P5 - Two Special Cases Of Refraction

Question 1

Why are different colours of light refracted by different amounts?

Answer 1

• Different colours of light are refracted by different amounts.
• This is because they travel at slightly different speeds in any given medium ( but the same speed in a vacuum ).

Question 2

What is the refractive index of a medium?

Answer 2

• The refractive index of a medium is the ratio of speed of light in a vacuum to the speed of light in that medium.
• Any material has a different refractive index for each different speed.

Question 3

What happens to red light when it travels from air into glass?

Answer 3

• Red light slows down the least when it travels from air into glass, so it is refracted the least and has the lowest refractive index.

Question 4

What happens to blue light when it travels from air into glass?

Answer 4

• Blue light has a higher refractive index ( 1.523 ) so it is refracted more.

Question 5

What can a prism be used for?

Answer 5

A prism can be used to make the different colours of white light emerge at different angles.

Question 6

What is the effect of dispersion?

Answer 6

• A prism can be used to make the different colours of white light emerge at different angles.
• This produces a spectrum showing all the colours of the rainbow, this effect is called dispersion.

Question 7

When does total internal reflection happen?

Answer 7

• Total internal reflection only happens when light travels from a more dense medium with a higher refractive index to a less dense medium with a lower refractive index ( e.g. from glass to water ).
• The ray of light won’t come out all, but it will reflect back into the glass, this is total internal reflection.

Question 8

What happens if the angle of incidence is larger enough ( greater than the critical angle )?

Answer 8

• The ray of light won’t come out all, but it will reflect back into the glass, this is total internal reflection.

Question 9

What happens if the angle of incidence is less than the critical angle?

Answer 9

• If the angle of incidence is less than the critical angle, most of the light passes through into the air, but only a bit of it is internally reflected.

Question 10

What happens if the angle of incidence is equal to the critical angle?

Answer 10

• The emerging ray travels along the surface, there is a bit of internal reflection.

Question 11

What is the correlation between refractive index and critical angle?

Answer 11

• Different materials have different critical angles.

* The higher the refractive index of the medium, the lower the critical angle will be.

Question 12

What is the critical angle for glass, and why is it useful?

Answer 12

• The critical angle for glass is 42 degrees.
• This is useful because it means 45 degree angles can be used to get TIR, as in optical fibres, prisms in binoculars, reflectors, road signs and cat eye son roads.