– cm. The parallel incident beam appears to diverge from a point cm on the left of the centre of the two-lens system. Clearly, the answer depends on which side of the lens system the parallel beam is incident. Further we do not have a simple lens equation true for all u (and v) in terms of a definite constant of the system (the constant being determined by f and f , and the separation between the lenses).
T he notion of effective focal length, therefore, does not seem to be meaningful for this system. (b) u = – cm, f = cm, gives v = cm. Magnitude of magnification due to the first (convex) lens is . u = + ( – ) cm = + cm (object virtual); f = – cm which gives v cm Magnitude of magnification due to the second (concave) lens = / .
Net magnitude of magnification = . Size of the image = . cm . If the refracted ray in the prism is incident on the second face at the critical angle i c , the angle of refraction r at the first face is ( °– i c ).
Now, i c = sin – ( / . ) ~ ° Therefore, r = ° sin i = . ; i ~ ° . Two identical prisms made of the same glass placed with their bases on opposite sides (of the incident white light) and faces touching (or parallel) will neither deviate nor disperse, but will mearly produce a parallel displacement of the beam.
(a) To deviate without dispersion, choose, say, the first prism to be of crown glass, and take for the second prism a flint glass prism of suitably chosen refracting angle (smaller than that of crown glass prism because the flint glass prism disperses more) so that dispersion due to the first is nullified by the second. (b) To disperse without deviation, increase the angle of flint glass prism (i.e., try flint glass prisms