Science 8th grade
UNIT 12: Light 12.2 How shadows form
Science 8th grade
UNIT 12: Light 12.2 How shadows form
When light strikes an object, different things can happen. It depends on the material the object is made of.
• The light may pass straight through the object. We say that it has been transmitted The material is transparent.
• The light may be absorbed by the object.
(The object gets a little warmer.) The material is opaque.
• The light may bounce off the object. We say that it has been reflected.
The black plastic is opaque, and absorbs light.
The car windscreen is transparent.
The metal is shiny and reflects light.
Sometimes two of these things can happen, or all three. For example, glass transmits light but it may also reflect some of it.
In the shade
On a sunny day, you may want to stay in the shade. You need to find somewhere where the Sun's hot rays cannot reach you.
A shadow forms when light from a light source is blocked by an opaque object. The area where there is a shadow is darker because less light is reaching it.
Forming a shadow
A shadow forms when an opaque object blocks the light. It forms because light travels in straight lines - it cannot bend round corners.
The diagram shows one way to understand this. A pencil stands in front of a white card screen.
When light from a torch is directed at the pencil, the shadow of the pencil appears on the screen.
Questions
1) The pencil has a shadow. Is the pencil opaque or transparent?
2) A shiny object reflects light. Will a shiny object make a shadow when light shines on it? Explain your answer.
Rays of light
When we want to understand light, we often draw diagrams with straight lines, showing the paths of the light rays.
To understand where the tree's shadow will fall, we need to draw lots of rays of light, all coming from the Sun. Then we can see which rays are blocked by the tree. This will show us where the tree's shadow will be.
Questions
3) Explain why we draw straight lines to show rays of light.
$A+I$: 4) Look again at the diagram of the tree and its shadow. Later in the day, the Sun will move round so that it is shining from the top right-hand corner of the diagram. Redraw the diagram with the Sun's rays coming from the top right and work out where the tree's shadow will be.
Activity 12.2 (Shadows large and small)
Shadows change during the day as the Sun moves across the sky. Their direction changes and their size changes.
Look at the photograph of the lemurs. They have long tails, but the shadows of their tails are even longer. How can this be?
Explain why shadows change their size, according to the Sun's position in the sky. Here are some things you might use to investigate this:
• a toy animal or doll
• a bright light
• a ruler, pencil and protractor
• a sheet of paper.
You could demonstrate your ideas to the rest of the class. You could make diagrams to show your ideas.