Science 8th grade
UNIT 12: Light 12.1 How light travels
Science 8th grade
UNIT 12: Light 12.1 How light travels
- UNIT 1: Plants
- UNIT 2: Food and digestion
- UNIT 3: The circulatory system
- UNIT 4: Respiration
- UNIT 5: Reproduction and development
- UNIT 6: Stated of matter
- UNIT 7: Elements and compounds
- UNIT 8: Mixtures
- UNIT 9: Material changes
- UNIT 10: Measuring motion
- UNIT 11: Sound
- UNIT 12: Light
- UNIT 13: Magnetism
- References
Light is important to us. We see things because light from them enters our eyes.
Sight is one of our most important senses. We use it to find out about the world around us.
Sources of light
A light source is an object that emits its own light. Hot objects such as flames and the Sun are light sources. A torch bulb has a hot filament, so it is also a light source.
Some light sources are not hot-for example, a computer screen.
Question
1) Give four examples of objects that are light sources.
Straight lines
The photograph of the Sun behind the clouds shows us how light travels. You can see the straight rays of light spreading out from the Sun in the photograph. This tells us that light travels in straight lines.
Question
$A+I$: 2) Look again at the photograph. You cannot see the Sun, but you can casily work out where it is in the sky. Explain how you could use a ruler to work out the Sun's position in the sky. Make a sketch to show your idea.
Activity 12.1 (Light travelling in straight lines)
Here is one way to show that light travels in straight lines.
Each of the three cards has a small hole in it. Pull the string so that it is taut (tight and straight). Then the three holes will be arranged in a straight line.
Look through the holes and you will see the candle flame. Light from the flame passes through the three holes, one after another.
Make a prediction: what will happen if you move one of the cards sideways?
Does it matter which card you move? Test your ideas.
Luminous and non-luminous
An object which is a source of light is described as luminous.
An object which is not a source of light is described as nonluminous.
We see non-luminous objects because they reflect light to our eyes.
For example, the chair in the diagram is a non-luminous object.
The person can see it because light from the lamp reflects off it.
In the diagram, we show the light from the lamp travelling in straight lines.
Questions
$A+I$: 3) Imagine that you are reading a book. A lamp is nearby to help you see the pages. Draw a diagram to show how light from the lamp makes it possible for you to read the book.
4) In Stage 7 Unit 11, you learnt that we see the Moon and planets by reflected light.
a: What is the source of this light?
b: Is the Moon a luminous or non-luminous object?
c: Draw a diagram to show how somcone on Earth sees the Moon by reflected light.