[WK95]

An excited hydrogen atom emits a photon with a wavelength of 656.4 nm.
 In what region of the spectrum is this photon?
What is the frequency of this photon?

The region of the spectrum is visible orange.

As for the frequency, using c=λv where c=3x10^8 and λ=656.4nm=656.4x10^-9m=6.564x10^-7

From that, I get v=4.57x10^14 Hz. However, that is wrong.

[Corribus]

From that, I get v=4.57x10^14 Hz. However, that is wrong.

Are you sure?

[WK95]

LONCAPA says its wrong. I tried it with and without units.

[Borek]

Perhaps you are expected to enter different number of significant digits, or you are using incorrect format for input, as the number is definitely OK.

[WK95]

If there were an error in signifcant figures, LONCAPA would indicate that I need more or less digits.

[Borek]

http://www.wolframalpha.com/input/?i=wavelength+656.4+nm+frequency+

[WK95]

I also tried that value but it's also wrong. But how come it differs from my value?

[Borek]

how come it differs from my value?

4.5672×10¹⁴ and

4.57x10^14

don't look different to me.

[DrCMS]

They are different because Wolfram Alpha used the speed of light as 299792458 m/s (i.e. the actual value) rather than approximate value of 3x10⁸m/s you used.