbows

A rainbow is formed when light gets dispersed and internally reflected by water droplets present in the air.

• For a rainbow to be visible:
• The Sun should be shining brightly.
• There should be rain or tiny droplets in the air.
• The Sun needs to be positioned behind the person observing the rainbow.

For example, on a sunny day, if a person stands behind a hose spraying water, a rainbow can be seen forming as the Sun's light passes through the water droplets.

Colours of Light

White light is made up of seven colours, which can be seen in a rainbow. By mixing three primary colours of light, all other colours can be created.

• The primary colours of light are red, green, and blue.
• These colours cannot be produced by mixing other colours of light together.

Adding Colours of Light

• When beams of the three primary colours of light overlap, they produce new colours:
• Red + Green = Yellow
• Red + Blue = Magenta
• Blue + Green = Cyan
• Red + Green + Blue = White

Devices like phones, computers, and televisions create different colours by combining the three primary colours of light. When you look closely at the screens of these devices, you can see individual sources of red, green, and blue light. Most screens are capable of displaying 256 different colours by adjusting the brightness of the primary colours.

Subtracting Colours of Light

Coloured filters are used to remove specific colours from light, similar to the filters used in chemistry experiments.

• When a transparent coloured filter is placed in front of white light, it transmits only its own colour while absorbing the other six colours.
• Traffic lights use white lamps with coloured filters (red, yellow, green) to create specific colours.

For instance, in a red traffic light, the white light passes through a red filter, which transmits only red light and absorbs the other six colours.

• This process is known as the subtraction of light, where white light has colours removed to leave just one.
• Yellow and green filters work in the same way, each transmitting one colour and absorbing the others.

Stage lights and photographers also use coloured filters to achieve different effects. Using multiple filters can block all light; for example, shining green light through a green filter and then through a red filter would result in no light being transmitted.

Seeing Colours

Non-luminous objects, which do not emit light on their own, are visible because of the light they reflect.

• A flower appears yellow because it reflects yellow light and absorbs the other six colours.
• A white object reflects all colours equally, while a black object absorbs all colours and reflects none.

Objects have different appearances based on the colours they reflect and absorb. For example:

• A red ball reflects red light and absorbs the other colours.
• A white ball reflects all colours.

In different lighting conditions, objects may look different:

• A red cube appears black in green light because it absorbs green light and reflects red light, which is not present in this case.
• A blue cube looks black in red light because it absorbs red light and reflects blue light, which is not present.

Galaxies

On a clear night, away from the bright lights of cities, people can see a milky band stretching across the sky. This band is part of the Milky Way, which is our galaxy. The word “galaxy” comes from a Greek word that means “milky.”

• The reason the Milky Way appears as a band is that we are inside it, and it has a spiral shape. We see only one part of this spiral from our location within the galaxy.

Shapes of Galaxies

• The Milky Way is a spiral galaxy, and if we could see it from a distance, it would look like a beautiful spiral.
• Most of the stars that people see at night are actually in the Milky Way. This galaxy is huge, containing about 250 billion stars, and our Sun is one of them.
• There are other galaxies too, and they come in different shapes. Some are spiral like ours, while others are elliptical (oval-shaped) or irregular (no definite shape).
• The galaxy that is closest to the Milky Way is called the Canis Major Dwarf Galaxy, and it is about 25,000 light-years away from us. A light-year is the distance light travels in one year, which is very far!

What are Galaxies Made of?

• Galaxies are made up of a mix of materials, including stellar dust, gas, stars, and solar systems, all held together by the force of gravity.
• Stellar dust is not very dense; there is about one particle of dust for every million cubic meters of space.
• Gravity is what keeps everything in a galaxy together, and it is very strong because galaxies are massive. The Milky Way, for example, is about 1.5 trillion times more massive than the Sun, which is why its gravity is so powerful.

How Many Galaxies are There?

• Scientists believe there are about 100 billion galaxies in the universe. This is a rough estimate and can change based on how densely packed the galaxies are or what we think the total size of the universe is.

Asteroids

Asteroids are rocky objects that orbit the Sun, and they can vary in size from as small as 2 metres to as large as 975 kilometres across.

• Most asteroids have irregular shapes, and their forms are often compared to that of potatoes.
• The majority of asteroids are found in the asteroid belt, which is located between the orbits of Mars and Jupiter.
• There are millions of asteroids in space, and some of them have been given specific names.

The largest asteroid in our solar system is Ceres, which was discovered in 1801. Ceres has a diameter of 975 kilometre s.

• Ceres is round in shape, covered with craters, and has a core, mantle, and crust, making it similar to a small planet.
• Scientists think that Ceres could have been a full-fledged planet if it had continued to grow during the early stages of the Solar System's formation.

Another asteroid, Itokawa, measures about 530 metres in length. In 2005, a spacecraft visited Itokawa to collect samples from its surface.

• Itokawa is believed to be made up of loosely bound rocks that are held together by weak gravity. These rocks likely come from broken planets or moons.
• Some asteroids are made of a single piece of rock, which is indicated by their small size and the fact that they spin quickly.

Asteroids and Earth

Large asteroids collide with Earth approximately every 130,000 years, while smaller asteroids hit more frequently.

• Between 1994 and 2013, small asteroids measuring less than 20 metres across disintegrated in the atmosphere without reaching the ground.
• Asteroids impact Earth because:
• Earth’s strong gravity pulls in nearby objects.
• Many asteroids have orbits that bring them close to Earth.

Primary Colors of Light

Color Fusion

• When the three primary colors of light— red, green, and blue —overlap, they create new colors.

Combinations of Primary Colors

• Red + Green = Yellow
• Red + Blue = Magenta
• Blue + Green = Cyan
• Red + Green + Blue = White
• Screens on devices like mobile phones, computers, and televisions create a variety of colors by mixing these three primary colors of light.
• If you look closely at some computer monitors, TV screens, or phone displays, you can see the individual sources of red, green, and blue light.
• Most screens can display a total of 256 different colors by adjusting the brightness of these primary colors.
• For instance, orange is made by combining red and green light, with red being brighter than green.
• Studies show that while the human eye can see millions of colors, most digital displays use a palette of 256 colors to represent a wide range of shades.