Australia's Science Forum

Ok guys need a little help here so i dont have to get equipment out to work it out myself......

with lenses - concave is the focal length longer or shorter with the thickness of the lens
and
same question with Convex lenses...


i trust i have not confused the question.

thanks in advance for any help

Focal length will be shorter in lenses with a greater curvature of the surface. In many cases the thicker ones will therefore have a shorter focal length.

Hi All,

Just to reiterate what worktofish has mentioned..


The thicker the lens whether it be Concave or Convex will have a much shorter Focal Length this is because the light coming into the lens (Incident Ray) will have a much more depth of solid material to travel through. And as this light (Refracted Light) path exits the lens it continues parallel to the incident ray initially entering the lens.

The space between the incident Ray and the Refracted Ray will be considerably larger (Although Parallel) hence the gap after exiting will be smaller hence the Focal Point will merge a lot closer.

If the lens was thinner, the Incident Ray and Refracted Ray will be parallel but much closer together as there is less material for the light to travel through (Slow down in) hence the incident Ray will merge in the centre as the Focal Point Further away.

As mentioned above this same thing happens Convex lenses as well it is just a little harder to measure as the reflected rays coming from partial reflection off the lens will be scattered and difficult to measure but all-in-all still measurable by all means.

I hope this has not been too confusing.

Cheers,

Robb...

Hey guys thanks for your help - makes stock take a lot easier

Hi

I addition to this question about focal lengths. I have a heap of lenses that the focal length is not known and before I update our stock wanted to see what we actually had. Is there a simple method to determine this?

I'm sure there is but as it is a long time since I did any physics I can't remember the procedure.

Cheers
Nikki

Hi Nikki,

The best option is to shine light through each lens and measure each Focal Length from the surface of the lens to the Foc. Pt.

Hope this helps,

A bit of physics to get you on the way....

Cheers,

Robb....

Hey Robb I think you were confused on first post, ( refracted light parallel to incident light) is for prism. for lens
Light passing from less dense to more dense medium is refracted towards the normal (Perpendicular line to tangent) thats air to glass then from glass to air is refracted away from normal.
Convex lens cross at focal point.
Concave lens Diverge light but are quoted as having a virtual focal point onsame side as source if lines drawn backwards.

Hi Ocker,

Thanks for the note regarding light being refracted from the Norm Perpendicular to the tangent. Thanks for the correction, had typed all info in and had sent it off without checking what I had done. I will make sure I read through before sending off.

The thicker the lens the more refration there is due to the glass slowing light down enough to change the angle of refraction. (convex), opposite side of the diverged light will produce the focal point but fainter than what light first was allowed to enter the lens.

Thank again..

Robb.......