- Question 3 Over 8 Of People In The Uk Wear Contact Lenses For Vision Correction The Most Common Type Is Made From Sof 1 (163.59 KiB) Viewed 8 times
- Question 3 Over 8 Of People In The Uk Wear Contact Lenses For Vision Correction The Most Common Type Is Made From Sof 2 (50.04 KiB) Viewed 8 times
Question 3. Over 8% of people in the UK wear contact lenses for vision correction. The most common type is made from soft hydrogels, which are oxygen- permeable. Many factors can affect the hydration in the lens and therefore the perceived comfort level, including environmental elements such as tem- perature. A cross-sectional schematic of how the lens sits over the cornea is shown in the figure below. Contact lens (L) contact lens retina R₂ cornea (a) Air-conditioned offices are a challenging environment for contact wear- ers due to the dry air and the direct breeze in the eye, though it is unclear whether the colder temperature itself is also a contributing fac- tor to the discomfort. In the following, you will estimate the temperature at the outer surface of the cornea (R = R₁). It can be assumed that at the interior surface of the cornea (R = Ro) the temperature is constant T(Ro) To, while on the outer surface of the lens, the temperature is equal to the ambient air T(R₂) = T₂. i. Assuming a spherical symmetry and other necessary simplifica- tions, reduce the appropriate form of the heat diffusion equation and find the general expression for the temperature distribution. ii. The thermal conductivity in the corneal (kc) and the lens (kr) layers are distinct, since they are made of different materials. Write down the two equations that describe the temperature in the lens-cornea layers. iii. How many unknown coefficients must be found in order to fully Cornea (C) Ro R₁ [3 marks] [2 marks]
determine the temperature profile? iv. List all the boundary conditions necessary to solve the system. v. Calculate the temperature at the corneal surface; i.e. T(R₁) = T₁. (b) The assumption of spherical symmetry in this case is, of course, an unrealistic one given that only a part of the eye is exposed to the air (or covered by the contact lens). Discuss how this might have led to errors in the estimation of T₁. Is the actual cornea surface temperature likely to be higher or lower, and why? [1 mark] [4 marks] [6 marks] [2 marks]