Scientists such as Heinrich Hertz, Philipp Lenard, Max Planck, and Albert Einstein made scientific contributions that ultimately demonstrated that light is electromagnetic radiation, and that it has a “dual nature.” Some electromagnetic phenomena are best explained with a particle model, and some with a wave model. In a sense, “wave” and “particle” are just easy mental models for light. We employ them because we are used to seeing waves and particles—such as those in water waves and baseballs—in our daily lives. Electromagnetic radiation is a basic concept in physics, but it doesn’t fit completely into one of these neat little boxes. Research and discuss at least one modern technology that employs electromagnetic radiation and that can be explained by the wave model, the particle model, or a combination of the two. Some possible technologies include solar panels, burglar alarms, cameras, and cell phones.​

Aheat engine running backward is called a refrigerator if its purpose is to extract heat from a cold reservoir. the same engine running backward is called a heat pump if its purpose is to exhaust warm air into the hot reservoir. heat pumps are widely used for home heating. you can think of a heat pump as a refrigerator that is cooling the already cold outdoors and, with its exhaust heat qh, warming the indoors. perhaps this seems a little silly, but consider the following. electricity can be directly used to heat a home by passing an electric current through a heating coil. this is a direct, 100% conversion of work to heat. that is, 19.0 \rm kw of electric power (generated by doing work at the rate 19.0 kj/s at the power plant) produces heat energy inside the home at a rate of 19.0 kj/s. suppose that the neighbor's home has a heat pump with a coefficient of performance of 4.00, a realistic value. note: with a refrigerator, "what you get" is heat removed. but with a heat pump, "what you get" is heat delivered. so the coefficient of performance of a heat pump is k=qh/win. an average price for electricity is about 40 mj per dollar. a furnace or heat pump will run typically 200 hours per month during the winter. what does one month's heating cost in the home with a 16.0 kw electric heater? what does one month's heating cost in the home of a neighbor who uses a heat pump to provide the same amount of heating?

Could we see a galaxy that is 20 billion light-years away?

If there's a small amount of friction between two surfaces, the result could be select all that applya. no movement b. heatc. a little bit of movementd. sliding around