Light

Grade 8 Atlantic                                                                                        Teacher : Ibu Nuri Martini

L I G H T

You can see an object only if LIGHT from it enters your EYES. Some objects such as the Sun, Electric Lamps and Candles make their own light. We call these LUMINOUS sources. Most things you see do not make their own light but reflect it from a luminous source. They are NON - LUMINOUS objects. 

SHADOWS

Shadows are formed for two reasons. First, because some objects which are said to be Opaque, do not allow light to pass through them. Second, because light travels in straight lines.

SPEED OF LIGHT 
Proof that light travels very much faster than sound is provided by a THUNDERSTORM. The flash of lighting is seen before the thunder is heard. The speed of light has a definite value.

LAW OF REFLECTION 
The law of reflection states:

1.  The incident ray, the reflected ray and the normal all lie in the same plane. (This means that they can all be drawn on a flat sheet of paper)
2. The angle of incidence equals the angle of reflection 

PLANE MIRRORS 
When you look into a plane mirror on the wall of a room you see an image of the room behind the mirror, it is as if there were another room. The position of the image formed by a mirror depends on the position of the object.
A Real image is one which can be produced on a screen and is formed by rays that actually pass through it.
A Virtual image cannot be formed on a screen and is produced by rays which seem to come from it but do not pass through it.
The image in a plane mirror is :

• as far behind the mirror as the object is in front and the line joining the object and image is perpendicular to the mirror,
• the same size as the object, 
• virtual, 
• laterally inverted

LENSES 
Lenses are used in optical instruments such as cameras, spectacles, microscopes and telescopes; they often have spherical surfaces and there are two types,

1. A Converging (or convex) lens is thickest in the center and bends light inwards. 
2. A Diverging (or concave) lens is thinnest in the center and spreads light out. It always gives a diminished image 

PRACTICAL WORK 

Practical work : Images formed by a converging lens

Practical work : Images formed by a conerging lens

Practical Work : Images formed by a converging lens

Sound

Grade 8 Atlantic                                                                           Teacher : Ibu Nuri Martini

SOUND

Sources of sound all have some part which vibrates. A guitar has strings, a drum has a stretched skin and the human voice has vocal cords.
The sound travels through the air to our ears and we hear it. Evidently soound cannot travel in a vacuum as light can. Other materials, including solids and liquids, transmit sound.

Sound waves are reflected well from hard, flat surfaces such as walls or cliffs and obey the same laws of reflection as light. The reflected sounds forms an echo.
The speed of sound depends on the matrial through which it is passing. It is greater in solids than in liquids or gases because the molecules in a solid are closer together than in a liquid or a gas.
In the air the speed increases with temperature and at high altitudes, where the temperature is lower, it is less than at sea-level.