Unit 3 - Lesson 7: Critical Angle & Total Internal Reflection Flashcards
Snell’s law only applies to refraction to a denser material. True or false?
Snell’s law only applies to refraction to a denser material. True.
When light is refracted to a less dense material, what happens to the angle of refraction if you keep increasing the angle of incidence?
When light is refracted to a less dense material, if you keep increasing the angle of incidence, the angle of refraction will get closer and closer to 90 degrees.
Eventually it reaches a critical angle (C) for which r = 90 degrees.
When light is refracted to a less dense material, if you keep increasing the angle of incidence, the angle of refraction gets closer and closer to 90 degrees. Eventually it reaches a critical angle (C) for which r = 90 degrees. At this point, the light is reflected in line with what?
At this point, the light is reflected in line with the boundary. In glass, for example, it would skim the edge of the glass.
Above the critical angle (C), what type of reflection do you get?
Above this critical angle (C) you get total internal reflection, where the light can never escape.
https://i.pinimg.com/736x/ce/fe/78/cefe78fb2ffc43fe457f80d27a7a591d.jpg
What is the principle of total internal reflection used in?
The principle of total internal reflection is used in fibre optic cables, where information is transported via light. This is the principle of fibre optic internet.
What are other uses for total internal reflection?
Periscopes, binoculars & the cats eye reflectors on the roads.
What can we use to demonstrate total internal reflection?
We can demonstrate total internal reflection by using a semi-circular block and a ray box.
What is the equation of an adapted Snell’s law, which we can use to find the critical angle?
sin C = 1/n
How do we rearrange Snell’s law to find refractive index?
sin C = 1/n
sin C x n = 1/n x n
sin C x n = 1
sin C x n/sin C = 1/sin C
n = 1/sin C
How do we rearrange Snell’s law to find the critical angle of a material?
sin C = 1/n
C = sin^-1 (1/n)
What is the critical angle for water?
49 degrees
What is the critical angle for glass?
42 degrees
When the angle of incidence equals the critical angle, the angle of refraction will be… (complete the sentence)
90 degrees
What is the refractive index of glass to 2sf?
1.5
We can only find the critical angle of a material if we take measurements from an incident ray moving from a (less/more) dense to a (less/more) dense material.
We can only find the critical angle of a material if we take measurements from an incident ray moving from a more dense to a less dense material.
What does the > symbol mean?
Greater than
r = 90 degrees
n = ?
C = 30 degrees
sin C = 1/n
sin C x n = 1
n = 1 / sin C
n = 1 / sin (30)
n = 2
What materials will we need for the practical: use a semi-circular block to show total internal reflection?
Semi circular glass or acrylic block.
Protractor.
Ruler.
Pencil.
Paper.
Ray Box.
What are the instructions for the practical: use a semi-circular block to show total internal reflection?
- Cover the end of your torch with card. The card must have a small 1-2mm hole in to let light out. The more powerful the torch, the better.
- Place the semi-circular block on to the paper and draw around it with a pencil.
- Place the ray box or torch on the surface directed at the curved edge of the semi-circular block.
- Mark the positions of the rays on the paper with a pencil.
- Use a protractor to measure the angle of refraction with different angles of incidence. Fill in the table.
- Try to find the angle of incidence that results in an angle of refraction of 90 degrees. This is the critical angle of the glass block. What happens when you increase the angle of incidence past this? The ray should no longer be able to refract. It’ll reflect back into the block.
What materials will we need for the practical: use a rectangular glass block to show double refraction?
Rectangular glass or acrylic block.
Protractor
Ruler
Pencil
Paper
Ray Box
What are the instructions for the practical: use a rectangular glass block to show double refraction?
- Cover the end of your torch with card. The card must have a small 1-2mm hole in to let light out. The more powerful the torch, the better.
- Place the rectangular block on to the paper and draw around it with a pencil.
- Place the ray box or torch on the surface directed at the block.
- Mark the position of the rays entering and leaving the block on the paper with a pencil.
- Remove the block and connect the two rays. This’ll not be easy unless you aim to get as much refraction as possible. The change in direction can be very subtle.
- Use a protractor to measure the angle of incidence and angle of refraction. Remember, these are the angles between the ray and the normal.
- Calculate the refractive index (n) of the block using Snell’s law.
- Check your answer. It should be around 1.5.
What materials do you need for the practical: use a triangular prism to investigate refraction?
Triangular prism.
Protractor.
Ruler.
Pencil.
Paper.
Ray Box.
What are the instructions for the practical: use a triangular prism to investigate refraction?
- Direct the ray from the ray box at the prism as shown (in the top image). It’ll hit the surface at an angle of 45 degrees. As the critical angle of the glass is 42 degrees, the ray will be totally internally reflected.
https://i.pinimg.com/736x/80/3c/cd/803ccd915a0e0ab92e25161eaab5097a.jpg - Now shine the ray towards the prism as shown (in the bottom image). It’ll be reflected through an angle of 180 degrees.
