Unizor - Physics4Teens - Energy - Light as Energy Carrier

Notes to a video lecture on http://www.unizor.com

Light as Energy Carrier

Sun is a source of light and a source of heat. Just looking at light, we don't see energy it carries, but, touching a surface that was under Sun's light for some time, we feel its temperature, which is a measure of inner energy of the molecules inside the object. The object warms up under the Sun's light, which can only be explained by energy the light carries.

Let's conduct an experiment with light to prove that it carries energy.
On YouTube there are a few videos that show it, one of them demonstrates a radiometer.

The radiometer ("solar mill") looks like this:

Four small plates are arranged on a spinning wheel in the vacuum to avoid interference of air motion. Each plate has two surfaces, one silver and another black.
When light is directed on this "solar mill", black sides absorb light and are heated more than silver ones that reflect light.
As a result, the molecules near the black surface are moving more intensely than those near the surface of the silver side, thus pushing the silver side more and causing the rotation of the "solar mill".

So, there is no doubt that light carries energy. An important question is, how it does it.
We used to think about heat as the intensity of molecular motion. In case of light there is no such motion, light travels from Sun to Earth through vacuum.
Apparently, the situation is similar to gravity in a sense that gravity carries energy, but does not require any medium, like molecules, to carry it. Recall that we have introduced a concept of a field as a certain domain of space were forces exist and energy is present without any material substance. Somewhat similar situation is with light. Its nature is the waves of electromagnetic field.

Electromagnetic field is a completely different substance than gravitational field, but both are capable to carry energy without any material presence.

Classifying light as the waves of an electromagnetic field, we are opening the door to using this wave model for explanation of different kinds of light.
First of all, let's consider the light we see with a naked eye. The vision itself is possible only if the light carries some energy, that agitates some cells inside our eye, that, in turn, send an electric signal to a brain - one more argument toward a light as a carrier of energy.

Light that can be seen by a naked eye is called visible. But what about different colors that we can differentiate by an eye and different intensity of light that we view as "bright" or "faint"? The only explanation within a wave theory of light is that different intensities and colors that our eye sees are attributable to different kinds of waves of an electromagnetic field.

Any wave has two major characteristics: amplitude and frequency. This is similar to a pendulum, where amplitude is the maximum angle of deviation from a vertical and frequency is measured as a number of oscillations per unit of time. Light, as a wave, also has these two characteristics. The amplitude is an intensity of light, while frequency of the visible light is viewed as its color.

We all know that the photo laboratories, developing old fashioned films, are using rather faint red light during the developing process in order not to overexpose the film to light. The obvious reason is that red light carries less energy than white one, that is known to be a combination of many differently colored kinds of light. So, the color-defining frequency, as well as an intensity-defining amplitude of electromagnetic waves, determine the amount of energy carried by light.

Not only visible light is a manifestation of electromagnetic waves. From cosmic radiation called gamma rays to x-rays to ultra-violet light to visible light to infra-red light to microwave to radio waves - all are electromagnetic waves of different frequencies.

The unit of measurement of frequency is called Hertz, abbreviated as Hz with 1 Hz meaning 1 oscillation per second.

The range of frequencies of electromagnetic waves is from a few oscillations per second for very low frequency radio waves to 10²⁴ oscillations per second for very high frequency gamma rays. The visible spectrum of frequencies is close to 10¹⁵ oscillations per second with the light perceived as red having a smaller frequency around 0.4·10¹⁵ Hz, followed in increasing frequency order by orange, yellow, green, blue and violet with a frequency around 0.7·10¹⁵ Hz.

Amount of energy carried by light depends on the amplitude and frequency of electromagnetic waves that constitute this light. Generally speaking, the higher the amplitude - the higher the energy is carried by light in a unit of time and, similarly, the higher the frequency - the higher the energy is carried by light in a unit of time.

Dependency of the energy carried by light on the frequency of electromagnetic waves that constitute this light is a more complex problem, that was solved in the framework of the Quantum Theory of light. According to Quantum Theory, electromagnetic waves propagate in packets called photons. Each photon carries an energy proportional to the frequency of waves that constitute this photon, and the amplitude of electromagnetic waves is, simply, a measure of the number of photons participating in these electromagnetic waves. That's why the picture of a light as a sinusoidal wave is a very simplified view on the nature of light as it is understood by contemporary physics.