Question

Why are Newton rings dark?

Answer 1

Newton's rings appear dark due to interference effects, which occur when light waves reflect off the top and bottom surfaces of a thin film, creating conditions for destructive interference.

Here’s a step-by-step breakdown of why they appear dark:

1. **Formation of Newton's Rings:**
   Newton's rings are a pattern of concentric circles created when light is reflected from a thin air film between a convex lens and a flat glass surface. The film thickness changes radially from the center, which causes different path lengths for the reflected light waves from different points.

2. **Interference of Light Waves:**
   Light can be considered a wave, and when two light waves from different parts of the thin film (top and bottom surfaces) meet after reflection, they can interfere with each other. This interference can be constructive or destructive.

3. **Destructive Interference (Dark Rings):**
   At certain points, the difference in the path length of the two reflected waves results in **destructive interference**. Destructive interference occurs when one wave is out of phase with the other, causing the two waves to cancel each other out. For light waves, this happens when the path difference is an odd multiple of half-wavelengths (i.e., \(\Delta L = (2n + 1)\lambda/2\)), where \(\lambda\) is the wavelength of light and \(n\) is an integer. As a result, the intensity of light at these points is minimized, producing dark rings.

4. **Constructive Interference (Bright Rings):**
   On the other hand, there are regions where the path difference results in **constructive interference**, where the waves amplify each other, leading to bright rings.

So, Newton's rings are dark in places due to destructive interference, where the light waves reflected from the lens and the flat surface cancel each other out, creating regions of low intensity.