# Physics Part 2

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

#### chandler742

##### Senior Member
7+ Year Member
15+ Year Member

Here is another example of using your library of physics concepts.

Scientists have deduced that a distant galaxy is in rotational motion because they observed that two EM waves emitted from two different locations within the galaxy have different frequecies. Which law did they use to deduce?

A) Conservation of Angular momentum
B) Law of Gravitation
C) Hyguen's Principle
D) Doppler Effect

#### rCubed

##### taiko master
10+ Year Member
15+ Year Member
what exactly is hyugen's principle, for the life of me, i can't remember it

#### freakazoid

##### Guy Friend Extraordinaire
7+ Year Member
15+ Year Member
I'd go with D too. Red shift on one side, blue shift on another.

Ditto on the Hugyen's principle. I looked it up online, but it doesn't make any sense to me.

#### chandler742

##### Senior Member
7+ Year Member
15+ Year Member
freakazoid, you are on a roll.

If one point is red shifted and another point is blue shifted that means two different parts of the galaxy is moving away from another like points on a wheel. Red shift means it is moving away and blue shift is moving toward the source.

Similarly, in a laser doppler probe to measure RBC in a blood vessel, if RBC is moving toward the probe the frequency is blue shifted and if RBC is moving away from the probe it is red shifted.

Hence, it uses the doppler effect.

Oh by the way, I got this question as a free response Q when I took the real mcat before i started working at kaplan.

Hyguen's principle states that light behaves like a wave because any point on a wavefront can act like a secondary wave.

#### freakazoid

##### Guy Friend Extraordinaire
7+ Year Member
15+ Year Member
Ummm . . . what does that mean? Referring to the secondary wave thing . . . and how does that help you come to the conclusion that light acts like a wave? Thanks.

#### chandler742

##### Senior Member
7+ Year Member
15+ Year Member
I'll give you an example of the secondary wave.

Classically light has been thought to have dual properties i.e. matter and wave.

Einstein's Photoelectic effect supported the matter hypothesis.

On the other hand, interference and diffraction supported the wave theory.

What I mean by a secondary wave is as follows. Suppose you had a small slit that allowed a monochromatic light to enter the slit.

IF light behaved like matter the light would go throught the slit in a straight line to the other side.

However, light doesn't behave like matter in this experiment.
When a monochromatic light enters a slit. The slit acts like a point source of light i.e. the waves move radially away from the slit as if the slit was the source of the wave. This is what I mean by secondary wave.

Furthermore, when light moves out radially you will see the classic constructive and destructive interference characteristic of waves on the other side(dependant upon the angle on the other side). Thus, this proved that light behaves like a wave.