What type of waves require a medium in order to travel

With the simplified model of the eye, what corrective lens (specified by focal length as measured in air) would be needed to enable a person underwater to focus an infinitely distant object? (be careful-the focal length of a lens underwater is not the same as in air! assume that the corrective lens has a refractive index of 1.62 and that the lens is used in eyeglasses, not goggles, so there is water on both sides of the lens. assume that the eyeglasses are 1.90

Research and discuss the aurora borealis in terms of electric charges, magnetic fields, and forces. which times of the year are best for seeing the northern lights, and where are the best places to view them? explain your answers. also identify links to good images. if you’ve ever seen the aurora borealis, describe your experience and note the time and place that you saw it.

Which of the following is an example of a compound machine. a. a bicycle chain. b. a coffee mug. c. a paper cutter