Light is a form of energy, it is obtained from many sources.
These sources may be natural or artificial.
Look at the following pictures.
Describe each picture above.
Use the pictures to predict what you are going to learn in this unit.
14.1: LIGTH PROPAGATION
Propagation means to move, spread or travel.
Activity 14.1: To investigate how light is propagated
• A torch or candle
• 3 equal pieces of cardboard
What to do:
(i) Make small holes in the centre of the cardboards.
(ii) Arrange the pieces of cardboard so that all the holes at their centres are in a straight line.
(iii) Place a lit candle or torch at one end of the pieces of cardboard as shown alongside.
(iv) Observe though the other end of the cardboard. What can you see?
(vi) Write short notes concerning this experiment.
(vii) Explain how light from the following sources of light travels.
(a) Light from a lamp
(b) Light from the sun
(c) Light from a car’s head lamps
(viii) What is the name given to a group of rays?
Light travels in a straight line. Light also travels in all directions. Light travels in the form of a ray or rays. What is a ray?
14.2: Medium for light transmission
Activity 14.2: Comparing how light travels through different media
Various materials for example:
• Oiled paper
• Clear polythene paper
• Block of wood
• Clear glass
What to do:
(i) Get into your working groups.
(ii) Let one of you shine a torch on the materials listed above one by one.
(iii) Let the rest of you stand on the opposite side of the materials being illuminated by the torch.
Observe what happens.
(iv) Interchange so that all of you make observations.
(v) Let your group secretary record the observations made.
(vi) Share your report with the rest of the class.
Compare your observations with the following: • Light was seen at the other end when the clear polythene and clear glass were put in the path of light. • Some light (not much light) was seen when an oiled paper was put in the path of light. • No light was seen when an notebook and block of wood were put in the path of light.
What conclusion can you make concerning transparent, translucent and opaque materials, from the above investigation?
Materials or medium that transit light can be classified as follows:
Transparent media These are materials that allow light to pass through them. We can see through them clearly
Examples of transparent materials include glass windows, colourless drinking glasses, air, clear water
Name 3 other examples of transparent materials.
These are materials that allow some light to pass through them. We cannot see through them clearly.
Examples of translucent materials include frosted glass, waxed paper and thin pieces of cloth or paper.
Practice Activity 14.1
1. Frosted glass is used in making modern windowpanes and toilet windows.
What is the advantage of using such a glass?
2. Name 2 other examples of translucent materials.
These are materials that do not allow light to pass through them.
We cannot see through opaque materials because they block the light.
Examples of opaque materials include stones, wood, metal sheets, books, timber and the human body. Name 3 other examples of opaque materials.
14.3: Laws of light propagation
Reflection of light
Activity 14:3: To demonstrate reflection of light
• A plane mirror
• Source of light e.g. the sun or torch
• Wall What to do:
1. Hold a mirror in the sun as shown alongside.
2. The light is reflected to the wall by the mirror.
3. Now, change the angle of the mirror. Does the light patch change its position? When light falls on the mirror, the direction in which it will be reflected, depends on the angle at which
the light hits the mirror.
Reflection is the bouncing back of light when it falls on a surface. Smooth shiny surfaces reflect most of the light that falls on them. They are good reflectors of light.
Rays falling on the mirror are called incident rays.
Rays that bounce off the surface are called reflected rays. Brightly coloured surfaces reflect light better than dull coloured surfaces.
Types of reflection
There are two types of reflection. These are: (a) Regular reflection (b) Irregular reflection
(a) Regular reflection
Regular reflection occurs when light falls on a flat shiny surface such as a mirror.
The beam of light is reflected as parallel beams
(b) Irregular reflectors (diffuse reflection)
Irregular reflection occurs when light falls on a rough surface. Reflected light spreads in different directions. Identify other incident rays and reflected rays on the picture.
Practice Activity 14.1:
1. Name 4 examples of smooth shiny surfaces.
2. Comment on the reflection of light on dull and bright surfaces.
3. In what areas is knowledge on reflection of light useful?
4. Research on your XO laptop or from books in your school library.
Refraction of light
Activity 14.4: To demonstrate refraction of light
• Pencil or ruler
• Glass of water
What to do:
(i) Put a pencil into a clear glass of water.
(ii) Observe the appearance of the pencil or ruler at various positions.
Light travelling from the air is bent at the surface of the water (meeting point of air and water).
Refraction of light makes the pencil appear bent. Refraction occurs because light travels at different speeds in different media.
Refraction is the bending of a light ray when it travels from one medium to another different medium.
The following are examples of refraction of light:
• A coin at the bottom of a container with water appears raised. • Riverbeds appear shallow. • The floor of the swimming pool appears raised
Revision Activity 14
1. A group of rays form a . ...............
2. You have been provided with a list of materials:
Frosted glass, oiled window pane, stone, wood, drinking glass, polythene paper.
Group the materials as transparent, translucent or opaque in the table like the one shown below.
4. Name the two types of reflections.
5. Demonstrate the refraction of a light using a ruler and a glass of water.
6. What kind of surface reflects most of the light falling on it?
7. (a) Name the type of reflection shown below.
(b) Name rays: (i) A........................ (ii) B..........................
8. Describe briefly where refraction is displayed.
9. Rose and James observed lit candles through different metallic pipes as shown below
(a) What happened in each setup?
(b) What did the experiments that Rose and James did confirm?
10. The following figure shows a model of a periscope. A periscope utilises light.
(a) What law of light propagation does it use?
(b) What is a periscope used for?
11. Describe briefly the uses of the following media in daily life.
(a) Transparent media.
(b) Translucent media.
(c) Opaque media.
1. Read the following words in pairs. • Energy
2. Spell 3 words while your friend writes them in his or her notebook.
Let your friend also spell 3 other words as you write them in your notebook.
3. Discuss with your friend the meaning of any 3 words in the word list. Refer to notes in your textbook.