Module 4 - Section 2 - Refraction And Refractive Index Flashcards
What is refraction ?
Refraction is the way a wave changes direction as it enters a new medium - The change in direction is caused by the wave slowing down or speeding up
When does refraction occur ?
Refraction occurs when the medium a wave is travelling in changes
What happens if the Ray bends towards the normal ?
This means the wave is slowing down - and this is because the wave is going from a less optically dense material to a more optically dense material
What happens if the ray bends away from the normal ?
This means the wave is speeding up - this means that the wave is going from a more optically dense material to a less optically dense material
Why does the speed of a wave change when it changes mediums ?
This is because the wavelength of the wave is changing and the frequency stays constant - therefore using the equation v = f x wavelength —> the speed must change
How can you investigate refraction ?
You can investigate refraction by using a Ray box and a glass block:
1) Place a glass block on a pice of paper and draw around it
2) Using a ray box to shine a beam of light into the glass block - also turn of any other lights and close the blinds to make sure that no other light is interfering with the investigation
3) Trace the path of the incoming and outgoing beams of light on both sides of the block
4) remove the block and join up the lines that you have drawn with a straight line that follows the path the light beam took through the glass block - you should be able to see from the drawing, the path of the ray bent when entering and leaving the block
5) Then measure the angles of incidence and refraction - where the light enter and exits the block - remember that the normal line is at 90 degrees to the point where your light enters and leaves
What is the refractive index ?
The refractive index of a material measures how much it slows down light
So for e.g. light travels fastest in a vacuum and it slows down in other materials because it interacts with the different particles in them and therefore slows down - the more optically dense a material is the more light it slows down when it enters it
Whats the equation of the refractive index ?
n = c / v
n = absolute refractive index of a material
c = speed of light in a vacuum - which is just the speed of light - 3.00x10*8
v = speed of light in that specific material
Whats is Snell’s law ?
N sin theta = constant —> n = refractive index of the material where the light travels in and out of theta = the angle the light ray makes with the normal of the boundary
The real law actually means ( this is what you need to know ) :
n1 sin theta1 = n2 sin theta2 —> n2 is the refractive index of the second material that the light is crossing into and theta2 means the angle of refraction
What device can be used to measure the refractive index of a material ?
refractometer - the machine shines a beam of light at the object - then you view the refracted beam through a microscope and measure the angle of refraction
What happens when light goes from a more dense material to a less dense material ?
If you shine a ray of light at a glass to air boundary and then keep on increasing the angle of the incidence ray then eventually the angle of refraction becomes 90 degrees and this causes light to be refracted along the boundary - at 90 degrees to the normal. When this happens the angle of incidence is called the critical angle - this is where the angle of refraction is 90 degrees
This means that any angle over the critical angle for the angle of incidence will cause there to be no refraction as at any angles over the critical angles there is no refraction
Why is there no refraction over the critical angle ?
This is because all the light is reflected back into the material - this effect is called total internal reflection
For a material to air boundary at the critical angle what does snells law simplify to ?
Sin C = 1 / n
C being the critical angle —> and n being the refractive index of the material which is not the air
How can you investigate angles and total internal reflection with glass blocks ?
Shine a light ray into the curved face of a semi-circular;at glass block so that it always enters at right angles to the edge - this means that the ray wont refract as it enters the block, just when it leaves from the straight edge to
Then vary the angle of incidence until the light beam refracts so much that it exits the block along a straight edge - this angle of incidence is called the critical angle C for a glass-air boundary
If you increase the angle of incidence so its greater than C, you’ll find that the ray is reflected from the straight edge of the block
