Optics

Question 1

What is optics

Answer 1

Branch of physics which describes the behaviour of light, instruments that use detect and manipulate light, and its interactions with other forms of matter

Question 2

Define the following

1. The “normal”
2. Angle of incidence
3. Angle of reflection
4. Angle of refraction

Answer 2

1. The “normal” - perpendicular line to interface of two media
2. Angle of incidence - Angle between incident ray and normal
3. Angle of reflection - Angle between reflected ray and normal
4. Angle of refraction - Angle between refracted ray and normal

Question 3

When does angle of incidence = angle of reflection

Answer 3

Perfectly smooth interface between two media of different density

The reflected rays reflect with random angles off a rough interface

Question 4

Define Snell’s law

Answer 4

Snell’s law is the law of refraction which predicts the angle of refraction when a ray of light passes from one medium to another. The amount of refraction is dependent on the two indices of refraction determined by the different densities of the media. When light passes from low density to high density media, light will bend towards the normal and vice versa.

n1 sin theta 1 = n2 sin theta 2

n1 is density of medium 1
theta 1 is the angle of incidence

n2 is the density of medium 2
theta 2 is angle of refraction

Question 5

What is the critical angle of incidence and why is it relevant in fibreoptic scopes

Answer 5

The critical angle of incidence is the angle at which all of the light is reflected and none passes through into the second medium. I.e. there is no angle of refraction.as light is not refracted, only reflected.

sin critical angle theta = n2/n1

This concept is applied in optic scopes to ‘pipe’ light down a curved path. Light repeatedly reflects of a fibre air interface at a large incident angle larger than the critical angle.

Question 6

What is the difference between the optic bundles that transmit light toward the patient and those that receive light from the patient and return it to eye piece in a fibreoptic scope.

Answer 6

Toward patient - non-ordered (incoherent) bundles used
- cheaper and easier to manufacture

Back to eye piece - Ordered (coherent) optic bundles

• expensive
• 5000 - 40 000 fibres aligned so that the position of the fibres is preserved, each fibre generating an element of the image much like pixels on a computer screen

Question 7

What is the difference between a fibreoptic scope and a ‘chip in tip’ endoscope

Answer 7

Fibreoptic scope contains ordered bundles which are difficult to manufacture, expensive and easily damaged

Chip-in-tip endoscopes contain small video camera sensors in the tip of the scope while the light source and non-coherent bundles have been replaced with one or more light emitting diodes at the end of the scope