question_answer

(a) People usually prefer light-coloured dresses during summer and dark dresses during winter. Why? (b) How would a blue object appear under sodium lamp light? (c) What does a welder protect against when he wears a mask? (d) Explain why the sky is blue, and the sun appears red at sunset.          

Answer:

                (a) A white surface reflects all the light incident on it and a black surface absorbs all the light incident on it. In summer, light coloured dresses absorb very little energy and one is saved from heat. In winter, dark dresses absorb most of the light energy which gets converted into heat and keeps warm. (b) Sodium lamp gives yellow light. A blue object absorbs yellow light falling on it from the lamp. So the object appears black. (c) The welder protects his eyes from ultraviolet radiations from the welding arc. Filter of the mask absorbs ultraviolet rays. (d) Blue colour of the sky. The scattering of light by the atmosphere is colour-dependent. According to Rayleigh's law, the intensity of scattered light,\[I\propto 1/{{\lambda }^{4}}\]. Blue light is scattered much more strongly than red light. The blue component of light is proportionately more in the light coming from different parts of the sky. This gives the impression of the blue sky. During sunrise or sunset, the sun is near the horizon. Sunlight has to travel a greater distance. So shorter waves of blue region are scattered away by the atmosphere. Red waves of longer wavelength are least scattered and reach the observer. So the sun appears red.