LIGHT ENERGY

Light energy is a kind of kinetic energy with the ability to make types of light visible to human eyes. Light is defined as a form of electromagnetic radiation emitted by hot objects like lasers, bulbs, and the sun

SOURCES OF LIGHT:

  1. Light is a form of energy that enables us to see.
  2. This form of energy can be obtained from very many sources. The things that give off light are known as sources of light.
  3.  These sources can be natural sources of light or artificial sources of light.

Natural Sources of Light:

These are sources that exist without peoples influence or control.

        Examples of natural source of light.

  • Sun
  • Stars
  • Fire flies
  • Glow worm
  • Volcanic eruptions
  • Lightning
  1. The Sun is the main source of light on earth.
  2. The moon is sometimes not regarded as a source of light because it does not produce light of its own but reflects it from the sun.
  3. Sources that give off their own light are called Luminous sources.
  4. Sources which reflect light but do not give their own light are known as non luminous sources.

Artificial Sources of Light:

  1. These are source of light that were made by people.

Examples of Artificial sources of light

  • Electric bulbs
  • paraffin lamps
  • Candles etc.
  • Some of these sources produce light after producing heat.
  • These are called Incandescent sources.
  • Those which give light without producing heat first are ——————-.

USES OF LIGHT:

  1. Light can be used in the following ways:-
  2. Sunlight is used by plants to make their own food
  3. Light is used by  animals to see.

How we are able to see

i)      We are able to see objects when these objects reflect light into our eyes.

ii)      light from the source moves to the object, the object reflects some of the light    into our eyes and we see the object.

iii)     We cant see objects in darkness because there isn’t light for the objects to reflect  into our eyes.

  • Sunlight is converted into Solar electricity by the solar panels.
  • Our bodies use light from the sun to make vitamin D.
  • Light from artificial sources like electric bulbs, candles is used to see at night.
  • It is used for protection to scare away enemies or wild animals.
  • Light in general is used in photography and in any other optical instrument.

PROPERTIES OF LIGHT:

  1. Light comes from different colours.
  2. Light can travels in all directions from the source e.g. the sun.
  • Light travels along a straight line.

4.     A path along which light travels is called a Ray.

5.     A ray of light is represented as a straight line with an arrow head to indicate the direction of movement of light.

A ray of Light.

          Experiments to show that light travels in a straight line

        a)

light travel in stringht line

        The light can be seen because there is a straight line from the candle to the eye.

a

        The light from the electric bulb will not be seen because light does not bend around corners.

Light in c can be seen because the holes in the cardboards is straight while the light cannot be seen because the holes in the cardboards are not in a straight line and so light does not travel around corners.

RAYS AND BEAMS OF LIGHT:

1.     A ray is a straight path taken by light.

2.     A collection of rays of light or group of rays moving in the same direction is called a beam.

        TYPES OF BEAMS.

        i)  Parallel beams, 

ii)  Diverging beam

iii) Converging beam

  •  

Parallel beams

It is a group of rays moving in one direction at an equal distance from each other.

pa

Diverging beams:  

It is a group of rays moving in different directions from one source.

diev beam

Torches and car head lamps give off diverging beams.

Converging Beam

It is a group of rays moving towards one point from different directions.

cove

Convex Lenses and Solar cookers give off converging beams

LIGHT AND SOUND.

1.     Light travels faster than sound.

2.     The speed of light is 300,000km/sec.

        Examples to show that sound travels faster than sound

        a) We see lightening first before we hear sound during thunder.

        b) In fire works, we see light spread first before hearing the sound.

        c) If a gun fired in the air at night, we see the light from it first before we hear the sound.

        d) The moving aeroplane in the air is actually seen in front of where sound is heard.

3.     The speed of light in glass and water is much slower. 

Effects of Different materials on light:

When light rays fall on an object, it may be;

a) Allowed through the object.

b) Reflected by the object.

c) diffused/altered in different directions.

d) Obstructed.

TRANSPARENT OBJECTS:

1.     These are objects that allow all the light to pass through them.

2.     We are able to see through transparent materials because they allow all light rays to pass through them.

translucent object

        Examples of transparent materials.

  • air
  • clear polythene papers
  • Clear glass
  • Clear water.

TRANSLUCENT OBJECTS:

  1. These are objects that allow only little light to go through them.
  2. When light meets a translucent material it is scattered/diffused.
  3. We cannot see clearly through translucent objects because they diffuse/scatter light rays.
trasparent object

Examples of translucent materials

a)  Frosted glass (glass with rough surface).

b)  Waxed or oiled paper,

c)  Thin cloth, and

d)  Coloured water.

e) Misty air/ Air with fog

OPAQUE OBJECTS:

1.     These are materials that do not allow any light to go through them.

2.     We cannot see through opaque objects because they do not allow any light ray to pass

        through them.

opaque object

3.     When light meets an opaque object, it is blocked or stopped.

4.     Some of the light is absorbed and some is bounced back.

5.     The bouncing back of light rays is called reflection of light.

Examples of opaque objects

  1. Stones
  2. Bodies of animals,
  3. Concrete
  4. Wood
  5. Metal etc.

Effects of light when it meets opaque objects:

  1. When light meets an opaque object it is obstructed.
  2. Once light is obstructed a Shadow is formed to the opposite of the source of light.
  3. A shadow is a region of darkness formed when light rays are obstructed by an opaque object.

Types of Shadows

There are two types of shadows and these are:

  1. Umbra
  2. Penumbra

UMBRA

1.     This is the darker region of a shadow.

2.     This is caused when the source of light is far smaller than the object.

         PENUMBRA

1.     Penumbra is a partially dark/ lighter region of a shadow.

2.     In most cases it surrounds the Umbra-shadow.

Diagrams showing umbra and penubmra shadows
penubra shadow

     How a shadow is formed

     a) When the source of light is small, the shadow formed is dark allover.

aa

b) When the source of light is larger, the shadow formed has two regions, Umbra and Penumbra.

bb

     Size of Shadow in Relation to position of source of light.

  1. The size of the shadow depends on the angle of the source of light in relation to the object and screen.
  2. Shadows caused by sunlight are longest in the morning at sun rise and at evenning at sun set because of the angle of the sun and the objects.
  3. They are shortest at noon.
  4. The size of the shadows also depends on the distance of the object from the source and distance from the screen.

Importance of shadows.

  1. Shadows are used to tell direction.
  2. Shadows provide shelter to animals and some plants.
  3. We also use the shadow to estimate time.

ECLIPSES:

  1. Eclipses are also shadows. The word Eclipse means cut off.
  2. Eclipses are formed when the moon or earth obstructs light from the sun.

Types of Eclipses

i).     Lunar Eclipse (Eclipse of the moon)

  1. Solar Eclipse (Eclipse of the sun)

Lunar Eclipse (Eclipse of the moon

Lunar Eclipse takes place when the Earth comes between the moon and the sun. The Earth obstructs light from reaching the moon.

eclipes

Solar Eclipse (Eclipse of the sun)

  1. Solar Eclipse takes place when the moon comes between the sun and the Earth.
  2. The moon obstructs light from reaching some parts of the earth.
  3. The parts which receive Umbra shadow get total darkness and those which get penumbra shadow receive partial darkness.
soral ec