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When white light passes through a diffraction grating, what is the smallest value of \(m\) for which the visible spectrum of order \(m\) overlaps the next one, of order \(m+1 ?\)

Short Answer

Expert verified
The smallest value of \(m\) is 2.

Step by step solution

01

Understand the Principle of Diffraction Grating

When white light passes through a diffraction grating, it diffracts into its component colors (spectrum). For a diffraction grating, the condition for constructive interference is given by the equation: \( d \sin \theta = m \lambda \), where \(d\) is the distance between slits, \(\theta\) is the angle of diffraction, \(m\) is the order of the spectrum, and \(\lambda\) is the wavelength of light.
02

Define the Overlapping Condition

For the visible spectra of order \(m\) and \(m+1\) to overlap, the maximum wavelength of the spectrum from order \(m\) (red light) should coincide with the minimum wavelength from the order \(m+1\) (violet light). Let's denote the minimum visible wavelength as \(\lambda_{violet} = 400\,nm\) and the maximum as \(\lambda_{red} = 700\,nm\).
03

Establish the Overlap Equation

The overlap condition can be expressed as: \[ m \lambda_{red} = (m+1) \lambda_{violet} \] Substitute \(\lambda_{red} = 700\,nm\) and \(\lambda_{violet} = 400\,nm\) into the equation: \[ m \times 700 = (m+1) \times 400 \]
04

Solve for the Order \(m\)

Rearrange and solve the equation: \[ 700m = 400m + 400 \] \[ 300m = 400 \] \[ m = \frac{400}{300} = \frac{4}{3} \] Since \(m\) must be an integer, take the smallest integer greater than \(\frac{4}{3}\), which is \(m = 2\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

White Light Dispersion
White light is a mixture of all the colors in the visible spectrum. When it passes through a diffraction grating, this light spreads out into its component colors. Think of it like a prism effect, where white light splits into a rainbow.
A diffraction grating is specially designed to have many closely spaced slits or grooves. As light encounters these slits, different wavelengths (or colors) of light bend or 'diffract' at different angles.
  • Shorter wavelengths (like blue or violet) diffract at smaller angles.
  • Longer wavelengths (like red) spread out more, bending at larger angles.
Therefore, by using a diffraction grating, we can observe a full spectrum of colors spread out based on their wavelength.
Constructive Interference
In physics, when waves combine, they interfere. There are two main types: constructive and destructive interference. In the case of a diffraction grating, constructive interference is key to producing a visible spectrum.
Constructive interference occurs when waves align in such a way that their crests and troughs match up perfectly, producing a stronger combined wave.
  • For a diffraction grating, the condition for constructive interference is: \(d \sin \theta = m \lambda\).
  • Here, \(d\) is the spacing between the grating's slits, \(\theta\) is the angle of diffraction, \(m\) is the order of the spectrum, and \(\lambda\) is the wavelength of the light.
This equation ensures that the different wavelengths (colors) are separated out, producing visible bands.
Order of Spectrum
When discussing diffraction gratings, we often hear about 'orders' of the spectrum. This refers to the different sets or 'orders' of the separated spectrum that appear when light is diffracted. Each order represents a distinct pattern of colors.
Higher orders equate to more spread out colors, but this also leads to overlapping of colors from different orders.
  • Order \(m = 1\) is the first occurrence of the spectrum.
  • Order \(m = 2\) is the second, but occurs at a larger angle than the first.
  • Overlapping occurs when the red end of one order (higher wavelength) coincides with the violet end (lower wavelength) of the next, e.g., \(m\) and \(m+1\).
Understanding these orders helps in knowing how to set up and analyze experiments using diffraction gratings.

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Most popular questions from this chapter

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