Nature of waves

Starter Look at the video: Can you identify areas of “calm” water? What may have caused this? What may be occurring at other areas in the tank?

Learning Objectives Know what reinforcement and cancellation of waves means. Know a demonstration to show interference. Know what constructive and destructive interference is, and when it will occur.

Success criteria Label areas of reinforcement and cancellation. (Grade C) Describe a demonstration which shows reinforcement and cancellation. (Grade C) Explain why constructive or destructive interference occurs. (Grade A)

Interfering waves What will happen if the crests of two identical waves meet? The wave height will double. This is called REINFORCEMENT. What will happen if the crest of one wave meets the trough of another identical wave? The waves will subtract from each other, giving calm water. This is CANCELLATION.

Interfering waves On the diagram given: put an R on areas of reinforcement. put an C on areas of cancellation.

Demonstration of interference using sound waves Take part in the demonstration. Write a brief explanation of how the demonstration works, what you heard and why. Include a diagram to aid your explanation. (Page 197, fig 3. – old course).

To produce a stable interference pattern, the wave sources must be coherent (must have the same frequency and therefore wavelength).

Task Foundation: complete worksheet P5f2 – old course, then answer questions 3 and 4 on page 197. Higher: you will need to work on explaining interference patterns on the next few slides.

 When two waves from the same source meet in step (or in phase)... ...constructive interference occurs.

When two waves from the same source meet out of step (or in antiphase)...  ...destructive interference occurs.

Constructive or destructive? Constructive interference occurs when the path difference from the two sources is a whole number of wavelengths. Destructive interference occurs when the path difference from the two sources is an odd number of half wavelengths.

Imagine a light source pointing at a screen with two slits in it, and projecting onto a second screen. (the two light waves are given a different colour to make it easier to see) LIGHT SOURCE

Some of the waves will bend as they pass through the slits. Why? LIGHT SOURCE

Why are the waves in step? Where diffracted waves meet up, constructive and destructive interference occurs. Because, in this case, they have travelled the same distance (count the peaks!). A LIGHT SOURCE Waves are in step. There will be a bright spot at this point (Point A)

B A There will be other bright points apart from A. LIGHT SOURCE Why are the waves in step? Waves are in step. There will be a bright spot at this point (Point B) The difference in distance the two waves have travelled is a whole number of wavelengths (count them!)

B C A Destructive interference can also occur. Why are the waves out of step? Destructive interference can also occur. The difference in distance the two waves have travelled is an odd number of half wavelengths (count them!). i.e. Troughs and crests line up. B C A LIGHT SOURCE Waves are out of step. This point (Point C)will be unlit.

Look at figure 6 on page 197 – old course Make a note of the path difference equations at the bottom of the page.

Tasks Foundation: answer questions 1 and 2 on worksheet P5f1 – old course. Higher: answer questions 1 and 3 on worksheet P5f1 – old course.

Answers 1. Alternate loud and quiet areas of sound; at some points the sound waves reinforce and at others they subtract from each other Sound heard is an octave higher in pitch; the areas of loud and quiet sound are closer together or have half the separation of in a 2. Bright light No light (or darkness) 3. Waves must diffract to overlap; this requires a gap of similar size to the wavelength; light has a very small wavelength (400–600 nm) Path lengths from each slit to P differ by exactly one wavelength so waves interfere constructively (crests or troughs arrive together) Z; path lengths from each slit to Q and Z differ by exactly two wavelengths At M – darkness (or no light); path lengths from each slit to M differ by exactly one and a half wavelengths so waves interfere destructively (crest and trough meet) Increase Decrease

Learning Objectives Know what reinforcement and cancellation of waves means. Know a demonstration to show interference. Know what constructive and destructive interference is, and when it will occur.

Success criteria Label areas of reinforcement and cancellation. (Grade C) Describe a demonstration which shows reinforcement and cancellation. (Grade C) Explain why constructive or destructive interference occurs. (Grade A)

Can you explain this? (reward point for correct explanation) Plenary Can you explain this? (reward point for correct explanation)