Diffraction

1. Diffraction 2 January, 2020 Keywords Wavelength, wave-front, shadow region

2. All students should be able to: Be able to describe what diffraction is. Most students should be able to: Be able to draw diagrams of diffraction and describe how different situations alter the amount of diffraction. Some students should be able to: Give examples of where diffraction may be noticed in everyday life. Outcomes

3. We will answer this at the end. Starter: How come we can hear sounds round corners but can’t see round corners?

4. Your teacher will show you an example of Diffraction on an online Ripple Tank. (offline simulator click here) Or perhaps in real life!! What happens when the slit is the same size as the wavelength of the wave? What about when the slit is a lot bigger? Can you describe what happens when there is an obstacle in the middle of the wave and to one side? Demonstration time!!

5. How would you describe how the gap width alters the amount of diffraction? Diffraction diagrams - Copy NB these should be small circular arcs centred on the edge of the gap

6. What is diffraction? Diffraction is a property of all waves, which can be shown by water waves in a ripple tank. Here, the waves travel along until they reach a gap. The width of the gap is similar to the wavelength of the waves. The waves pass through the gap and spread out. This is diffraction. So, when radio waves travel past an obstacle or through a narrow gap, they bend or spread out due to diffraction.

7. How does wavelength affect diffraction? The amount of diffraction depends on how the wavelength compares with the size of the gap. What happens if the wavelength is much smaller than the width of the gap? In this case, only the edges of the wave front are diffracted. The amount of diffraction depends on wavelength. The longer the wavelength, the greater the diffraction.

8. wavelength waves diffraction obstacles gap smaller Copy and complete the following – use the keywords below to help: Diffraction is a property of all ………. When they pass through a ……… they spread out. The amount of diffraction depends on how the ……….. compares with the size of the gap. The amount of …………. increases as the wavelength becomes ………… and similar the the size of the gap. Diffraction also occurs around ………….. too.

9. The same thing happens in harbours too. Waves come from the sea this way Notice the circular ripples due to the waves diffracting through the gap

10. Diffraction in a harbour Waves come from the sea this way Notice the circular ripples due to the waves diffracting through the gap

11. The wave was produced from a long fault on the sea bed. This is why Sri Lanka was badly affected Notice the diffraction around Sri Lanka towards southern India Tsunami December 2004

12. How do radios work in hilly areas? In hilly areas there may not be a clear path between the transmitter and the radio, so how can a signal be received?

13. Why can I receive radio but not TV?

14. Problems with Radio and TV Reception Long wavelength radio waves diffracted into the “shadow” region Short wavelength radio waves (for TV) diffracted less

15. Draw 2 diagrams to help (one showing light waves and one showing sound waves) and then write an explanation using these keywords: light, small, wavelength, similar, doorframe, sound, large, diffraction. Plenary: So - How come we can hear sounds round corners but can’t see round corners?

16. Most spreading out of waves (diffraction) occurs when the wavelength is a similar size to the size of the gap. So the sound spreads out and can be heard around the corner, But the light does not spread out and so the band cannot be seen. Plenary: So - How come we can hear sounds round corners but can’t see round corners? • My Answer: • Light has a small wavelength. • The wavelength is a lot smaller than the size of the gap (the door frame) • Sound has a large wavelength similar to the gap size. • How did you do? WWW/EBI