2.2 Refraction

Question 1

What is refraction?

Answer 1

Refraction is the change of direction a light ray undergoes when it enters a medium with a different refractive index.

Question 2

What are the 2 requirements for refraction?

Answer 2

• Ray passes into a material with a different refractive index.
• At an angle of incidence > 0֯.

Question 3

What is Snell’s law?

Answer 3

At the boundary between two media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction (sin i / sin r) is constant.

Question 4

What happens when white light enters a prism?

Answer 4

The different colours have different wavelengths. As white light enters the prism and refracts the frequency of each colour stays constant and the wavelength decreases.

Question 5

What happens during the dispersion of light?

Answer 5

• When the wavelength is smaller (e.g. violet light), the angle of refraction will be smaller (bends most). Therefore, it will refract closer to the normal.
• When the wavelength is larger (e.g. red light), the angle of refraction will be larger, so the ray will be further from the normal (bends the least).

Question 6

What is meant by the critical angle?

Answer 6

The angle of incidence for which the angle of refraction is 90°

Question 7

What is total internal reflection?

Answer 7

For any angles of incidence greater than the critical angle, all of the light gets reflected.

Question 8

What are the 2 conditions for total internal reflection?

Answer 8

1. The wave must be travelling from a more optically dense medium to a less optically dense medium (e.g. from glass to air, not the other way around).
   (higher refractive index medium to a lower refractive index medium)
2. Angle of incidence > critical angle.

Question 9

What is the structure of Monomode fibre optic cables?

Answer 9

• Monomode fibres only use one mode of light (essentially one wavelength/one path), therefore very little dispersion.
• They are expensive and difficult to make.
• Sharp change in refractive index at core cladding boundary.

Question 10

What is the structure of Multimode/Step-Index fibre optic cables?

Answer 10

• Relatively wide core, thus ‘light’ has various paths.
• As a result, different paths take different lengths of time. This leads to dispersion of the signal.
• Sharp change in refractive index at core cladding boundary.

Question 11

What are the uses of total internal reflection in industry?

Answer 11

• Fibre optic cables
• Endoscopes

Question 12

How do flexible endoscopes use fibre optic cables?

Answer 12

1. Deliver non-coherent light into the body.
2. Take coherent light from the body back to an eyepiece to form a clear image.

Question 13

What is coherency?

Answer 13

The fibres going from the body / object to eye or computer need to be coherent. It means they have the Same Spatial Relationship.
A coherent bundle means that the force ends at each end must be in the same relative position.

Question 14

What happens during the use of flexible endoscopes?

Answer 14

• Incoherent light travels down a bundle of optical fibres to illuminate the inside of a patient.
• The light reflects off the surfaces inside the body.
• Coherent bundle is used to produce images by using fibres with the same spatial relationship.
• This bundle is connected to an eyepiece/camera for observation.
• Control cables allow view to be manipulated/tools to be operated (eg cutting samples).
• Often tubes present to facilitate irrigation.