# Problem about application of E = hv to white light

The reason why we see white light is because we see all the light in the visible lights spectrum.

So let's say I have a light bulb that gives out white light, the reason why it gives out light its because it keeps emitting a constant amount of energy. By E = hv, we should only be able to see one kind of light. ( for instance, the energy emitted by light is 100 J and the frequency of the light wave should be 100/h, which means one kind of light) So why can we still see white light?

What is wrong with my concept? Thanks a lot!

## Answers and Replies

bhobba
Mentor
By E = hv, we should only be able to see one kind of light. ( for instance, the energy emitted by light is 100 J and the frequency of the light wave should be 100/h, which means one kind of light) So why can we still see white light?

You logic for that conclusion escapes me.

The reason we see white light is we have receptors for many different photon energies and when they are all present we perceive that as white.

Thanks
Bill

A 100 watt light bulb radiates 100 Joules per second in a wide range of frequencies, only some of which are in the visible part of the spectrum. So in 1 second 100 Joules of photons are emitted in many colors. E=hv gives the energy of just *one* photon, and v=E/h its color. Since, there is no such thing as a white photon you cannot use use v = E/h this way. Using the formula for v with E = 100 Joules would only make sense, for example, if the 100 watt bulb radiated just one 100 Joule photon per second. And that would not be white light!

• vanhees71
Using the formula for v with E = 100 Joules would only make sense, for example, if the 100 watt bulb radiated just one 100 Joule photon per second. And that would not be white light!

This made me laugh I mean, it's true of course, but my God is it an understatement.

• vanhees71 and James_Harford