- Using different sources of monochromatic light demonstrate that:
- Increasing the wavelength increases the width of the fringes
- The angle of diffraction of the first minima can be found using the equation:
- θ = the angle of diffraction (radians)
- λ = wavelength (m)
- b = slit width (m)
- This equation explains why red light produces wider maxima
- It is because the longer the wavelength, λ, the larger the angle of diffraction, θ
- It also explains the coloured fringes seen when white light is diffracted
- It is because red light (longer λ) will diffract more than blue light (shorter λ)
- This creates fringes which are blue nearer the centre and red further out
- It also explains why wider slits cause the maxima to be narrower
- It is because the wider the slit, b, the smaller the angle of diffraction, θ
Slit width and angle of diffraction are inversely proportional. Increasing the slit width leads to a decrease in angle of diffraction, hence the maxima appear narrower
Single Slit Geometry
- The diffraction pattern made by waves passing through a slit of width b can be observed on a screen placed a large distance away
The geometry of single-slit diffraction
A group of students are performing a diffraction investigation where a beam of coherent light is incident on a single slit with width, b.
The light is then incident on a screen which has been set up a distance, D, away.
A pattern of light and dark fringes is seen.
The teacher asks the students to change their set-up so that the width of the first bright maximum increases.
Suggest three changes the students could make to the set-up of their investigation which would achieve this.
Step 1: Write down the equation for the angle of diffraction
- The distance between the slit and the screen will also affect the width of the central fringe
- A larger distance means the waves must travel further hence, will spread out more
- Therefore, moving the screen further away would increase the fringe width