Physics Question: On Velocity-Wavelength- Frequency (1 Viewer)

[theonly1]

hey,

recently i got a question from my teacher below and we seem to be coming to two different answers. Hopefully you may be able to shed some light on the issue. The question is

1) The velocity of red light with wavelength of 550nm and a period of 5.0 is?

My Working:

Since i need to find the velocity and i have the wavelength and can find the frequency i would use the formula V= f λ

Period = 5.0 so to convert this to frequency is 1/t so
frequency= 1/5

Now need to convert 550nm into m: (550nm = 5.5 × 10^7m)

Now need to apply the formula: V=f λ

V= (5.5 x 10^7) x (1/5)
= 0.00000011
= 1.1 x 10^-7

Did i do anything wrong? my teacher believes the answer to be 110m/s

 

[someth1ng]

Looks fine to me but I'd give you 0 because you don't have "-" in front of some 7s and no units.

The question is really, really shit. It doesn't make sense. You don't get red light with those conditions.

 

[hjed]

the answer is 3x10^8 m.s^-1 in the positive direction or something very close to it. In physics exams light always travels at c

 

[theonly1]

Looks fine to me but I'd give you 0 because you don't have "-" in front of some 7s and no units.

The question is really, really shit. It doesn't make sense. You don't get red light with those conditions.

Lucky for me the question was a multiple choice;

a) 110m/s
b) 2750 m/s
c) 1.1 x 10 ^-7
d) 1.1 x 10^-9

My teacher said it is (a).... she must of got her conversions mixed up. Which (a.b.c.d) would you have picked?

 

[theonly1]

the answer is 3x10^8 m.s^-1 in the positive direction or something very close to it. In physics exams light always travels at c

even with red light? just act like we dont know what c is, what would the answer be in this question?

 

[nerdasdasd]

c is ALWAYS 3 x 10^-8......

 

[AnimeX]

c is ALWAYS 3 x 10^-8......

not in another medium (e.g. glass) o.o is it?

 

[hjed]

No but they will usually specify it is in a different medium. In a real exam this question would be about sound waves.

 

[superSAIyan2]

you'd need a REALLLLLLY DENSE medium for light to travel at either answer

 

[someth1ng]

C is the best answer.

So you guys know, c is only 3x10⁸ ms^-1 in a vacuum. The velocity is different in other mediums - that's why we get refraction.

The density of the object must be VERY high - probably not even an ordinary state of matter.

 

[mahmoudali]

you said period is 5... ?? what units?

 

[theonly1]

you said period is 5... ?? what units?

Seconds