Index of refraction and focal length

[pm1]

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How does the index of refract affect the focal length of a lens???

Thanks!

 

[gettheleadout]

Think about what the index means. It's a measure of how refractive a material is, meaning a measure of how much it bends light, right? So if we have a convex lens, we expect it to focus incoming parallel rays to a focal length on the transmitted side. The lens material refracts the light inward. If we increase the refractive index, the lens will bend the light more. Considering which direction the light is being bent, what do you think will happen to the focal point?

You can try it yourself to see, here: http://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html

 

[FeinMS]

Quantitative method: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenmak.html

I don't think we're expected to know this though.

 

[gettheleadout]

Quantitative method: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenmak.html

I don't think we're expected to know this though.

Nah, lens maker's equation isn't required knowledge, but a qualitative question on a change of n is definitely something everyone should be able to answer.

 

[pm1]

Think about what the index means. It's a measure of how refractive a material is, meaning a measure of how much it bends light, right? So if we have a convex lens, we expect it to focus incoming parallel rays to a focal length on the transmitted side. The lens material refracts the light inward. If we increase the refractive index, the lens will bend the light more. Considering which direction the light is being bent, what do you think will happen to the focal point?

You can try it yourself to see, here: http://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html

ooh I see..

so by increasing the index of refraction, the light will be more bent, which is the same as decreasing my focal length (since f is where the light must intersect in the other side of the lens).

bigger n = smaller f ?

thank you!

one more question, then why doesn't n affect a mirror's focal length? is it because the light is not going through a different medium and potentially refracted, but being reflected instead?

 

[gettheleadout]

ooh I see..

so by increasing the index of refraction, the light will be more bent, which is the same as decreasing my focal length (since f is where the light must intersect in the other side of the lens).

bigger n = smaller f ?

thank you!

one more question, then why doesn't n affect a mirror's focal length? is it because the light is not going through a different medium and potentially refracted, but being reflected instead?

Yep! We only consider the reflective properties of thin mirrors, not whatever refractive properties exist at the surface interface (though I imagine you could model this as a thin film at the glass-silver surface). For MCAT purposes, a mirror has no index of refraction.