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Microgravity: living on the International Space Station
Microgravity: living on the International Space Station

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3 Diffraction of waves

Before you journey into the quantum world, you need to consider some physical effects that can be seen in your everyday life. Diffraction of waves is one of them. For example, when the entrance to a harbour is of the right size, water waves diffract, or spread out, as they move into the harbour (Figure 4).

A colour photograph taken from the air of a harbour.
Figure 4 Water wave diffraction in a harbour.

If you click here [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] , it will take you to an online animation where you can change the gap size for single slit and ripple tank simulations. Note carefully what happens to the waves, then try Activity 2.

Activity 2 Exploring diffraction

Timing: Allow approximately 15 minutes

Choose the one correct option to complete the following statements based on the animation.

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You can also see diffraction effects using LASER light. Watch Video 2 where this effect is demonstrated using LASER pens and diffraction gratings. Then complete Activity 3.

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Video 2 LASER pen diffraction experiment.
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Here, the concept of light behaving as a particle and a wave is introduced. Later on, you will see how electrons, as small particles of matter, can also behave as particles and waves.

Activity 3 The LASER diffraction experiment

Timing: Allow approximately 15 minutes

Choose the one correct answer from the options given to complete the following statements.

1. Moving out from the centre of the board, the order of the colours was as follows.

a. 

red and green


b. 

blue, green and red


c. 

yellow and red


d. 

green and blue


e. 

green, red, blue and yellow


The correct answer is b.

2. The diffraction gratings used, in terms of lines per millimetre (mm), were:

a. 

300


b. 

1200


c. 

600 and 1200


d. 

300 and 600


e. 

300


The correct answer is d.

3. As the number of lines per mm on the diffraction gratings decreased, the spots on the board:

a. 

moved further away


b. 

moved closer


c. 

were unchanged


The correct answer is b.

As the number of lines per millimetre on the diffraction grating decreases, the distance d between the lines increases. When this happens, the angle θ increases which affects the distances between the spots in Question 3 above. But how are these terms related to each other? Video 2 introduced Equation 1 which can now be explained in more detail.

equation left hand side n times lamda equals right hand side d postfix times sine of postfix times theta
Equation label: Equation 1

n is an integer (whole number) as you count the dots from the centre outwards; the central dot is labelled n = 0.

λ is the wavelength of the LASER and is measured in m.

d is the separation between adjacent lines on the diffraction grating, again measured in m.

θ is the angle of diffraction from the LASER pen to the individual dots.

sin(θ) is the sine of the angle θ; this can be calculated by using the ‘sin’ button on your calculator.

You will now look at sodium D-lines and the simplest atom of all, hydrogen.