Topic outline
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In the previous course, Sources of Light, you learnt that many light rays make up a light beam.
One of the properties of light is that it travels in straight lines and this is referred to as the rectilinear propagation of light. The arrows on the light beam image indicate the direction in which the light is travelling.
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Parallel, divergent and convergent beams of light
Beams of light can be either:
Watch the YouTube video below which will take you through an experiment which demonstrates the rectilinear propagation of light.
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Transparent materials
Transparent materials allow light to pass through them. Examples include glass windows and doors, clear plastics and prisms. You will be able to see through a transparent object.
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Translucent materials
Translucent materials do not allow all the light to pass through therefore when looking through, for example, a translucent window (frosted window) you will only be able to see a blurred image since all the light is not able to penetrate through the frosted window. Examples of translucent materials included frosted glass and tinted car windows. You are able to only partially see through a translucent object.
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Opaque materials
Light is unable to pass through opaque materials. Examples include brick walls, humans, concrete, books etc. You are unable to see through an opaque object.
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Shadows
You know about shadows, you see them often, even your own but have you ever actually asked the question: 'What exactly is a shadow?'
You know that an opaque object does not allow light to pass through it e.g. a person. So if the sun shines on a person, it's light is unable to pass through the person and a shadow is formed behind him/her.
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Types of shadows
The light source will determine the type of shadow created. A point light source will produce a clearly defined shadow. A larger source of light will produce different types of shadows viz.:
- Umbra
- Penumbra
But what's the difference? This is best answered by way of an image:
Image: Wikipedia (CC BY-SA)
Umbra
This is the 'totally dark' shadow portion. The edges of the umbra are clearly defined. If you are standing in the umbra shadow you will be unable to see the light source.
Penumbra
This shadow type is partially dark and may have slightly blurry edges. If you are standing in the penumbra shadow, you will be able to see part of the light source. -
Activity
Try the following experiment at home:
- The aim of this experiment is to demonstrate how a shadow is formed from a source of light
- You will need the following materials:
- a piece of cardboard with a hole in it
- a source of light e.g. a torch or candle (handle with care)
- an opaque object
- a screen or piece of paper on which the shadow will be cast
Shine the light through the hole in the cardboard. What do you observe? What type of shadow is formed?Remove the cardboard and just shine the light in line with the opaque object. do you see a difference in the types of shadows created? Can you name them? What is your conclusion?
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Eclipse
If you think back to the course on Sources of Light, you will recall that both the earth and moon are opaque and non-luminous objects, meaning that light cannot pass through them. Therefore, shadows, called eclipses, can be created. Let's find out more:
Lunar eclipse
A lunar eclipse will occur when the earth comes between the sun and the moon thus forming a shadow on the moon. The shadow can be partial or complete. A total (complete) lunar eclipse occurs when the moon lies in the umbra of the shadow of the earth.
Image: Royalty free (CC) -
Solar eclipse
A solar eclipse occurs when the moon comes in between the earth and the sun. The sun is therefore blocked by the moon either totally (total solar eclipse as this image portrays) or partially (partial solar eclipse). Remember: never look directly at the sun!Image: Wikimedia Commons (CC BY-SA)
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