(a) Viewing a solar eclipse with the naked eye can damage your sight so it is very important to wear protective glasses. A suitable pair-of glasses can be made by evaporating aluminium on to plastic film. Given that the coefficient of absorption of aluminium at optical frequencies is about 6 x 107 m-1 calculate the thickness of aluminium through which it is possible to look at the Sun without damaging the eyes. (b) Instead of looking up the absortion coefficient of aluminium in tables it is tempting to approach this problem from first principles by calculating the skin depth for electromagnetic waves impinging on a metallic boundary. Why does this yield an answer for the required thickness that is an order of magnitude too small?