Question

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?

Answer 1

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

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

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

Answer 6

That's how I did it

Answer 7

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.

Answer 8

so the 4.0/3.9?

Answer 9

yes

Answer 10

1.0256 x 10-36

Answer 11

you're pluggin it into the calculator incorrectly

Answer 12

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

Answer 13

What order did you put it into your calculator?

Answer 14

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

Answer 15

havent used a normal, handheld calculator in a long time

Answer 16

Think my calculator is just messing up.

Answer 17

Thanks for the help.

Answer 18

yeah, it's tricky pluggin in negative exponents.

no problem

Answer 19

@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