AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Diffraction: Plane transmission diffraction grating at normal incidence.
A plane transmission diffraction grating is a slide with many equally spaced parallel slits.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A plane transmission diffraction grating is a slide with many equally spaced parallel slits. At normal incidence, monochromatic light arrives perpendicular to the grating surface, so the path difference between waves from adjacent slits is d sin θ, where d is the slit spacing and θ is the angle from the normal. Constructive interference occurs when d sin θ = nλ, giving sharp principal maxima for integer n. The zero-order maximum is at θ = 0°, and higher orders appear symmetrically on both sides. You use this equation to calculate wavelength, slit spacing or order, and you should describe how increasing the number of illuminated slits sharpens the maxima and how white light produces a spectrum in each order.
What earns the marks
- State that at normal incidence the path difference between adjacent slits is d sin θ.
- Apply the grating equation d sin θ = nλ to calculate wavelength, slit spacing or order.
- Identify the zero-order maximum at θ = 0° and explain that higher orders occur symmetrically about it.
- Explain that maxima are sharp because many slits contribute, with destructive interference between them.
- Describe the use of a spectrometer or laser to measure angles of diffraction and determine wavelength.
- Explain that white light gives a continuous spectrum in each non-zero order, with violet diffracted least and red most.
Common mistakes and how to put them right
Using the single-slit equation for minima when analysing a grating.
Put it right: Use d sin θ = nλ for grating maxima.
Confusing the slit spacing d with the number of lines per metre N.
Put it right: d = 1 ÷ N.
Assuming the zero-order maximum is dispersed into a spectrum.
Put it right: At θ = 0° all wavelengths overlap, so only non-zero orders show dispersion.
Exam tips
- Write the grating equation as d sin θ = nλ and define each symbol before substituting values.
- Check that the angle you use is measured from the normal to the grating, not from the grating surface.
- When calculating d from lines per metre, use d = 1 ÷ N and keep units consistent, converting to metres where needed.
Try the lesson
The start of the Diffraction: Plane transmission diffraction grating at normal incidence. lesson, made from checked teaching notes for this specification point.
Have a go first
Light is incident normally on a plane transmission diffraction grating. What is meant by normal incidence in this arrangement?
Listen
A plane transmission diffraction grating is a slide with many equally spaced parallel slits. At normal incidence, monochromatic light arrives perpendicular to the grating surface, so the path difference between waves from adjacent slits is d sin θ, where d is the slit spacing and θ is the angle from the normal. Constructive interference occurs when d sin θ = nλ, giving sharp principal maxima for integer n. The zero-order maximum is at θ = 0°, and higher orders appear symmetrically on both sides. You use this equation to calculate wavelength, slit spacing or order, and you should describe how increasing the number of illuminated slits sharpens the maxima and how white light produces a spectrum in each order. A strong answer earns marks for these points. State that at normal incidence the path difference between adjacent slits is d sin θ. Apply the grating equation d sin θ = nλ to calculate wavelength, slit spacing or order. Identify the zero-order maximum at θ = 0° and explain that higher orders occur symmetrically about it. Explain that maxima are sharp because many slits contribute, with destructive interference between them. A common slip: Using the single-slit equation for minima when analysing a grating. Instead, use d sin θ = nλ for grating maxima. Exam tip: Write the grating equation as d sin θ = nλ and define each symbol before substituting values.
Key words
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Diffraction grating equation
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Normal incidence
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Zero-order maximum
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Dispersion of white light
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See every definition
- Diffraction grating equation
- Constructive interference occurs when d sin θ = nλ, where d is the slit spacing, θ is the angle from the normal and n is an integer.
- Normal incidence
- Monochromatic light arrives perpendicular to the grating surface.
- Zero-order maximum
- The maximum at θ = 0°, where all wavelengths overlap.
- Dispersion of white light
- White light gives a continuous spectrum in each non-zero order, with violet diffracted least and red most.
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