P6 Refraction (page 221) Flashcards
Grab a glass of water and put a straw in it. The straw looks like it’s bending, what is this called?
Refraction.
Refraction is what?
waves changing direction at a boundry.
When a wave crosses a boundary between two materials whap happens?
it changes speed.
If the wave is travelling along the normal it will change speed, but is it refracted?
No its not refracted.
If the wave hits the boundary at an angle it changes direction, is it refracted?
Yes
The wave bends towards the normal if it slows down. It bends away from the normal if what?
if it speeds up.
How much it’s refracted by depends on what?
how much the wave speeds up or slows down, which usually depends on the dinsity of the two materials (using the higher the density of a material, the slower a wave travels through it).
What is the opitcal dinsity of a material?
it is a measure of how quickly light can travel it - the higher the optical density, the slower light waves travels through it.
The wavelength of a wave changes when it is refracted, but what happens with the frequency?
it stays the same.
What are Rays?
they are straight lines that are perpendicular to wave fronts. They show the direction a wave is travelling in.
Ray diagrams show the path of a wave.
How can you construct a ray diagram for a refracted light ray?
1) First, start by drawing the boundary between your two materials and the normal. the normal is an imaginary line that’s perpendicular (at right angles) to the point where the incoming wave hits the boundary.
2) Draw an incident ray that meets the normal at the boundary
3) The angle between the incident ray and the normal is the angle of incidence. {If you’re given this angle, make sure to draw it carefully with a protractor).
4) Now draw the refracted ray on the other side of the boundary.
5) The angle of refraction is the angle between the refracted ray and the normal.
6) If the second material is optically denser than first, the refracted ray bends towards the normal (see diagram on page 221) and the angle of refraction is smaller than the angle of incidence. If the second material is less optically dense, the angle of refraction is larger than the angle of incidence.
You can also explain refraction using wave front diagrams.
What is a wave front diagram?
it is a line showing all of the points on a wave that are in the same position as each other after a given number of wavelengths.
When a wave crosses a boundary at an angle, what happens?
only part of a wave front corsses the boundary at first. If it’s travelling into a denser material, that part travels slower than the rest of the wave front.
so by the time the whole wave front crosses the boundary, the faster part of the wave front will have travvelled further than the slower part of the wave front. see diagram on page 221.
(this difference in distance travelled (caused by the difference in speed) by the wave front causes the wave to bend (refract).
The wave fronts being closer together shows what in diagram 2 on page 221?
it shows a change in wavelength (and so a change in velocity).
On diagram 3 page 221, it shows the part of the wave front slower than the rest, what do this mean?
this part of the wave front will have travelled further by the time it crosses the boundary.
(refraction is a common behaviour of waves, so make sure you really understand it before moving on).
