Question

calculate number of photons in 1000 litre of radiation in a thermal equilibrium at 500 k

Answer 1

Number of photons =

Total energy

Energy of one photon

Few instructors will make the question so simple. Instead, they might disguise it as follows.

EXAMPLE

A common laser pointer produces 1.0 mW at a wavelength of 670 nm. Calculate the number of photons produced per millisecond.

Solution

Step 1. Calculate the energy of a photon.

E

=

h

c

λ

=

6.626

×

10

34

J⋅s

×

2.998

×

10

8

m⋅s

−

1

670

×

10

−

9

m

=

2.965

×

10

−

19

J

( 3 significant figures + 1 guard digit)