physics chapter 5.6-5.7 optics - diffraction

diffraction

the spreading of waves when they pass through a gap or by an edge - less diffraction occurs when a wave passes through a wide gap than through a narrow gap

when do waves diffracted through a gap spread out more

when the gap is made narrower; or when the wavelength is made larger

when water waves are diffracted through a gap, why does each diffracted wavefront have breaks either side of the centre

due to waves diffracted by adjacent sections on the gap being out of phase + cancelling each other out in certain directions

pattern of light diffracted by a single slit, observed on a white screen

shows a central fringe with further fringes either side of the central fringe - intensity of the fringes is greatest at the centre of the central fringe, + the intensity of the diffracted light varies with the distance from the centre of the fringe pattern

how the peak of each fringe varies with distance when light is diffracted by a single slit

peak intensity of each fringe decreases with distance from the centre

how the central fringe is different from each of the outer fringes when light is diffracted by a single slit

the central fringe is twice as wide as each of the outer fringes, with each of the outer fringes being the same width, + the outer fringes are much less intense than the central fringe

what does using a monochromatic source of light show in single slit diffraction

the greater the wavelength, the wider the fringes

what does using an adjustable slit show in single slit diffraction

making the slit narrower makes the fringes wider

equation for width of single fringe

W = 2 x wavelength of light x distance to screen, D / width of single slit, a

what needs to be the case to ensure that interference occurs in single slit diffraction

each slit must be narrow enough to make the light passing through it diffract sufficiently; the 2 slits must be close enough so the diffracted waves overlap on the screen

equation for the fringe spacing of the interference fringes

w = wavelength x distance to screen, D / slit separation, s

a diffraction grating, + what happens when a parallel beam of monochromatic light is directed normally at one

consists of a plate with many closely spaced parallel slits ruled on it - when a parallel beam of monochromatic light is directed normally at one, light is transmitted by the grating in certain directions only

why, when a parallel beam of monochromatic light is directed normally at a diffraction grating, is light transmitted by the grating in certain directions only

the light passing through each slit is diffracted - the diffracted light waves from adjacent slits reinforce each other in certain directions only, and cancel out in all other directions

when does the angle of diffraction between each transmitted beam and the central beam increase when light is directed at a diffraction grating

if light of a longer wavelength us used e.g. replacing a blue filter with a red filter; or if a grating with closer slits is used

diffraction grating equation for the angle of diffraction of the nth order

grating spacing, d, x sin(θ) = order, n, x wavelength, λ --> dsin(θ) = nλ

equation for number of slits per metre on the grating, N

N = 1 / d, grating spacing

how the angle of diffraction varies with grating spacing, d

for a given order and wavelength, the smaller the value of d, the greater the angle of diffraction - the larger the number of slits per metre, the bigger the angle of diffraction

how to find the maximum number of orders produced

substitute θ=90' (sinθ=1) in the grating equation, + calculate n using n = d/λ --> maximum number of order = d/λ, rounded down to the nearest whole number

spectrum analyser

an electronic spectrometer linked to a computer that gives a visual display of the variation of intensity with wavelengtj

how can a diffraction grating in a spectrometer be used

to study the spectrum of light from any light source, + to measure light wavelengths very accurately

what type of spectrum is the spectrum of light from a filament lamp

a continuous spectrum, from deep violet at about 350nm to deep red at about 650nm

what does the most intense part of the spectrum of light from a filament lamp depend on

the temperature of the light source - the hotter the light source, the shorter the wavelength of the brightest part of the spectrum

what can you measure by measuring the brightest part of a continuous spetrum

the temperature of the light source

what type of light spectrum is emitted from a glowing gas in a vapour lamp / discharge tube

emits light at specific wavelength, so its spectrum consists of narrow vertical lines of different colours = a line emission spectrum

what are the wavelengths of lines produced by a glowing gas in a vapour lamp/discharge tube characteristic of

the element that produced the light - if a glowing gas contains more than one element, the element in the gas can be identified by observing its line spectrum

a line absorption spectrum

a continuous spectrum with narrow dark lines at certain wavelengths e.g. observed after the spectrum of light from a filament lamp is passed through a glowing gas

what is the pattern of dark lines after the spectrum of light from a filament lamp is passed through a glowing gas due to

due to the elements in the glowing gas - the elements absorb light of the same wavelength they can emit at, so the transmitted light is missing these wavelengths - the absorbed light will then be emitted, but not necessarily in same direction as transmitted light