# Question Video: Water Bottle Thermal Radiation Physics

A bottle containing water is placed in constant-intensity sunlight and absorbs infrared radiation. During the bottle’s exposure to sunlight, the temperature of the water remains constant. Which of the following statements is correct? [A] The water does not receive any energy from the sunlight. [B] Infrared radiation absorption and emission have no effect on temperature. [C] The water absorbs more energy from infrared radiation than the energy it loses by cooling. [D] The water loses exactly as much energy by cooling as the energy of the infrared radiation that it absorbs. [E] The water loses more energy by cooling than the energy of the infrared radiation that it absorbs.

02:34

### Video Transcript

A bottle containing water is placed in constant-intensity sunlight and absorbs infrared radiation. During the bottle’s exposure to sunlight, the temperature of the water remains constant. Which of the following statements is correct? (A) The water does not receive any energy from the sunlight. (B) Infrared radiation absorption and emission have no effect on temperature. (C) The water absorbs more energy from infrared radiation than the energy it loses by cooling. (D) The water loses exactly as much energy by cooling as the energy of the infrared radiation that it absorbs. (E) The water loses more energy by cooling than the energy of the infrared radiation that it absorbs.

So we have here this bottle of water, and it’s absorbing infrared radiation from the Sun. At the same time, we’re told that the water in the bottle has a constant temperature. We want to pick which of our answer options explains how it is that this water can absorb radiation and yet maintain a constant temperature.

The first answer choice claims that the water does not receive any energy from the sunlight. We know though that this is not true. Any time, say, we go stand out in the Sun, we can feel the warmth of sunlight. Sunlight transfers energy to whatever absorbs that light.

Option (B) says that infrared radiation absorption and emission don’t affect temperature. We know, however, that this isn’t true either. If we leave an ice cube, for example, out in the sunlight, the infrared radiation from the light that is absorbed by the cube may well raise the temperature of that cube so that it melts.

Answer option (C) says that the water in our bottle absorbs more energy from infrared radiation than the energy it loses by cooling. That energy lost through cooling happens through the emission of infrared radiation. So the water absorbs infrared radiation, and then it emits some. If the water absorbed more energy from infrared radiation than the energy it loses though, then that would mean its temperature would increase. Since the temperature of the water is constant, we know that this doesn’t actually occur.

In contrast to option (C), option (D) says that the water loses exactly as much energy by cooling as it receives from infrared radiation. In other words, the amount of energy transferred to the water is the same as the amount of energy transferred away from it. When those amounts were the same, we would expect the water’s temperature to remain constant. Answer option (D) looks like it will be our answer. But just to make sure, let’s look at the last choice.

Option (E) says that the water loses more energy by cooling than the energy of infrared radiation that it absorbs. If this happened though, the temperature of the water would decrease. The water will be transferring more energy away from it than it takes in through absorption.

This confirms answer option (D). The water loses exactly as much energy by cooling as the energy of the infrared radiation that it absorbs.