Light behaves as both a wave and a particle, and you need both descriptions to fully explain how it interacts with matter. James Clerk Maxwell showed that light is an electromagnetic wave, while later experiments revealed that light also comes in discrete packets called photons. Together, these models form the foundation for how astronomers extract information from starlight.

Maxwell's Electromagnetic Model of Light

Maxwell's equations unified electricity and magnetism into a single framework. They predicted that oscillating electric and magnetic fields could travel through space as a wave, and that this wave would move at exactly the speed of light.

In an electromagnetic wave, the electric and magnetic fields oscillate perpendicular to each other and perpendicular to the direction the wave travels.
Maxwell's prediction was confirmed experimentally by Heinrich Hertz, who generated and detected radio waves in the lab.
Polarization provides further evidence for the wave model. Because light is a transverse wave, its electric field can oscillate in a preferred direction. Polarized sunglasses work by blocking light oscillating in certain orientations, reducing glare.

Wavelength, Frequency, and the Speed of Light

Three quantities describe any electromagnetic wave, and they're locked together by one key equation.

Wavelength ( $\lambda$ ): the distance between two consecutive crests (or troughs). Red light has a longer wavelength (~700 nm) than blue light (~450 nm).
Frequency ( $f$ ): the number of complete wave cycles passing a point each second, measured in hertz (Hz). Blue light has a higher frequency than red light.
Speed of light ( $c$ ): in a vacuum, all electromagnetic waves travel at approximately $3 \times 10^8$ m/s.

These three are related by:

$c = \lambda f$

Because $c$ is constant, wavelength and frequency are inversely related. If frequency goes up, wavelength must go down. Gamma rays, for example, have extremely high frequencies and very short wavelengths, while radio waves have low frequencies and long wavelengths.

Maxwell's electromagnetic model of light, 16.3 Energy Carried by Electromagnetic Waves – University Physics Volume 2

Photons and the Particle Model

Sometimes light behaves not as a continuous wave but as a stream of individual energy packets called photons. Each photon carries an amount of energy set by its frequency:

$E = hf$

Here $h$ is Planck's constant ( $6.626 \times 10^{-34}$ J·s). Higher-frequency photons (like ultraviolet) carry more energy than lower-frequency photons (like infrared).

Two experiments were especially important in establishing the particle model:

Photoelectric effect: When light above a certain threshold frequency strikes a metal surface, it ejects electrons. Increasing the light's intensity (brightness) doesn't help if the frequency is too low. This showed that energy arrives in discrete chunks, not as a smooth wave. (This principle underlies how solar panels convert light into electric current.)
Compton scattering: X-ray photons colliding with electrons lose energy and shift to longer wavelengths, just as you'd expect from a particle-to-particle collision.

Wave-particle duality is the term for this dual nature. Light isn't purely a wave or purely a particle; which behavior you observe depends on the experiment you're running.

Wave Properties of Light

Several phenomena only make sense if light is a wave:

Refraction: Light changes speed when it moves from one medium to another (say, from air into glass), causing it to bend. This is how a prism spreads white light into a rainbow of colors, a process called dispersion.
Diffraction: Light waves bend around obstacles or spread out after passing through narrow openings. This effect becomes noticeable when the opening is close in size to the wavelength.
Interference: When two light waves overlap, they can reinforce each other (constructive interference) or cancel each other out (destructive interference), producing characteristic bright and dark patterns.
Reflection: Light bounces off a surface, changing direction. The angle of reflection equals the angle of incidence.

Maxwell's electromagnetic model of light, 16.2 Plane Electromagnetic Waves – University Physics Volume 2

Brightness and Distance

Distance and Apparent Brightness

A star's true energy output and how bright it looks from Earth are two different things. Sorting out the difference is one of the central tasks in astronomy.

Luminosity is the total energy a star (or any object) radiates per second. It's an intrinsic property that doesn't change no matter how far away you are.
Apparent brightness is how bright that object looks to an observer on Earth. It depends on both the object's luminosity and its distance from you.

The connection between them follows the inverse square law:

$B \propto \frac{1}{d^2}$

If you double your distance from a light source, its apparent brightness drops to one-quarter. Triple the distance, and brightness falls to one-ninth.

This is why the Sun, a fairly ordinary star, dominates our sky: it's only about 93 million miles (1 AU) away. Meanwhile, the Andromeda Galaxy contains hundreds of billions of stars yet appears as a faint smudge because it's roughly 2.5 million light-years from Earth. Knowing the inverse square law lets astronomers work backward: measure a star's apparent brightness, compare it to its known luminosity, and calculate how far away it is.