Which instrument is used to measure altitudes in aircraft's ?

Answer: Option B

Explanation:

Doppler's Effect - The Doppler effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding.

Kirchhoff's Law - Kirchhoff's Law signifies that good absorbers are good emitters.

Stefan's Law - The radiant energy emitted by a black body per unit area per unit time is directly proportional to the fourth power of its absolute temperature.

Answer: Option D

Explanation:

Critical Velocity - The maximum velocity upto which fluid motion is streamline is called Critical Velocity.

Streamline Flow - If velocity of all fluid particles is same all the time then it can be said in streamline flow.

Terminal Velocity - When a body falls in viscous medium, its velocity first increases and finally becomes constant. The constant velocity is called terminal velocity.

Answer: Option C

Explanation:

Cohensive force - Force of attraction between the molecules of same substance is called Cohensive force.

Answer: Option A

Answer: Option C