Light Energy
Discover how light travels, bounces, bends, and creates the colours we see every day.
What is Light?
Light is a form of energy that allows us to see. It travels in straight lines called rays and moves incredibly fast — about 300,000 kilometres per second! That is fast enough to travel around the Earth more than seven times in just one second.
Light comes from sources such as the Sun, light globes, fire, and torches. Objects that make their own light are called luminous. Objects that we can see only because light bounces off them (like the Moon, books, and people) are called non-luminous.
Luminous Objects
- ● The Sun
- ● Light globes and torches
- ● Fire and candles
- ● Glowworms (found in Australian caves!)
Non-Luminous Objects
- ● The Moon (reflects sunlight)
- ● Trees and rocks
- ● People and animals
- ● Books and buildings
Reflection
Reflection happens when light bounces off a surface. Smooth, shiny surfaces like mirrors reflect light very well and produce a clear image. Rough surfaces scatter light in many directions, so you do not see a clear reflection.
How a Mirror Works
When you look in a mirror, light from your face travels to the mirror and bounces straight back to your eyes. The angle at which light hits the mirror equals the angle at which it bounces off.
Angle in = Angle out (This is the Law of Reflection.)
Mirror surface
Reflection is used all around us: rear-view mirrors in cars, telescopes, periscopes, and even the reflectors on road signs and bike helmets keep us safe at night.
Refraction and Colour
Refraction is the bending of light as it passes from one material into another — for example, from air into water or glass. This is why a straw in a glass of water looks bent or broken at the surface.
Rainbows and White Light
White light (like sunlight) is actually a mixture of all colours! When light passes through a prism (or raindrops in the sky), each colour bends by a slightly different amount, spreading the light into a spectrum: red, orange, yellow, green, blue, indigo, and violet.
The visible spectrum — red through to violet
Why Do Objects Have Colours?
Objects appear coloured because they absorb some colours of light and reflect others. A red apple reflects red light and absorbs the rest. A white surface reflects all colours equally. A black surface absorbs almost all colours.
How Our Eyes Work
Your eyes are amazing organs that detect light and send signals to your brain. Here is what happens when you look at an object:
Did you know? The image on your retina is actually upside down! Your brain automatically flips it the right way up.
Key Vocabulary
Reflection
The bouncing of light off a surface. Smooth surfaces (like mirrors) reflect clearly; rough surfaces scatter light.
Refraction
The bending of light as it moves from one material into another, such as from air into water or glass.
Spectrum
The range of colours that make up white light: red, orange, yellow, green, blue, indigo, and violet.
Retina
The light-sensitive layer at the back of the eye that converts light into signals sent to the brain.
Worked Examples
Why does a straw look bent when placed in a glass of water?
Step 1: Light travels from the straw through the water to the air above.
Step 2: As light crosses from water into air, it changes speed and bends (refracts).
Answer: The straw looks bent because refraction changes the direction of the light rays as they leave the water, making the straw appear to be in a different position than it actually is.
Why do we see rainbows after rain in Australia?
Step 1: Millions of tiny water droplets remain in the air after rain.
Step 2: Sunlight enters each droplet, refracts, reflects off the inside, and refracts again as it exits.
Answer: Each colour in white light bends by a slightly different amount, so they separate into a spectrum — the rainbow we see. You always see a rainbow on the opposite side of the sky from the Sun.
Why does a green leaf look green?
Step 1: White light (which contains all colours) shines on the leaf.
Step 2: The leaf absorbs most colours but reflects the green wavelengths.
Answer: The leaf looks green because it reflects green light back to our eyes and absorbs the other colours of the spectrum.
Knowledge Check
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Key Concepts Summary
- ●Light is a form of energy that travels in straight lines (rays) at 300,000 km/s.
- ●Reflection is the bouncing of light off a surface; angle in equals angle out.
- ●Refraction is the bending of light as it passes between materials of different density.
- ●White light is a mixture of all colours; a prism or raindrops separate it into a spectrum.
- ●Our eyes work by focusing light onto the retina, which sends signals to the brain via the optic nerve.