Sources of light
We see an object only when light from it enters our eyes.
Luminous objects give out their own light: the Sun, other stars, a burning candle, a glowing bulb, a firefly.
Non-luminous objects do not make light. We see them because they bounce back light that falls on them: the Moon, a book, a table, your friend's face.
The Moon looks bright at night, but it is non-luminous. It shines with sunlight reflected from its surface.
Light travels in a straight line
Look at a burning candle through a straight pipe. You can see the flame. Now bend the pipe; the flame disappears. Light travels in straight lines, a property called rectilinear propagation of light. Beams of sunlight through a gap in a roof, or the beam of a torch in a dusty room, show these straight paths clearly.
Transparent, translucent and opaque
Type | What happens to light | Examples |
|---|---|---|
Transparent | lets almost all light through | clear glass, water, air |
Translucent | lets some light through | butter paper, frosted glass |
Opaque | stops light completely | wood, cardboard, your body |
Shadows
When an opaque object blocks the path of light, a dark region forms behind it on a screen. This is a shadow.
To get a shadow you need three things:
a source of light,
an opaque object,
a screen (the ground, a wall or a sheet) on which the shadow falls.
Facts about shadows:
A shadow is always dark, whatever the colour of the object. A red ball and a blue ball give the same black shadow.
A shadow shows the outline of the object, not its details.
The size of a shadow changes with the position of the light. Bring a torch closer to your hand and the shadow on the wall gets bigger.
Our shadow is shortest at noon, when the Sun is high overhead, and longest in the early morning and evening, when the Sun is low.
Shadows can be different shapes depending on how an object is held. A cylinder can cast a circular shadow from one direction and a rectangular shadow from another.
The pinhole camera
A pinhole camera is a box with a tiny hole on one side and a screen of tracing paper on the opposite side. Point it at a bright object, such as a tree in sunlight, and an image appears on the screen.
The image is:
inverted (upside down), because light rays travel in straight lines through the hole and cross over,
real, because it forms on a screen,
in natural colour, unlike a shadow.
On a sunny day, the round patches of light under a tree are images of the Sun formed by small gaps between the leaves acting as pinholes.
Reflection from mirrors
A mirror changes the direction of the light that falls on it. This bouncing back of light from a surface is called reflection. Shine a torch on a mirror in a dark room and the beam appears on the opposite wall.
Shiny, smooth surfaces such as mirrors, polished steel and still water reflect light well and form clear images. Rough surfaces scatter light in all directions. The image in a plane mirror is upright, the same size as the object, and left-right reversed.
Summary
Luminous objects make their own light; non-luminous objects reflect light.
Light travels in straight lines, which is why shadows and pinhole images form.
A shadow needs a light source, an opaque object and a screen; it is always dark.
A pinhole camera gives an inverted, coloured image; a mirror reflects light.