When a healthy eye encounters bright light, the pupil contracts to cause less light to enter the eye. The pupil, appearing as the black center in the colored part of the eye, is actually a hole in the iris, and it is the iris that causes the pupil to change size. In dim light, the pupil gets bigger (dilates) to let more light into the eye. Wearing dark sunglasses decreases the amount of sunlight entering the eye, so the pupil responds by dilating. If the sunglasses decrease the visible light but do not block UV, then significantly more UV enters the eye compared to when not wearing sunglasses. Using a darkened room as a simulation of the amount of light entering your eyes while wearing sunglasses, find the minimum amount of UV that must be blocked by sunglasses to ensure that increased levels of UV are not entering your eyes.
Ruler with millimeter increments
Strategies for Investigation
1. Students work in pairs. In a darkened room carefully hold the ruler close to your partner’s eye. Measure the diameter of the pupil. Switch so each student may make a measurement.
2. Repeat the procedure, but turn on bright lights to contract your pupils as much possible. Do not make the lighting uncomfortable for the people. If the day is sunny and warm enough, you might want to do this outside.
3. Calculate the area of the pupil in both pictures. Assume that the pupil is a perfect circle.
4. Find the ratio of the area of the dilated pupils to that of the constricted pupils. This is an estimate of the amount of UV that must be blocked by a pair of sunglasses.
Results and Conclusions
1. What is the minimum percentage of UV that must be blocked so your sunglasses are not enhancing the damaging effects of UV on your eyes? As an example, if the area of the pupils were twice as large when dilated compared to contracted, the sunglasses would need to block 50% of the UV in order to compensate for the enlarged pupils. If the pupils grew three times in area when sunglasses were put on, then 67% of the UV would need to be blocked.
2. What assumptions are you making in order to draw this conclusion? Discuss the strengths and weaknesses of these assumptions.
3. How could you modify your experiment to eliminate these assumptions? If you can carry out a new experiment, compare the results of the two.
4. Can you design an experiment that would relate the SPF of sunscreen to the UV-blocking ability of sunglasses? This experiment could be used to test the rating of your sunglasses! Can this be done mathematically without the experiment?
5. Do you feel your sunglasses adequately protect you? If you wear untinted eyeglasses, would you benefit from a lens coating that blocks UV?
6. If the amount of UV reaching the ground increased 4% due to thinning of the ozone layer, what is the minimum UV-blocking required to keep the harm to your eyes at the same level as before? Remember unprotected eyes are being harmed at the current levels of UV radiation.