Question

a light source of wavelength 2000°A emits 0.5 J energy , the number of photons emitted from the souce will be
​

Answer 1

Explanation:

The energy of a photon is E= bc/λ

=

632.8×10

−9

(6.62×10

−34

)(3×10

8

)

=3.14×10

−19

J

Given, the energy emitted per second by laser =5mW=5×10

−3

J

Thus the number of photons emitted per second =

3.14×10

−19

5×10

−3

=1.6×10

16

Answer 2

Given: A light source of wavelength 2000°A emits 0.5 J energy.

To find: Number of photons emitted

Explanation: Let the wavelength of the light source be denoted as l, energy by E and number of photons as n.

E = 0.5 J

l = 2000 A

= 2000 * 10^-10 m ( 1 A = 10^-10 m)

Energy of a single photon=$\frac{hc}{l}$

where h is Planck's constant whose value is $6.6 \times {10}^{ - 34}$ and c is speed of light.

Using formula:

=$\frac{6.6 \times {10}^{ - 34} \times 3 \times {10}^{8} }{2000 \times {10}^{ - 10} }$

=$9.9 \times {10}^{ - 19}$

Number of photons

= Total energy/ Energy of a photon

=$\frac{0.5}{9.9 \times {10}^{ - 19} }$

=$5.05 \times {10}^{17}$

Therefore, $5.05 \times {10}^{17}$ photons are emitted from the source.