VISUAL PHYSICS ONLINE

8.3 FROM THE UNIVERSE TO THE ATOM

P83 002

The frequency of light measured by an observer moving at a relative velocity v along the line of sight to the source which emits light at a frequency is given by

Ultraviolet radiation from hydrogen in the laboratory is found to have a frequency of 2.47x10¹⁵ Hz. The same type of radiation coming to Earth from hydrogen atoms in interstellar space is found to have a frequency of 2.44x10¹⁵ Hz.

(A)

Are the atoms approaching or receding from the Earth? Justify your answer.

(B)

Calculate the speed of the hydrogen atoms relative to the Earth.

View solution below only after you have completed answering the question. The solution is not in a form that you would answer in an examination. The answers are often in more detail to help improve your appreciation and understanding of the physics.

Solution

(A)

The hydrogen atoms are receding (moving away) from the Earth since the observed frequency is less than the frequency measured in the laboratory .

(B)

Measurements using the Doppler Effect such as given in this example, provided evidence for the expansion of our Universe.