Question

A diffraction pattern is obtained by making blue light incident on a narrow slit. If blue light is replaced by red light then

• A. there is no change in diffraction pattern.
• B. diffraction bands become broader.
• C. diffraction bands disappear.
• D. diffraction bands become narrower.

Answer 1

The Correct Option is B

Solution: Diffraction Pattern Change When Blue Light is Replaced by Red Light

In a diffraction experiment, light passes through a narrow slit and forms a diffraction pattern on a screen. This pattern consists of bright and dark bands, known as diffraction fringes. The position and width of these bands depend on the wavelength of the incident light.

Explanation:

The diffraction fringe (or band) width is related to the wavelength of the light. The relationship can be understood using the single-slit diffraction formula:

y = ^{m λ L / d}

Where:

• y is the position of the m-th diffraction band (measured from the center).
• λ is the wavelength of the light.
• L is the distance from the slit to the screen.
• d is the width of the slit.

Step-by-Step Explanation:

1. Effect of Wavelength on Diffraction:

The diffraction fringe width y is directly proportional to the wavelength λ. Therefore, if the wavelength increases, the diffraction bands will become broader.

Blue light has a shorter wavelength compared to red light. Since blue light has a smaller wavelength, the diffraction bands produced by blue light will be narrower compared to those produced by red light.

2. Replacing Blue Light with Red Light:

Red light has a longer wavelength than blue light. As a result, when we replace blue light with red light, the diffraction bands will become broader.

Conclusion:

When blue light is replaced by red light, the diffraction bands become broader.

Answer:

The correct answer is Option 2: Diffraction bands become broader.

Why?

This happens because red light has a longer wavelength than blue light, and the width of diffraction fringes increases with the wavelength of the light used. Thus, replacing blue light (shorter wavelength) with red light (longer wavelength) leads to broader diffraction bands.