WAVE BEHAVIOUR DIFFRACTION

This is the tendency of waves to bend around objects and spread out to fill spaces

DIFFRACTION Diffraction happens most strongly when the wavelength is comparable to the size of the gap or the size of the barrier This is why light coming through a door into a darkened room doesn’t spread out to illuminate the whole room, because light has a very small wavelength compared to the width of the door Sound, however, does spread through the room because sound has a much longer wavelength

DIFFRACTION PROBLEMS Diffraction problems are usually describe or explain problems They relate mostly to a physical implication of the diffraction of given wave, or ask you to compare the different diffraction behaviour of two waves

PROBLEM ONE A breakwater in a harbour has a large gap in it to allow boats through, as shown below. State what type of waves it will allow through and why

PROBLEM ONE Diffraction around barriers and through gaps occurs most when the wavelength is comparable to the size of the gap. The gap in the breakwater is large, therefore it will let through waves with a long wavelength (long ocean swells) but not short choppy waves (from wind). This is good, because it is the short, choppy waves that cause seasickness and problems on boats

PROBLEM TWO At the beach, Hine stands behind the changing rooms. She can hear the waves crashing, but she can’t see them. Explain why.

PROBLEM TWO Diffraction around barriers and through gaps occurs most when the wavelength is comparable to the size of the gap. Visible light has a very short wavelength, so it doesn’t diffract round the changing rooms very much, so Hine can’t see the waves. Sound has a longer wavelength, so it will diffract around the changing rooms so that Hine can hear the waves

PROBLEM THREE A student lives in a house with poor radio reception, as it is in a valley with hills all around. Her friend suggests that she is likely to have better reception if she listens to an FM station (frequency 100MHz) than an AM station (frequency 1MHz). Do you agree with her friend? Explain your answer.

PROBLEM FOUR Diffraction around barriers and through gaps occurs most when the wavelength is comparable to the gap. AM has a lower frequency, meaning a longer wavelength. The AM signal will diffract more around the hills and have a better chance of reaching the house. FM has a higher frequency and a shorter wavelength. The FM signal will not diffract much around large barriers like hills. Her friend is wrong; AM has a better chance of being picked up