All notes to be written and where possible print pictures and paste in your notebooks with your notes.
What is an Eclipse?
An eclipse of the moon or a lunar eclipse occurs when the Earth blocks out the light of the Sun and casts its shadow on the Moon.
A solar eclipse occurs when the Moon blocks out the light of the Sun from parts of the Earth, and casts a shadow on a part of the Earth.
Reflection of Light
Objects like mirrors reflect or bounce back light. The bouncing back of light is called reflection. We can see all objects because light rays bounce off their surfaces and travel towards our eyes.
Some materials reflect light very well and are called good reflectors. These materials have a smooth, shiny or glossy surface, for example, a shiny metal or mirror. When straight light rays hit against these objects, all the rays bounce back in a straight line in one direction only.
Rusty metal, cloth, and carpet are examples of poor reflectors. When straight light rays hit against the surface of a poor reflector, the rough bumps bounce or reflect light in all directions.
|Lighter colours reflect more light, and darker colours absorb more light|
Absorption of light
Look at this picture of a ladybird. Why is it red and black? And why is the leaf so green? How do we see the different colours? It all has to do with what happens when light hits a surface.
When light hits a surface, some of the light is absorbed and the rest is reflected. It is the reflected light that reaches our eyes and allows us to see the object. Previously, we learned that white light is a mixture of different colours. When white light from the Sun hits the red shell of the ladybird all of the colours are absorbed, except red. Red light is reflected back to our eyes and so we see a red ladybird.
The green leaf absorbs all the colours except green which it reflects back into our eyes.
What about the black spots of the ladybird? Is black a colour? The black spots on the ladybird absorb all the colours and no light is reflected. That is why they appear black.
Do you remember learning about heat as energy transfer in Gr 7? We looked at the absorption of heat. We saw that black, matt objects absorbed all of the light energy, while white objects reflected all of it. Black, matt (not shiny) objects absorb all of the colours of light and reflect none and so appear black to our eyes.
What about a white object? Why do you think white objects look white? Have a look at the following diagram for a clue.
Light travels at different speeds through different materials. It travels faster in gases than in liquids, and faster in liquids than in solids. Light slows down when it goes from a gas to a liquid. It speeds up when it moves from a solid to a liquid or from a liquid to a gas. When light rays move from one matter to the next, their speed change. This causes the light rays to bend. The bending of light rays is called refraction. Light rays are refracted only if they enter at an angle into the new material.
The diagram shows light rays going from air to water. When light travels from one substance to another, air to water in this case, it changes speed and the light rays are usually bent.
This figure shows a coin in a bowl. Notice that the eye cannot see the coin.
Figure 2 shows the container with water. The coin can now be seen because light from coin passed from the water into the air and is refracted towards the eye.