# B Reflections of Light in Contact lens

1. May 2, 2016

### cube137

It is said that whenever index of refraction varies, there are reflections and it can reach 4% for uncoated surfaces.. how about in thin contact lens used for eyes.. is there also 4% reflection on each surface if they are uncoated?

2. May 2, 2016

### Simon Bridge

The short answer is "yes". The exact reflection depends on the material the contact is made out of.
examples: http://www.ncbi.nlm.nih.gov/pubmed/3231673

But can contact lenses be uncoated? i.e. what about the liquid on your eyes, does that coat the contact lens?

3. May 3, 2016

### Andy Resnick

It depends on what the contact lens is made of- there are lots of 'flavors':
http://www.allaboutvision.com/contacts/faq/contact-materials.htm

And as Simon Bridge points out, there is a tear film that cover the lens when it is in your eye, so the refractive index mismatch is fairly small.

4. May 3, 2016

### Simon Bridge

Tear refractive index between 1.334 and 1.336 (±0.0011) depending on salt content.
http://www.ncbi.nlm.nih.gov/pubmed/8570161

Refractive index of "soft" contact lenses about 1.412 to 1.424 (±0.0005 typ uncertainty) depending on material.
http://www.ncbi.nlm.nih.gov/pubmed/20009942

Maybe there is a thin-film interference question in there?
The lens has tear-water either side of it about 2.3$\mu$ m thick.
http://www.otticamartini.it/wp-content/uploads/2014/03/68.full_.pdf
... the hydrogel contacts in the above paper are typically 5-6$\mu$ m thick.

Unsurprisingly these things are studied in painstaking detail.

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