Calculating Power of Tear Layer in Contact Lens Prescription

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The discussion focuses on calculating the power of the tear layer in a contact lens prescription for a mildly farsighted individual with a near point of 29.0 cm. To determine the necessary power of the tear layer, it is essential to consider the combined power of the contact lens and the tear layer, which together must correct the vision to achieve the normal near point of 25.0 cm. Participants express confusion about how to set up the problem, particularly regarding the signs of the lens powers and the combination of multiple lenses. The conversation emphasizes the need to understand the basic lens formula and how to calculate the overall power of a compound lens system. Ultimately, the accurate calculation of the tear layer's power is crucial for achieving ideal vision correction.
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Homework Statement


The contact lens prescribed for a mildly farsighted person is 1.25 D, and the person has a near point of 29.0 cm. What is the power in D of the tear layer between the cornea and the lens if the correction is ideal, taking the tear layer into account? Assume that a normal human has a near point of 25.0 cm.

Homework Equations


P = 1/do + 1/di = 1/f
 
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The tear+contact must make a compound lens with the power needed to correct eyesight with the stated near-point.
How do the powers of lenses combine?
 
Simon Bridge said:
The tear+contact must make a compound lens with the power needed to correct eyesight with the stated near-point.
How do the powers of lenses combine?
The two lenses would bee a double convex (in the eye) and the contact would be a convex, right?
But I'm confused about how to set up the problem.
One would be negative and one would be positive, right?
 
Go back to basics:
When you are given the nearpoint, and you want to know what power of lense will correct that eyesight - how do you work that out?
When you have two lenses one after the other with P1 and P1 ... how do you find the overall power P of the combined lens?

If you do not answer questions I cannot help you.
Note: the shape of the lens is irrelevant.
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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