Earth as a System: Energy, Matter, and Life: CBSE Class 9 Physics practice questions

C. Killing germs in water purifiers

UV rays ($100$–$400\ \text{nm}$, where $1\ \text{nm} = 10^{-9}\ \text{m}$) carry more energy than visible light. They are used to kill germs in water purifiers and help power fluorescent lights. Because long exposure harms the eyes and skin and raises cancer risk, people use UV-protective glasses and sunscreen. Photosynthesis uses visible light, signals use radio waves, and night-time warmth is infrared trapped by greenhouse gases.

B. $500\ \text{s}$

$$t = \dfrac{d}{c} = \dfrac{1.5 \times 10^{11}}{3 \times 10^{8}} = 0.5 \times 10^{3} = 500\ \text{s}$$ which is about 8.3 minutes. Light can cross a vacuum because it is an electromagnetic wave, while sound (a mechanical wave) cannot. $4.5 \times 10^{19}$ s comes from multiplying instead of dividing, $2 \times 10^{-3}$ s from dividing the wrong way round, and $50$ s from slipping a power of ten.

C. 29%

Loss $= 1400 - 1000 = 400\ \text{W m}^{-2}$, and $$\dfrac{400}{1400} \times 100 \approx 29\%$$ This energy is absorbed and scattered by gases, clouds and dust. 40% divides the loss by 1000 instead of by the starting value, 71% is the fraction that does reach the ground, and 0.4% writes the $0.4\ \text{kW}$ loss as if it were a percentage. The solar constant ($1.4\ \text{kW m}^{-2}$) is the energy per second per square metre facing the Sun before any absorption.