Question

At the first glance, the top surface of the Morpho butterfly’s wing appears a beautiful blue-green. If the wind moves the colour changes. Reason: Different pigments in the wing reflect light at different angles.
A. Both assertion and reason are true and reason is the correct explanation of assertion
B. Both assertion and reason are true but the reason is not the correct explanation of assertion
C. Assertion is true but reason is false
D. Both the assertion and reason is false
E . Reason is true but assertion is false

Answer 1

Morpho butterflies have long wings with length of $7.5cm$ to $20cm$ and the wings are covered with many tiny hairs and pores. The pores act like slits for which the interference of light occurs.

Complete step by step answer:
Interference of light is a phenomenon in which two light waves superpose to form a resultant wave of greater, lower or the same amplitude. There is a difference between the visible colour of the wing and the original colour of the wing because of the interference. Now in this particular case interference occurs in two ways.
Primarily, when sunlight which is having various wavelengths incidences on these pores or slits some constructive interference (superpose of two maxima waves) and destructive interference (superpose of two minima waves) take place. So we can see some constructive and destructive interference at the same time, this is called iridescence.
Secondly, the interference occurs between the incident ray and the ray reflected from the wings. So when the wind blows the tiny hairs also moves and the reflectivity of the upper surface of the wings changes which changes the reflected wave. And due the interference the resultant colour of the wings changes.
This is the reason for which we can see different colours of the wings when the wind blows. Hence the assertion is true but the reason is false.

The correct option is C.

Note: This phenomenon is basically a natural example of multiple slit interference where there are many maximas and minimas and in this case for the blowing wind the arrangement of maximas and minimas changes as a result the interference pattern also changes. Also interference varies with the wavelength and the angle of incidence.