Optics B/C

[0ddrenaline]

Here's my optics test from the recent UT-Austin invitational (key). If you have any questions about it, feel free to PM or email me!

I was looking at this test and am stuck on problem 5. How do you find the focal length of a lens without knowing the radii of curvature? Sorry if it's a bit of a dumb question, I'm new to this event.

Thanks

This is a thick lens problem, not thin lens. The approach will be different.

The problem asks for the focal length of the lens. We are given the following information.
Lens power on both sides: 2.25 diopeters
Index of refraction: 1.5
Thickness, or separation between the lens' sides: 10 cm or .1 meters.

I know that, with the given information, you can find the focal length of the lens with Gullstrand's Equation found here: http://hyperphysics.phy-astr.gsu.edu/hb ... lcal.html#


P is the lens' power, is the right side's power, is the right side's power, d is the thickness, and n is the index of refraction.

When you know this, the problem is simple.

This works out to a power of 4.1625 diopeters. The focal length is the reciprocal of the power, so the final answer is 24 centimeters.

Thick lens is not a topic covered in the rules, but you should be familiar with it if you want to succeed.

[Bob_117]

Thanks Oddrenaline.

[jkang]

Under 2b it says "Competitors may bring any measuring tools, premade templates, writing utensils and any type of calculators for use during any part of the competition."

I interpreted this as things other than the binder, since 2a states that as "reference material," whereas 2b lists other things.

I was looking at this test and am stuck on problem 5. How do you find the focal length of a lens without knowing the radii of curvature? Sorry if it's a bit of a dumb question, I'm new to this event.
Thanks

As 0ddrenaline stated, this question should be approached as a thick lens problem through Gullstrand's equations. Although it's not explicitly stated in the rules as a topic, it can be implicitly included under lenses (at least that's where I chose to include it when writing this test). And for full points (accounting for significant figures), the answer would be 24.0 cm, not just 24 cm.

[amehrotra]

Hey Guys,

I've heard that for optics we get a paper as the base we are allowed to draw on to make our angular geometry easier? Is this true? I don't see it in the 2017 rules but when looking up event info I saw something of the sort? Does anyone know?

Thanks in advanced,
Aditya M

[DankMcIntosh]

So what exactly is an optic? I was a little confused.

[John Richardsim]

Hey Guys,

I've heard that for optics we get a paper as the base we are allowed to draw on to make our angular geometry easier? Is this true? I don't see it in the 2017 rules but when looking up event info I saw something of the sort? Does anyone know?

Thanks in advanced,
Aditya M

These are the "templates" referred to in the rules.

[Unome]

Hey Guys,

I've heard that for optics we get a paper as the base we are allowed to draw on to make our angular geometry easier? Is this true? I don't see it in the 2017 rules but when looking up event info I saw something of the sort? Does anyone know?

Thanks in advanced,
Aditya M

It's not that you get paper, but that you're allowed to make them and bring them in to use for the LSS.

[andrewwski]

Or you can bring in paper, put it in the LSS, and draw on the paper. Just make sure you don't draw on the LSS, and that you bring your own paper.

[Archaeopteryx]

On the rule sheet it says that the barrier has to be on the midline, but does that mean it's centered on the midline or just that it's touching the midline?

[John Richardsim]

On the rule sheet it says that the barrier has to be on the midline, but does that mean it's centered on the midline or just that it's touching the midline?

There's nothing saying it has to be centered, so it only has to be touching the midline.