1Seeing things
The Sun, lamps and flames are luminous: they give out light. You see everything else because light from a source reflects off it and travels into your eyes.
luminous
Lesson 24.1 notes
Light travels in straight lines
Sources and seeing, the speed of light, shadows and the pinhole camera.
2Straight lines
Light travels in straight lines. We draw it as rays: straight lines with arrows. You only see the lamp through the cards when all three holes line up.
ray
3The fastest thing there is
Light travels at about 300 000 km every second. Light from the Sun crosses 150 million km of empty space to reach us in about 8 minutes.
4Light and sound
Light and sound are both waves that carry energy and can be reflected. Sound needs particles to travel through, so it can't cross a vacuum. Light can, and is about a million times faster.
vacuum
5Through, scattered or blocked
Transparent materials let light straight through. Translucent ones let light through but scatter it, so things look blurred. Opaque materials absorb or reflect all the light.
transparenttranslucentopaque
6Shadows
An opaque object blocks the light, leaving a shadow the same shape as the object. Move the object closer to the lamp and it blocks more light, so its shadow grows.
shadow
7The pinhole camera
Rays from the top of the candle go through the tiny hole to the bottom of the screen, and rays from the bottom go to the top. The image is upside down.
pinhole cameraimage
Explore: Shadow lab
Drag the puppet between the lamp and the screen and watch its shadow grow and shrink. Then swap the small bulb for a big lamp.
Common mistake: You see things when light enters your eyes
Your eyes don't send anything out. You see an object when light from it, given out or reflected, travels into your eyes. In total darkness you see nothing.
Remember
- Luminous objects give out light. You see everything else because light reflects off it into your eyes.
- Light travels in straight lines at about 300 000 km/s. It can cross a vacuum; sound can't.
- Opaque objects block light and make shadows. A pinhole camera makes an upside-down image.
Going further at GCSE