Question

Interference is produced with two coherent sources of the same intensity. If one of the sources is covered with a thin film so as to reduce the intensity of light coming out of it to half, then:

• A. bright fringes will be less bright and dark fringes will be less dark
• B. bright fringes will be more bright and the dark fringes will be more dark
• C. brightness of both types of the fringes will remain the same
• D. dark region will spread completely

Answer 1

The correct option is A bright fringes will be less bright and dark fringes will be less dark

The sum of the intensities of the interfering waves equals the average intensity of the fringes. As the intensity of light coming out of one of the slit is reduced to half, the sum of intensities reduces. As the sum reduces the average intensity of the fringes reduces. Thus, Bright fringes will be less bright and dark fringes will be less dark.