What is the minimum coating thickness (but not zero) that will minimize the reflection at the wavelength of 705 nm where solar cells are most efficient?

d = 235 nm.

Explanation:

Let the layer be of glass. Refractive index of glass μ = 1.5. Let the required  thickness be d.

For minimum reflection , that is destructive interference in thin films , the condition is

2μd = λ ( for first order)

d = 705 / 2 x 1.5 = 235 nm [ λ =705 nm ]

luster is not a good characteristic to use by itself because luster is the property of minerals that indicates how much the surface of a mineral reflects light. the luster of a mineral is affected by the brilliance of the light used to observe the mineral surface.