## anonymous one year ago The retina of a human eye can detect light when radiant energy incident on it is at least 4.0 x 10^-17 J. For light of 505 nm wavelength, how many photons does this energy correspond to?

1. cuanchi

you have to calculate the energy of one photon E= h c /wavelength (in meters) convert the 505 nm to meters h= 6.626 x 10^(-34) Js (Plank's constant) c= 3.0 x 10^8 m/s (speed of light) After you calculate the energy of one photon of 505nm you have to divide the total energy 4.0 x 10^-17J by the energy of the one photon to calculate how many photons you have in that amount of energy

2. anonymous

I got 9.75 x 10^-37 but I think I did it wrong.

3. aaronq

you should post your work, but yeah, that number is not right - you should get a number of photons, not energy

4. anonymous

$E = \frac{ 6.626 x 10^{-34}J/s x 3.0 x 10^{8}m/s }{5.05x10^{-7} m }$

5. anonymous

$\frac{ 3.9 x 10^{-19} J }{ 4.0 x 10^{-17}J }$

6. anonymous

That's how I did it

7. aaronq

good stuff. it's good up until the division. you were supposed to the divide the threshold energy, 4.0*10^-17 J by the energy of the photon you can calculated.

8. anonymous

so the 4.0/3.9?

9. aaronq

yes

10. anonymous

1.0256 x 10-36

11. aaronq

you're pluggin it into the calculator incorrectly

12. aaronq

the answer is 102.5641, you should try to get this number

13. anonymous

What order did you put it into your calculator?

14. aaronq

i use this http://web2.0calc.com/

15. aaronq

havent used a normal, handheld calculator in a long time

16. anonymous

Think my calculator is just messing up.

17. anonymous

Thanks for the help.

18. aaronq

yeah, it's tricky pluggin in negative exponents. no problem

19. cuanchi

@staldk3 Try to use parentheses when you enter the denominator with a power of ten, otherwise the calculator will understand that you want to multiply everything by 10^-19 in place to divide by the 10^-19 (4.0 x 10^-17) / (3.9 x 10^-19) = 1.03 x 10^2 Otherwise the calculator understand (4.0 x 10^-17 / 3.9) x 10^-19 = 1.03 x 10^-36