Science 8th grade
UNIT 12: Light 12.4 How light bends
Science 8th grade
UNIT 12: Light 12.4 How light bends
The photograph shows a pencil standing in a beaker of water. The pencil looks as if it is broken at the point where it enters the water.
You may have noticed other strange effects when looking through water or glass-a river or pool may look shallower than it really is, for example.
Transparent materials
Water and glass are transparent materials. Light can pass through them, but something happens when light enters or leaves such a material.
We need to follow the path of a ray to see what happens. The photograph shows how a ray of light bends when it enters a glass block. It bends again when it leaves the block.
The change of direction of a light ray when it enters or leaves a transparent material is called refraction.
Question
1) Look at the photograph of the ray of light travelling through the glass block. Where does the ray bend? What path does it follow inside the glass block?
Rays changing direction
The diagram shows how someone can see a coin lying at the bottom of a pool of water. How does a ray of light travel from the coin to the person's eye?
The ray travels in a straight line to the surface of the water. This is where it bends.
The straight ray then travels to the eye.
To understand how it bends, we draw the normal to the surface at the point where the ray bends. The normal is drawn at ${90^ \circ }$ to the surface.
We can see that the ray bends away from the normal as it leaves the water.
Question
2) Look at the photograph above which shows the light ray passing through the glass block.
a: Find the point where the ray leaves the glass. Does it bend away from the normal?
b: Which way does the ray bend when it enters the glass?
Activity 12.4 (Refracted rays)
The diagrams show how a ray of light bends when it enters a glass block.
Check that these diagrams show correctly what happens when a ray of light is refracted by glass.
If a ray enters the glass along the normal, it does not bend. Is this correct?
If a ray enters the glass at bigger angle, it bends more. Is this correct?
(Take care! The angles are measured between the ray and the normal.)
The hidden coin
The picture shows a trick you can perform that makes use of refraction. A coin is placed in the bottom of a can. Move your head so that the coin is just hidden behind the rim of the can.
Now ask someone to carefully pour water into the can. The coin appears!
Question
$A+I$: 3) Use the idea of refraction to explain why the coin can be seen when there is water in the can.