# Energy of the photon

1. Sep 23, 2007

### leah0084

1. The problem statement, all variables and given/known data

An excited atom emits a photon of red light with a wavelength of 701. nm, what is the energy of the photon?----> _________________J

2. Relevant equations

and for this, my application was

firstly get landa from formula of c=landa * nu

and then landa * h, which is planck's constant which is 6.626e-34.

since my wavelength is 701 nm, it can be 701e-9 in order to change to meter format.

so my final application would be {(2.998e8m/s)/(701e-9m)} * ( 6.626e-34J*s)= 2.83e-19 (after rounding up to get significant figures which is 3 digits from 701nm)

3. The attempt at a solution

2.83e-19

2. Sep 23, 2007

### genneth

1. Units! 2.83e-19 even if it's correct numerically is still wrong. You *need* to always have units.

2. Stay in symbols! Write down $$E = \frac{hc}{\lambda}$$, then just substitute in the numbers, after conversion to correct units.

(Btw, it's "lambda" not "landa")

3. Sep 23, 2007

### leah0084

i did put J (unit) at the end as well and it kept telling me wrong. did i do any conversion wrong?

4. Sep 23, 2007

### genneth

5. Sep 23, 2007

### leah0084

oh, i got it.
the answer needed to be negetive because the question says "emits". so answer had to be negative. i replaced the answer with -2.84e-19 and it was correct. thank you for the help. now i can go to bed.