物理問題，關於光的 20點

When we are to judge whether a pair of sunglasses worths the value, we may consider several factors:

1) Quality of materials used
This is something to deal with the melleability of the metal and the resistance to chemical reactions. If the metal is melleable and has a high resistance to chemical reactions, then it is less likely to be broken.

2) Polarization power of sunglasses
Polarization power refers to the power of the sunglasses to block the sunlight. In particular, if it has a hig polarization power, it can block mosts of the light wave from the sun, and vise versa.
Precisely speaking, sunglases has long iodine chains which act as the polarizer. When the chains are placed along the electric field of the EM-wave emitted by the sun, energy is absorbed. Apparently less light passes through the sunglasses.

3) Power of blocking radiation
Our eyes are actually exposed to sunlight radiation directly, and it enters our body through the iris. As less light passes through the sunglasses, we tend to open our iris (by reflex action). This implies more radiation enters our body, and this is extremely dangerous. Thus the power of reflecting radiation is important for a pair of sunglasses. Usually the higher the power of blocking radiation, the better the sunglasses, and vise versa.

Base on these three factors, we can't really say for sure that a $200 pair of Serengeti sunglasses is better than any of a $10 pair from the flea market.