9.2 Lens Reflections and Coatings

How does refractive index affect reflection?

higher the index, the more light reflected

How does refractive index affect transmission?

higher the index, the less light transmitted

What can reflected light cause?

ghost images

falsification of image position

haze + glare

loss of contrast

increased presence of lenses

Why does a reflected image appear more blue?

index of refraction increases toward blue end of spectrum (shorter λ), therefore the reflected image contains more blue light

Why does transmitted light appear more yellow?

index of refraction increases toward blue end of spectrum (shorter λ), therefore the reflected image contains more blue light and transmitted light has less blue and appears yellow

How do segments affect reflections?

flat top or trifocals wearers may see ghost images + glare from seg top reflections

How do we decreased seg reflections?

add absorptive coating to seg top

How do we control reflections?

base curve

change pantoscopic tilt

change vertex distance

smaller lens size

AR coatings

Where are AR coatings placed?

on both sides of lens

What do AR coatings do?

decreased intensity of reflections and increase transmission

How do AR coatings work?

via interference

What is the amplitude condition for AR coating?

two reflective waves must be of equal amplitude

When is amplitude condition for AR coating true?

when reflective factors are the same for each surface

What material do we use for AR coating? Does this meet amplitude condition?

use MgF2 , does not meet amplitude condition

What is the path condition for AR coats?

two reflective waves must be 1/2λ out of phase

What does the path condition for AR coat do?

determines the thickness of coating

What λ do we use for path condition for AR coat? why?

555nm for max visibility

Best thickness for an AR coating is what?

optical thickness (any odd multiple of it)

What does a single AR coat look like?

appears purple

What does a single AR coat look like if it is too thin?

amber color

What does a single AR coat look like if it is too thick?

pale blue color