Electromagnetic Radiation:

Readings: Schneider & Arny: Unit 21, 22, 25, 30

Maxwell showed in the mid-1800's that light is energy carried in the form of opposite but supporting electric and magnetic fields in the shape of waves, i.e. self-propagating electromagnetic waves or electromagnetic radiation.

The wavelength of the light determines its characteristics. For example, short wavelengths are high energy gamma-rays and x-rays, long wavelengths are low energy radio waves. The whole range of wavelengths is called the electromagnetic spectrum.

Our eyes only see over the following range of wavelengths, called the visible portion of the spectrum:

The different wavelengths of visible light are divided into the colors


Due to its wave-like nature, light has three properties when encountering, or passing through, a medium:

1) reflection
2) refraction
3) diffraction

When a light ray strikes a medium, such as oil or water, the ray is both refracted and reflected as shown below:

The amount of refraction increases for a denser medium, meaning the angle of refraction decreases, which is why glass can form a lens. And the angle of refraction is also a function of wavelength (i.e. blue light is more refracted compared to red) which is why a prism breaks white light into its colors. This is also the origin to rainbows from drops of water.

Another example of refraction at sunset is the Green Flash.

Diffraction is the constructive and destructive interference of two beams of light that results in a wave-like pattern. When two peaks of a wave intersect they combine to make a bigger wave. When a peak and a trough intersect, they cancel.

click here to see interference movie

Inverse Square Law:

The brightness of an object varies inversely as the square of the distance. This means that objects farther away are dimmer.

But notice that the dimming does not progress in a linear fashion (i.e. 1, 2, 3, 4 ...) but rather in an inverse square (i.e. 1/2, 1/4, 1/8, 1/16 ...).

Doppler effect:

The Doppler effect occurs when an object that is emitting light is in motion with respect to the observer. If the object is moving towards the observer the light is ``compressed'', meaning that the wavelength of the light becomes smaller. Smaller wavelength means bluer light, so we say the object is blueshifted. If the object is moving away from the observer the light is ``expanded'', the wavelength is increased or redshifted.

Notice that the speed of light does not change, only the wavelength. It is a basic premise of the theory of relativity that the velocity of light never changes regardless of the motion of the observer.

Electromagnetic radiation is expressed in either wavelength, λ in centimeters, or frequency, ν in Hertz (sec-1). The relationship between wavelength and frequency is

νλ = c

where c is the speed of light, 3x1010 cm/sec.

The Doppler effect relates a change in wavelength, Δλ, to the original wavelength, λ, and the velocity of the source, v, such that

Δλ/λ = v/c

For example, the change in wavelength for a 50 cm radio wave from a car moving at 3000 cm/sec (about 45 mph) is

Δλ/(50 cm) = (3000 cm/sec)/(3x1010 cm/sec)

Δλ = 5x10-6 cm

a very small change.

Observational Astronomy:

Astronomy is a passive science, but observing phenomenon at different wavelengths has several advantages to overcome the lack of making experiments. There are different physics at different wavelengths, for example, high energy magnetic fields are seen in the x-ray, radiation from heat is seen in the infrared.

However, note that observing at different wavelengths requires vastly different technology and conditions. In particular, our atmosphere is opaque to certain wavelengths (good for us) meaning that they can only be observed from space (expensive for astronomers).