Energy - Heat Energy and Light Energy | Class 8 Science

There are natural and artificial sources of light

Natural Sources	Artificial Sources
Stars	Lamps
Sun	Torch
Fireflies	Candles
Glowworms	Bulbs

A Glowworm - Natural Source of Light
The process of glowing is known as bioluminescence

Basic uses of Light

1. Photosynthesis in plants
2. Sight
3. Used in photography
4. On roads, it is used for safety (traffic lights)

Properties of Light

i) Light travels in a straight line from the source

The experiment below shows how light travels in a straight line:

The diagram (a) above, the observer can see the candlelight through the holes made on the cardboards A, B and C.
When cardboard B is moved, the observer can no longer see the candlelight as before. Examine the figure below.

Also, looking through a rubber tube as shown below follows the same principle.

ii) Light travels in all directions

- For an experiment, consider a bulb or a candle.
- A lit candle in a room shows light in all sides.

Reflection of Light

- Reflection is the bouncing back of light on the surfaces of objects.
- There are two types of refection: Regular and Irregular (Diffused) reflection of light.

Characteristics of Images on a Plane Mirror

1. the image is virtual (not real)
2. the distance between the object and the mirror is the same as the distance between the mirror and the image.
3. the image is upright
4. the image is laterally inverted

A periscope uses the principle of refection to observe things around corners. The mirrors are placed at 45°. Observer the image below

Refraction of Light

Refraction is the bending of light waves as it moves from one medium to another e.g from air to water.
Apparent bending refers to the bending of light at the surface of water as it moves from air to water.

From the image above, the pencil appears broken. From the image below, the coin appears raised.

Attempt the experiments above to fully understand the effects of refraction of light.

Effects of Refraction of Light

1. A pool of water appears shallower
2. A ruler appears broken when placed in water
3. A coin in water appears raised
4. Formation of a mirage (as seen on a road during shinny days)

The image below shows a road mirage. In a road mirage, there appears to be water on the road.

Dispersion of Light

Dispersion of light refers to the splitting of white light into 7 different colours
The 7 colours form the spectrum. Can you name the 7 colours?

Try to remember the colours in order

Opaque, transparent and Translucent materials

- Transparent materials allow light to pass through and you can see through them (e.g Air, Clear Glass, Clear water)
- Translucent materials allow light to pass through but you cannot see through them clearly (e.g dirty water, oiled paper, plastics, frosted glass)
- Opaque materials do not allow light to pass through them (e.g metal, stone, wood, iron sheet).

- The sun is the main source of heat energy
- Other sources include firewood, electricity, gas and more
- The main uses of heat include warming, cooking, cooking, drying clothes and more.
- The main effects of heat include

• Increase in body temperature
• enables the change in state of substance
• causes expansion
• permanent changes of some substances

Heat passes through solids, liquids and gases in different processes.

Conduction

- This is the transfer of heat through solids

After sometime, the pin will fall. This is because, the heat is transferred from the point of heat on the metal to the rest of the metal rod. Once the heat reaches the waxed point, the wax will melt and the pin will fall. Please note that when multiple pins are attached to the metal rod, the pin closest to the heat source will fall first.

Convection

- This is the process by which heat travels through liquids and gases.

To experiment heat transfer through air, place a piece of paper just above a heat source such as a candle. You will observe that the piece of paper
is carried upwards for sometime. This is because the hot air from the candle rises carrying the piece of paper with it as cold air drops.

Radiation

- Radiation is the process of transfer of heat from one place to another, without a medium. For instance, the sun's heat reaches the earth through radiation.

- When seating near a fire, you can feel the heat coming from the fire through the radiation process.