- When light enters a glass block, it slows down, causing it to change direction
- When it leaves the block it speeds up again, changing direction once more
- As the light enters the block it bends towards the normal line
(Remember: Enters Towards)
- When it leaves the block it bends away from the normal line
(Remember: Leaves Away)
Exam Question: Refraction
- In your examination you might be asked to write a method explaining how you might investigate the refraction of light through different shaped blocks
- As part of this method you should describe:
- What equipment you need
- How you will use the equipment
- How you will trace the rays of light before, while and after they pass through the block
- Place the glass block on a sheet of paper, and carefully draw around the block using a pencil
- Take a ray box and carefully aim the box so that a single ray of light passes through the block
- Using a pencil, mark some points along the path of the ray:
- Before it reaches the block;
- Where it hits the block;
- Where it leaves the block;
- After it has left the block.
- Now remove the block from the paper and, using a ruler and pencil, draw straight lines connecting points: a and b; b and c; c and d. The resulting line will show the path of the ray
- Replace the block within its outline and repeat the above process for a ray striking the block at a different angle
Key things to remember include:
- Naming the apparatus that you need (remember the ray box)
- Explaining how to trace the rays
- When light enters a denser medium (such as glass) it slows down and bends towards the normal
- Snell’s law gives the relationship between the angle of incidence i, and the angle of refraction r:
- Where n is the refractive index of the material
- You can rearrange this equation with the help of the formula triangle:
- The refractive index is related to the speed of light in the material (which is less than its speed in a vacuum):
- The refractive index is a number that is always bigger than 1 and is different for different materials (n is about 1.5 for glass)
Important: (sin i / sin r) is not the same as (i/r). Incorrectly cancelling the sin terms is a common mistake.
When calculating the value of i or r start by calculating the value of sin i or sin r.
You can then use the inverse sin function (sin-1 on most calculators) to find the angle.