A very hot glass on putting in cold water cracks. What does this change indicate?

What does the diagram indicate ?

What are carbon resistors ? What does the colour indicate ?

What does higher and lower BOD levels indicate?

Refractive index of glass relative to water is 9/8. What is the refractive index of water relative to glass? (AS_(1))

When sound travels from air to water, which parameter does not change?