# Understanding wavelength and frequency.

1. Jan 22, 2010

### sphyics

"when light enters from one medium to other its frequency remains same whereas wavelength changes."
i'm not able to understand this relation as frequency has a inverse relation to wavelength

2. Jan 22, 2010

### f95toli

Yes, but what IS the relation between frequency and wavelength?
Have you looked at the formula?

3. Jan 22, 2010

### sphyics

frequency is inversely proportional to wavelength
and thats where my problem is if wavelength changes how come the frequency remains constant.

4. Jan 22, 2010

### DocZaius

wavelength * frequency = speed

If frequency remains the same and wavelength changes, then speed must change (this isn't a statement of causality, it's just a look at the equation). Light has different speeds in different mediums so it makes sense that it could change.

Last edited: Jan 22, 2010
5. Jan 22, 2010

### sphyics

i understand, why can't it happen other way frequency changes and wavelength remains same. (and this is what i was pondering about

Last edited: Jan 22, 2010
6. Jan 22, 2010

### DocZaius

Good question! I would be interested in the answer too. I have learned that frequency is determined by the source and wavelength by the medium, but I have not learned why each of those properties are tied to each of those factors.

7. Jan 22, 2010

### f95toli

Well, one "handwaving" argument is that the energy of light is proportional to frequency (E=h*f) and since energy has to be conserved it is the wavelength that must change.

I am sure one can think of a simpler reason, but this is probably as good as any.

Also, remember that -as with everything in physics- this is ultimately an experimental fact, we do know that light travels at different speed in different media, and we also know the if we send in a wave with a given frequency into a medium the wavelength changes whereas the frequency does not.

8. Jan 22, 2010

### sphyics

i was jus trying to picturise the statement
"when light enters from one medium to other its frequency remains same whereas wavelength changes."
http://en.wikipedia.org/wiki/Frequency see the first right hand diagram titled
by using defeniton of wavelength (one crest & one trough) i'm not able to conceptualize the idea of frequency remaining same and wavelength changes.

9. Jan 22, 2010

### mikelepore

The reason the frequency has to be constant when crossing a boundary is because a frequency is a count of individual events, which are, in this case, cycles. It's like if you have birds flying through an open window at a rate of ten per minute, that rate as counted from the outside and that rate as counted from the inside have to be the same.