dispersion TCA LCA

Question 1

outline dispersion (1)

blue vs red light (3)

Answer 1

separation of white light into colours according to wavelength

• refractive index of blue light > red light
• velocity in a material of blue light < red light
• blue light has shorter wavelength than red light

Question 2

how is dispersive power represented in opthalmic optics

Answer 2

represented by:

1. blue (F line) - 486.1nm
2. yellow (d line) -> mean deviation
3. red (C line) - 656.3nm

Question 3

outline V number (abbe) (2)

Answer 3

V number is used to describe the amound of dispersion by a material

• inverse of dispersive power

Question 4

identify the 6 digit specification of optical materials (2)

Answer 4

• first 3 digits give the first 3 decimals of the refractive index
• last 3 digits give the Abbe number without the decimal point before the last digit

eg: 617363
n = 1.617
V number = 36.3

Question 5

distinguish between chromatic and monochromatic (spherical) aberrations (2v1)

Answer 5

chromatic:

• when using more than one wavelength of light (dispersion)
• wavelength varies by refractive index
• transverse (TCA) or longitudinal (LCA)
• effect is increased by distance from optical centre

monochromatic:

• single wavelength of light
• increased refraction of rays as a function of radial distance from the optical centre
• occurs for incident light parallel to the optical axis

Question 6

distinguish between LCA and TCA

Answer 6

LCA:

• the dioptric interval between the foci for the different wavelengths
• occurs when the lens can’t focus different colours in the same focal plane -> the foci of the different colors lie at different points in the longitudinal direction along the optical axis.

TCA:

• the difference in the prism power for blue (F line) and red (C line)
• caused by obliquely incident light where all colors are in focus in the same plane, but the foci are not placed along the optical axis.

Question 7

which type of chromatic aberration is the human eye more affected by (2)

Answer 7

TCA

• lower tolerance to TCA (0.1 prism dioptre)
• human vision is not disturbed by LCA (1.00D)

Question 8

which type of aberration does the duochrome test rely on

Answer 8

longitudinal chromatic aberration

Question 9

what type of aberration may appear in the interface between zones of higher power multi-focal lenses? (2)

Answer 9

tranverse chromatic aberrations

• TCA may result in the formation of multicoloured images of an object side by side