Convex and Concave Lenses
Transparent plastic or glass materials that cause light rays to be refracted or bent are called lenses.
A concave lens is curved inwards. It is thin in the centre and thick at the edges. A concave lens makes objects appear smaller. When light rays pass through a concave lens, they spread out or diverge.
This diagram shows how light rays behave when they pass through a concave lens.
A convex lens is curved outwards. Similar to the lens in a magnifying glass, a convex lens is thicker in the middle than at the edges. It causes light rays to bend inwards until they come together at a point called the focus (focal point). The focal point is the distance from the lens to the point where light rays converge to form an image. Convex lenses form small upside down images. There is a natural convex lens in each of our eyes. The lens in each eye focuses light on the part of the eye that collects images. The diagram below shows how light rays behave when they pass through a convex.
Light rays and an object passing through a convex lens.
Note: the light rays bend before passing through the convex lens.
Instruments like telescopes and binoculars use combinations of lenses to make distant objects appear near. A combination of lenses is used in a microscope to make small things look big.
The Spectrum of White Light
When you use soapy water and a straw to blow some bubbles, you see colours in them. These are the colours of the rainbow. These colours are also seen when a drop of oil is placed in water. These seven (7) colours always appear in this order:
Red, orange, yellow, green, blue, indigo and violet. This group of colours is called the visible spectrum or the spectrum of white light. A scientist named Isaac Newton discovered that light is made up of all the colours of the spectrum. He discovered that when white light passes through a wedge-shaped glass (prism) it is separated into seven colours. They are the seven colours of the rainbow.
How are Rainbows Formed in the Sky?
. Straight light rays of white light from the Sun enter or go through rain drops at an angle.
. On entering the rain drops (at an angle), the straight light rays are bent or refracted.
. The smooth shine surfaces of the rain drops act like a mirror and reflect the light rays which break up or spread out into the colours of the rainbow. These colours are called the visible spectrum.
Breaking Up Light into Colours
We can break up light into the seven colours of the rainbow (the visible spectrum) by using any one of these: a soap bubble, a water spray or a drop of oil.
Separating the Colours that Make up Black
You can separate the colours making up black by using a black marker to make a small black dot a little above one end of a strip
Of white paper. If you hold the end of the strip in a container with water, you will notice that the paper absorbs the water. The absorbed water travels to the black dot which separates into all the colours of the visible spectrum-red, orange, yellow, green, blue, indigo and violet. This occurs because black absorbs all the colours in the spectrum.