ACARA v9 CONTENT DESCRIPTION “identify sources of light, recognise that light travels in a straight path and describe how shadows are formed and light can be reflected and refracted”
Builds on noticing that we need light to see things. Here we name where light comes from, find out that it travels in straight lines, and use that one idea to explain shadows, reflection in a mirror and the bending of light called refraction.
Some things make light and some only reflect it
A light source is something that makes its own light. The Sun, a lamp, a candle flame and a torch are all sources. Many other things look bright but make no light of their own. The Moon, a mirror and the page of a book only reflect light that reaches them from a source. If you turned every source off, those things would be completely dark, because there would be no light for them to bounce back to your eyes.
What makes its own light?
A light source makes its own light. Click each thing and find out if it is a real source or only bounces light.
A light source makes its own light, like the Sun, a lamp, a flame or a torch. The Moon, a mirror and a book look bright, but they make no light of their own. They only reflect light that lands on them from a real source. With the lights off, you could not see them at all.
Light travels in straight lines
Light travels away from a source in straight lines called rays. It does not curve or bend on its own. You can see this when a beam passes through a small hole: the light goes straight through and lands on a spot directly opposite the hole. Move the source up or down and the ray is still perfectly straight. This single idea, that light goes in straight lines, explains a lot of what light does.
Light travels in a straight line
Move the source up or down. The ray, the hole and the bright spot always stay in one straight line.
Light always travels in straight lines, called rays. The ray leaves the source, goes straight through the hole in the card and lands on the screen on the other side. When you move the source up or down, the ray bends nowhere. It stays straight, so the bright spot always sits directly opposite the hole.
Shadows form where an object blocks the light
Because light travels in straight lines, it cannot get around an object that it is unable to pass through. An object like that is called opaque. The straight rays are blocked, and the dark patch left behind the object is a shadow. Moving the light changes the shadow. A light close to the object makes a large shadow, and moving the light to one side slides the shadow across to the other side.
An object blocks light and casts a shadow
Move the light around. The post blocks the straight rays, and the shadow changes size and side.
Light travels in straight lines, so an object that the light cannot pass through, an opaque object, leaves a dark patch behind it. That dark patch is a shadow. Moving the light changes the shadow: a light up close makes a big shadow, and moving the light to a new side slides the shadow to the opposite side of the object.
Light reflects off a mirror
When light hits a smooth surface like a mirror, it bounces off in a new direction. We call this reflection. To describe it we measure the angle from the upright line that points straight out of the mirror. The neat rule is that the reflected ray leaves at the same angle as the ray that came in. A ray arriving at 45 degrees bounces away at 45 degrees, so the angle in equals the angle out.
A mirror reflects light at an equal angle
Pick an angle. The ray bounces off the mirror, leaving at the same angle it came in.
When light hits a mirror it bounces off, and we call this reflection. We measure each angle from the upright line that stands straight out of the mirror. The reflected ray always leaves at the same angle as the ray that came in. So a ray that arrives at 45 degrees bounces away at 45 degrees: the angle in equals the angle out.
Light bends when it enters water or glass
Light can also change direction as it passes from one clear material into another, such as from air into water or glass. This bending is called refraction. In air the ray runs straight, but at the surface of the water it bends to a new angle. This is why a straw standing in a glass of water looks broken at the waterline, and why a coin in a pool can look closer to the surface than it really is.
Light bends as it enters water
Switch the water on and off. In air the ray runs straight; entering water it bends at the surface.
When light passes from air into water or glass it changes direction at the surface. This bending is called refraction. In air alone the ray goes perfectly straight. As soon as it enters the water it bends, which is why a straw standing in a glass of water looks broken at the waterline.
Why this matters
Light is how we see everything around us, so understanding it explains a lot of the world. Straight-line light gives us shadows that tell the time on a sundial, mirrors that let us see around corners and check behind a car, and refraction that makes glasses, cameras and magnifying lenses work. Knowing that light has sources, travels straight, reflects and refracts gets you ready for more science about how we see and how lenses work later on.
Quick self-check
1. Which of these makes its own light?
2. How does light travel from a source to a screen?
3. Why does a post make a shadow on the wall behind it?
4. A ray hits a mirror at 45 degrees from the upright. What angle does it leave at?
5. Why does a straw in a glass of water look bent at the waterline?