A camera lens (refractive index 1.60) is to be coated on its outer surface with a…
PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics
Problem
A camera lens (refractive index 1.60) is to be coated on its outer surface with a single layer of magnesium fluoride (index 1.38). The goal is to minimize reflection of the most intense part of daylight, taken to be green light of λg=550nm in vacuum, when the lens is used under normal-incidence illumination.
At the same time, the designer wants to know what color, if any, will be most noticeable in the residual reflection once the coating is applied.
Answer the following.
(a) Show that there is a unique least-thickness coating that strongly suppresses the reflected green light under the stated conditions, and calculate that thickness.
(b) For the coating thickness found in part (a), determine the next longer visible wavelength (if any) that experiences constructive interference in the reflected light. Identify the corresponding perceived color.
(c) Discuss how the differing phase changes at the air–coating and coating–lens interfaces influence both the suppression of green and the enhancement of the wavelength identified in part (b).
(d) Suppose the coating were inadvertently deposited at twice the intended thickness. Predict qualitatively how the reflected color spectrum would change, and explain the physical basis for that change.