Light — Speed, Reflection, Refraction & Spectrum

Light in physics is electromagnetic radiation. The main ideas students usually need are simple: light has a fixed speed in vacuum, it reflects at a surface, it refracts when it enters a new medium, and visible light is only a small part of the electromagnetic spectrum.

In vacuum, light travels at

$$c \approx 3.00 \times 10^8\ \mathrm{m/s}$$

At a boundary, some of the light can reflect, some can refract, and some can do both. That depends on the materials and the angle.

If you want the fast picture, keep four ideas in mind:

• light has a definite speed in vacuum
• reflection means the ray stays in the same medium and bounces from a surface
• refraction means the ray enters a new medium and changes direction
• spectrum describes how light can be ordered by wavelength or frequency

What Light Means In Physics

In introductory physics, light is treated as an electromagnetic wave. In modern physics it also shows particle-like behavior, but for reflection, refraction, and the visible spectrum, the wave model is usually the one you need first.

The vacuum relation is

$$c = \lambda f$$

where $\lambda$ is wavelength and $f$ is frequency. A shorter wavelength means a higher frequency. That is why blue-violet visible light has a shorter wavelength than red visible light.

In a material, light usually travels more slowly than it does in vacuum. In the standard introductory model,

$$v = \frac{c}{n}$$

where $n$ is the refractive index of the material. This relation is the standard introductory model for light in a material, and it explains why refraction happens.

Reflection: Same Medium, Equal Angles

Reflection happens when light hits a boundary and stays in the original medium. A flat mirror is the simplest example.

The law of reflection is

$$\theta_i = \theta_r$$

where the angle of incidence and the angle of reflection are both measured from the normal, not from the surface. If you measure from the surface, the setup is wrong before the calculation even starts.

Refraction: New Medium, New Speed

Refraction happens when light crosses into a different medium and its direction changes because its speed changes. The main rule is Snell's law:

$$n_1 \sin \theta_1 = n_2 \sin \theta_2$$

If light enters a higher-index medium, it bends toward the normal. If it enters a lower-index medium, it bends away from the normal, provided refraction still occurs.

In an ordinary boundary problem, the frequency is treated as unchanged while the speed and wavelength adjust to the new medium. That is why light from the same source does not become a different color just because it entered glass.

Worked Example: Light From Air Into Glass

Suppose light goes from air into glass with

$$n_1 = 1.00, \qquad n_2 = 1.50, \qquad \theta_1 = 30^\circ$$

First find the speed in glass:

$$v = \frac{c}{n} = \frac{3.00 \times 10^8}{1.50} = 2.00 \times 10^8\ \mathrm{m/s}$$

Now find the refracted angle with Snell's law:

$$1.00 \sin 30^\circ = 1.50 \sin \theta_2$$

Since $\sin 30^\circ = 0.5$,

$$0.5 = 1.50 \sin \theta_2$$

so

$$\sin \theta_2 = \frac{1}{3}$$

and therefore

$$\theta_2 = \sin^{-1}\left(\frac{1}{3}\right) \approx 19.5^\circ$$

This result makes physical sense. The light slows down in glass and bends toward the normal because glass has the larger refractive index.

Visible Spectrum: Where The Colors Fit

The word "spectrum" can mean two closely related things.

In the broad physics sense, the electromagnetic spectrum is the full range from radio waves to gamma rays. Visible light is only one narrow band inside it.

In ordinary optics, the visible spectrum means the spread of visible wavelengths, often seen when white light passes through a prism or water droplets. Red light is at the longer-wavelength end of the visible range, and violet is at the shorter-wavelength end. The exact visible limits are not perfectly sharp, but a common rough range is about $400$ to $700\ \mathrm{nm}$ in vacuum.

Common Mistakes With Light Problems

Treating visible light as all of light

Visible light is only one part of the electromagnetic spectrum.

Measuring angles from the surface

Reflection and refraction angles are measured from the normal.

Assuming light always bends toward the normal

That only happens when it enters a higher-index medium.

Mixing up speed, frequency, and wavelength

In a medium, speed can change. At a boundary, introductory optics usually keeps the frequency the same and lets the wavelength change.

Where Reflection And Refraction Are Used

These ideas explain mirrors, eyeglasses, cameras, microscopes, rainbows, fiber optics, and many measurement tools. Even advanced optical systems usually build on the same core questions: how fast is the light moving here, and what happens when it meets a boundary?

Try A Similar Problem

Change the example from air-to-glass to glass-to-air or air-to-water and predict the bending direction before you calculate. If you want to try your own version with new angles or refractive indices, GPAI Solver is a practical next step.