A laser produces a 5.00 -mW beam of light, consisting of...

A laser produces a 5.00 -mW beam of light, consisting of photons with a wavelength of 632.8 $\mathrm{nm}$ (a) How many photons are emitted by the laser each second? (b) The laser beam strikes a black surface and is absorbed. What is the change in the momentum of each photon that is absorbed? (c) What force does the laser beam exert on the black surface?

A laser produces a 5.00 -mW beam of light, consisting of photons with a wavelength of 632.8 $\mathrm{nm}$ (a) How many photons are
emitted by the laser each second? (b) The laser beam strikes a black
surface and is absorbed. What is the change in the momentum of
each photon that is absorbed? (c) What force does the laser beam
exert on the black surface?

Key Concepts

Photon Momentum

Despite having no rest mass, photons carry momentum, which is given by the relation p = h/?. This concept is important as it illustrates the particle-like properties of light and explains how photons can exert pressure or transfer momentum upon interacting with matter, such as being absorbed or reflected by a surface.

Momentum Transfer and Force

The transfer of momentum during absorption or reflection of photons results in a force exerted on the material interacting with the light. This force is calculated as the rate of momentum change, derived from the product of the number of photons per second and the momentum per photon. This concept underpins the analysis of radiation pressure and its effects on materials.

Laser Power and Energy Rate

Laser power represents the rate at which energy is emitted from the laser and is measured in watts (joules per second). By relating the power of a light source to the energy per photon, one can determine the number of photons emitted per second. This connection is fundamental to understanding energy conversion in photonic systems.

Photon Energy

Photon energy is a measure of the energy carried by a single photon, defined by the relation E = hf or equivalently E = hc/?, where h is Planck’s constant, f is the frequency of the light, c is the speed of light, and ? is the wavelength. This concept is crucial for understanding how energy is quantized in electromagnetic radiation and forms the basis for calculating how many photons are produced based on the energy available.

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