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- Diffraction by a lens 2 answers
I've come across a question that I don't know how to tackle:
An alternative way of observing Fraunhofer diffraction uses lenses to provide appropriate conditions. Sketch an optical conﬁguration for observing Fraunhofer diﬀraction using a point source of light and two lenses.
A parallel beam of light is incident normally on a diﬀraction grating which consists of a regular array of 200 narrow slits per mm, each slit being 1 µm wide. Light emerging from the grating is focussed by a lens of focal length 300 mm onto a screen. The lens and screen are centred on a line perpendicular to the centre of the grating, and are parallel to the grating. It is intended to observe the 3rd diffraction order with light at a wavelength of around 460 nm. Where on the screen does this diffraction order appear?
I've done the first part of the question (sketching the optical configuration, with the aperture between two lenses, and the screen and source at a distance f away from each other lenses).
Everything I've learnt about lenses is used to magnify images, and I have no idea how to deal with a ray coming in at an angle. Surely you need to know at what distance the aperture is from the lens?
Any help/hints would be greatly appreciated.