You are taking pictures of the beach at sunset. Just before the Sun sets, a shutter speed of 1/100 and an aperture of ƒ/11 produces a properly exposed picture. Shortly after the Sun sets, however, your light meter indicates that the scene is only one-quarter as bright as before.
(a) If you don't change the aperture, what approximate shutter speed is needed for your second shot?
(b) If, instead, you keep the shutter speed at 1/100 s, what approximate ƒ-stop will be needed for the second shot?