The greenhouse effect is mainly due to increase in MCQ with Answer and Explanation

Explanation:
Echo is the reflection of sound waves from a rigid surface, returning to the listener after a time delay. For distinct echo, minimum distance to reflector is about 17 m (so time delay > 0.1 s, persistence of hearing). Refraction bends sound in media with varying density; interference combines waves; diffraction bends around obstacles. Memory tip: 'Echo = sound reflection like light in mirror'. This application-based question links wave properties to daily-life phenomena, frequently tested in competitive exams to assess conceptual clarity beyond definitions. Always distinguish echo from reverberation (multiple reflections).

Explanation:
Thin film interference produces colours.

Explanation:
First law of thermodynamics: Energy is conserved in an isolated system. It changes forms (kinetic to potential etc.) but total constant. Perpetual motion machine impossible.