The dioptric power (or focusing power) of a lens is the inverse of the focal length, or the refractive index of the surrounding medium divided by the focal length. Ordinary lenses as used in laser technology have focal lengths somewhere between 5 mm and many meters. Ball lenses and tiny as...
An object is at a distance of 20 m from a convex lens of focal length 0.3 m. The lens forms an image of the object. If the object moves away from the lens at a speed of 5 m/s, the speed direction of the image will be:
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