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1、Copyright Copyright 20122012A207 Physics Problem 02: Strange Glass6th PresentationThe problemExplain why a number of light spots are observed on the wall when a laser beam is shone through Glass B as compared to a single spot when the laser beam is shone through Glass A. What do you recognise? Glass
2、 A and Glass B are different. Glass A shows only one spot of light whereas Glass B shows multiple spots of light when the laser beam is shone through.The approachIdentify that light has wave-like properties and could undergo diffraction when it encounters multiple slits such as a diffraction grating
3、. Understand how waves could interfere to form resultant waves. Recognize that path difference equivalent to any multiple of a wavelength leads to constructive interference presented as those bright spots on the wall. Recognize that path difference involving half a wavelength (e.g. 0.5, 1.5, 2.5, 3.
4、5 .of a wavelength) leads to destructive interference. Derive a relationship involving the slit width, path difference, wavelength of light and angle of diffraction.Light Light undergoes different processes when it encounters different types of surfaces, e.g. reflection, refraction in A207 P01. Ligh
5、t can also behave as waves and exhibits wave-like properties. DiffractionWaves spread out when they pass through a small opening. The waves will spread out more when the opening becomes smaller. This phenomenon is known as diffraction.Waves passing through a smaller openingWaves passing through a sm
6、all openingWave parameters Wavelength is the length of one cycle of the wave. Amplitude is the maximum height of the wave. The path difference between two waves is the difference in the distance travelled by the waves from the starting points to a common point. Path Difference of 350 nmWavelength Am
7、plitude Distance from screenWave interferenceWave interference occurs when the wave from a source meets the wave from another source to give a resultant wave. The height at any point on the resultant wave is the sum of the height of the waves from the two sources at the same point.0.5 units-1.25 uni
8、ts-0.75 units-0.5 units 1.25 units0.75 unitsDistance from screenDistance from screenTotal constructive interferenceWhen two waves interfere with each other to give the maximum resultant amplitude, total constructive interference is observed.2.0 units2.0 units4.0 units-2.0 units-2.0 units-4.0 unitsTh
9、e maximum resultant amplitude is the sum of the amplitude of the two waves. In this case, the resultant amplitude is 4 units.Distance from screenDistance from screenTotal constructive interference If the wavelength of the light is 700 nm, total constructive interference occurs for path differences o
10、f: 0 nm, 700 nm, 1400 nm, 2100 nm, 2800 nm, Hence, for total constructive interference to occur: Path Difference = n, where n = 0, 1, 2, 3, and is the wavelength.Total destructive interferenceWhen two waves interfere with each other to give a resulting wave of zero amplitude, total destructive inter
11、ference is observed. - 2.0 units2.0 units0 unit2.0 units-2.0 units0 unitThe resultant amplitude is zero since each wave cancels out the other.Distance from screenDistance from screenTotal destructive interference If the wavelength of the light is 700 nm, total destructive interference occurs for pat
12、h differences of: 350 nm, 1050 nm, 1750 nm, 2450 nm, Hence, for total destructive interference to occur:Path Difference = (n + ), where n = 0, 1, 2, 3, and is the wavelength.Spreading of light waves Glass B consists of alternating transparent and opaque fine segments. Laser light incident on Glass B
13、 is transmitted through the transparent segments. As the light rays emerge on the other side, the light waves are diffracted in all directions.Transparent segments allowing light rays to pass throughOpaque segments blocking light raysInterference due to Glass B Path difference = 3333 sin 12.12 = 700
14、 nm = one wavelength of red light 3333 nm 1 = 12.12Transmitted red laser light rays which diffract and interfere to form bright spotsIncoming red light of wavelength 700 nmPath differenceInterference due to Glass B Path difference = 3333 sin 12.12 = 700 nm = one wavelength of red light (n , where n
15、= 1, = 700 nm) Total constructive interference Therefore, a light spot will be observed at this angle of 12.12.3333 nm Path differenceIncoming red light of wavelength 700 nmOther light spots will be observed as a result of total constructive interference when the transmitted red laser light rays int
16、erfere at other angles.Transmitted red laser light rays which diffract and interfere to form bright spotsGoing FurtherPath difference = Two wavelengths of light 3333 sin 2 = 2 x 700 nm ( ie. n , where n = 2, = 700 nm)3333 nm Path differenceIncoming red light of wavelength 700 nmTransmitted red laser light rays which diffract and interfere to form other
