# No links please Q. Find the focal length of the projector-lens which projects the image of size $60×60$ cm of a slide on a screen 2.4 m away, the slide being of the size $3×3$ cm. Light enters a prism of angle A at a grazing incidence to emerge at an angle $\theta$with the normal. Show that the refractive index of the material of the prism is given by,

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