Light waves Flashcards
What kind of waves are light?
Transverse, and they can be reflected and refracted
What is the law of reflection?
angle of incidence = angle of refraction
Practical: investigate the refraction of light, using rectangular blocks, semi-circular blocks and triangular prisms
- Shine a LIGHT RAY at an angle into the block of glass on top of a piece of paper. Some of the light is reflected, but a lot of it passes through the glass and gets refracted as it does so.
- TRACE the incident and emergent rays onto the piece of paper and remove the block.
- You should see that as the light passes from the air into the block, it bends TOWARDS the normal. This is because it slows down.
- When the light reaches the boundary on the other side of the block, it’s passing into a less dense medium. So it speeds up and bends AWAY from the normal.
- The light ray that emerges on the other side of the block is now travelling in the SAME DIRECTION it was to begin with - it’s been reflected towards the normal and then back again by the same amount.
What happens to the light when it enters an optically denser medium?
The light slows down and bends towards the normal
What happens to the light when it enters an optically less dense medium?
The light speeds up and bends away from the normal.
What is the refractive index formula?
n = sin i / sin r
What is another formula for the refractive index?
refractive index = speed of light in a vacuum / speed of light in a specified medium
n = c/v
Practical: Investigate the refractive index of glass, using a glass block
- Draw around a rectangular glass block on paper and direct a RAY OF LIGHT through it at an angle.
- TRACE the incident and emergent rays, remove the block, then draw in the refracted ray between them
- You need to draw in the normal at 90 DEGREES to the edge of the block, at the point where the ray enters the block.
- Use a PROTRACTOR to measure the angle of incidence and the angle of refraction.
- Calculate the REFRACTIVE INDEX using SNELL’S LAW.
What is total internal reflection?
When the angle of incidence is larger than angle C and all of the light is reflected back into the glass.
What is the critical angle?
The angle at which the ray is refracted along the surface of the block.
How do optical fibres use total internal reflection?
- Optical fibres made of plastic or glass consist of a central core surrounded by cladding with a lower refractive index
- The core of the fibre is so narrow that light signals passing through it always hit the core-cladding boundary at angles higher than C - so the light is always totally internally reflected.
- It only stops working if the fibre is bent too sharply.
How do prisms use total internal reflection?
- The ray of light travels into one prism where it is totally internally reflected by 90 degrees.
- It them travels to another prism lower down and its totally internally reflected by another 90 degrees.
- They ray is now travelling parallel to its initial path but at a different height.
What is the relationship between the critical angle and refractive index?
sin C = 1/n
