Snells law and critical angles

Question 1

what is the equation to use snell law to find critical angles

Answer 1

sin C = 1 / n

n is the refractive index of the material

The higher the refractive index, the lower the critical angle. For water, C is 49°.

Question 2

how does optical fibres use total internal reflection?

Answer 2

1) Optical fibres (see page 56) that are made of plastic or glass consist of a central core surrounded by cladding with a lower refractive index.
2) The core of the fibre is so narrow that light signals passing through it always hit the corecladding boundary at angles higher than C — so the light is always totally internally reflected.
3) It only stops working if the fibre is bent too sharply.

Question 3

how does Prisms use total internal reflection?

Answer 3

1) Total internal reflection also allows us to use prisms to see objects that aren’t in our direct line of sight. This is how a periscope works.
2) The ray of light travels into one prism where it is totally internally reflected by 90°.
3) It then travels to another prism lower down and is totally internally reflected by another 90°.
4) The ray is now travelling parallel to its initial path but at a different height.

Question 4

Define critical angle?

Answer 4

• Angle of incidence beyond which rays of light are totally internally reflected

Question 5

Define total internal reflection?

Answer 5

• When the angle of incidence is greater than the critical angle
• All light is reflected, none is refracted

Question 6

How do optical fibres work?

Answer 6

• Total internal reflection
• Core of fibre made out of glass with high refractive index
• Cladding is made out of glass with lower refractive index
• Light entering core is at an angle greater than the critical angle so light is TIR

Question 7

Give examples of the use of optical fibres?

Answer 7

• Endoscope - used in keyhole surgery
• Communication as less energy is lost when compared to using copper wiring