Explanation

The Doppler effect is applicable to all types of waves, including sound waves, light waves, and even water waves.

It describes the change in frequency or wavelength of a wave when there is relative motion between the wave source and the observer. 

Explanation

 The phrase "at time" means "precisely" or "exactly" when used with the verb "arrive."

So, "the train always arrives at time" means the train always arrives on schedule, without any delays.

Explanation

Oxygen ions (O²⁻) are highly reactive and readily combine with other elements to form more stable compounds

In aqueous solutions, they readily react with water to form hydroxide ions (OH⁻) and oxygen gas (O₂), effectively neutralizing their existence as free O²⁻ ions.

This makes it extremely difficult for O²⁻ to exist independently in a solution.

Explanation

Refractive index is both unitless and dimensionless. 

Refractive index is defined as the ratio of the speed of light in a vacuum to the speed of light in a medium.

Since both speeds have the same units (meters per second),

The units cancel out, making refractive index a dimensionless quantity.

It's also unitless because it doesn't have any associated units

Explanation

Secondary cells are rechargeable, meaning they can be recharged and used multiple times, unlike non-rechargeable primary cells. 

Explanation

“As for me, I would be happy to dedicate a few extra hours for the humanitarian cause.”

 As for → Used to introduce a new topic or refer to oneself/another in a sentence:

“As for me, I would be happy to help.”