what is diffraction?
the spreading out of waves when they pass through or around a gap.
when does the greatest diffraction occur?
-occurs when the gap is the same size as the wavelength. when the gap is smaller than the wavelength most waves are reflected, whereas when it is larger there is less noticeable refraction.
what happens when a wave meets an obstacle?
you get diffraction round the edges, the wider the obstacle compared to the wavelength, the less diffraction.
What happens when monochromatic light is diffracted through a single slit?
-forms an interference pattern of light and dark fringes. The pattern has a bright central fringe, with alternating dark and bright fringes on either side.
-the bright fringes are caused by constructive interference where the wave are in-phase and the dark fringes are caused by destructive interference where the waves arrive completely out of phase.
-The intensity of the fringes decreases from the central fringe
what happens if white light could be used instead of monochromatic light?
-the diffraction pattern you would see would be quite different as white light is made up of all colours
-so all different wavelengths of visible light, all of the different wavelengths would be diffracted by different amounts.
-so you get a spectrum of colour in the diffraction pattern, the diffraction pattern for white light has a central white maximum with alternating bright fringes which are spectra, violet is closest to the maximum and red is the furthest.
what does increasing slit width do?
-decreases the amount of diffraction, so the central maximum becomes narrower and it’s intensity increases.
what does increasing the light wavelength do?
-increases the amount of diffraction as the slit is closer in size to the light’s wavelength, therefore the central maximum becomes wider and it’s intensity decreases.
what is a diffraction grating?
a slide containing many equally spread slits very close together.
what happens when monochromatic light is passed through a diffraction grating?
-the interference pattern is much sharper and brighter than it would be after being passed through a double slit, this is because there are more rays of light reinforcing the pattern.
-this means the measurements of slit widths are much more accurate as they are easier to take.
what is the ray of light passing through the centre of a diffraction grating?
-It is the zero order line.
-lines either side of the zero order line are the first order lines, then the lines outside the two first order lines are the second order.
what is the max value of sin?
n=1 as numbers greater for sin n>1 are impossible.
uses of diffraction gratings?
-You can split up light from stars using a diffraction grating to get line absorption spectra which can be used to show which elements are presented in the star.
-x-ray crystallography, which is where x-rays are directed at a thin crystal sheet which acts as a diffraction grating to form a diffraction pattern.
