Question

Newton's rings are interference patterns caused by the reflection of light between two glass surfaces. What color is the center of Newton's rings produced with white light? a) white c) red b) black d) violet

Short answer

Answer: a) white

Step-by-step solution

Understand the concept of interference

Interference is a phenomenon that occurs when two or more waves meet and their amplitudes either add up (constructive interference) or cancel out (destructive interference). In the case of Newton's rings, light waves reflect between two glass surfaces and interfere with each other, resulting in the formation of a pattern of light and dark (constructive and destructive interference) rings.

Determine the conditions for constructive and destructive interference

In the case of Newton's rings, the rings are formed due to thin film interference. When light reflects from the top and bottom surfaces of the thin film (air gap between two glass surfaces), a phase shift may occur, leading to constructive or destructive interference. For constructive interference, the path difference between the two reflected waves must be a multiple of the wavelength (Lambda): 2 * t = m * Lambda, where m is an integer and t is the thickness of the film. For destructive interference, the path difference must be an odd multiple of half the wavelength: 2 * t = (m + 1/2) * Lambda.

Identify the condition at the center of the rings

At the center of the Newton's rings, the parallel glass surfaces are in contact, which means the thickness of the film (t) is zero. Therefore, using the constructive interference condition, we have: 2 * 0 = m * Lambda, or m * Lambda = 0. This means that the center of Newton's rings will have constructive interference for all wavelengths (m=0) of the incident white light.

Determine the color at the center of Newton's Rings

Since all wavelengths of the incident white light have constructive interference at the center, as shown in Step 3, each color component (red, green, and blue) contributes to the light intensity at this point. Thus, the central point will appear as the sum of these color components, resulting in the perception of white light. The correct answer is: a) white.