4.1: Spectroscopy

Interaction of Light and Matter

What happens when an electromagnetic wave impinges on a material? This depends on whether an object is transparent or opaque to that specific frequency of light. If the object is transparent the wave can pass through it, while if the object is opaque, the wave will bounce off the surface. We normally associate these properties with visible light, but they do apply to all electromagnetic waves. What is not obvious is that something that is transparent to one form of light may be opaque at other frequencies. For example, ordinary glass is transparent to visible light but largely opaque to ultraviolet radiation. Human skin is opaque to visible light, but transparent to X-rays.

Reflection

If a material is opaque to a frequency of light, then the light will bounce off the surface of the object in a process called reflection. The law of reflection states that the angle of reflection equals the angle of incidence. In Figure \(\PageIndex{1}\), the mirror is at the bottom of the image. An incident ray of light comes from the top left corner and strikes the mirror in the center. The angle between the light and imaginary line drawn upwards, or perpendicular, from the point where the light strikes the mirror is the angle of incidence. The angle of the reflected ray is also measured relative to the perpendicular line and is equal to the angle of incidence. This means that the ray of light bounces off the surface at the exact same angle that it hit the surface.

Figure \(\PageIndex{1}\): Reflection. The law of reflection states that the angle of reflection equals the angle of incidence. (CC BY 4.0; Andrew Park via Introduction to Physics (Park)). Accessible description of Figure \(\PageIndex{1}\).

We expect to see reflections from smooth surfaces, but a rough surface reflects light in a different way. Since the light strikes different parts of a rough surface at different angles, it is reflected in many different directions, or diffused. Diffused light is what allows us to see a sheet of paper from any angle. Many objects, such as people, clothing, leaves, and walls, have rough surfaces and can be seen from all sides.

Reflection is very important for use in telescopes. If a surface is smooth, as with a mirror, the direction of the reflected light beam can be calculated accurately depending on the shape of the reflecting surface. This is how curved mirrors are designed to gather and focus light.

Transmission and Refraction

If a material is transparent to a particular frequency of light, then the wave can mostly pass through the object. This process is called transmission. When light passes through a material, the path of the light is changed or bent. Why does light change direction when passing from one material to another? It is because light changes speed when going from one material to another. The amount of refraction, or bending of the light, varies between different materials.

An example of refraction is looking at a fish in a tank. In Figure \(\PageIndex{2}\), a person standing at the corner of a tank is looking at a fish that is floating close to the corner of the tank. To the observer, the same fish appears to be in two different places. This illusion is caused by refraction when light coming from the fish to the observer changes direction when it leaves the tank. Viewed from above, the light from the fish can travel two different paths to get to the observer's eyes, creating the image of two fish. Refraction is responsible for a tremendous range of optical phenomena, from the action of lenses to voice transmission through optical fibers.

Figure \(\PageIndex{2}\): Refraction. In a rectangular fish tank, a fish can appear in two different locations, because light changes directions when it passes from water to air. (CC BY 4.0; Andrew Park via Introduction to Physics (Park)). Accessible description of Figure \(\PageIndex{2}\).

The Visible Spectrum

Rainbows are an excellent example of the visible spectrum. A rainbow forms when light from the Sun passes through drops of rain, Figure \(\PageIndex{3}\). The raindrop breaks white light into the spectrum of colors. Suppose a ray of sunlight encounters a raindrop and passes into it. The light is refracted, or bent, when it passes from air to water. Blue and violet wavelengths are refracted more than the red wavelengths. Some of the light is then reflected at the backside of the drop and reemerges from the front, where it is again refracted. As a result, the white light is spread out into a rainbow of colors.

Figure \(\PageIndex{3}\): Rainbow Refraction. (a) A diagram of how light from the Sun, which is located behind the observer, can create a rainbow (b) A photo of a rainbow. (c) Refraction in a water droplet. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Accessible description of Figure \(\PageIndex{3}\).

In 1672, in the first paper that he submitted to the Royal Society, Sir Isaac Newton described an experiment in which he permitted sunlight to pass through a small hole and then through a prism. Newton found that sunlight, which looks white to us, is actually made up of a mixture of all the colors of the rainbow, Figure \(\PageIndex{4}\). Light is separated into different colors with a prism, a piece of glass in the shape of a triangle with refracting surfaces. Upon entering one face of the prism, the path of the light is refracted, but not all of the colors are bent by the same amount. The bending of the beam depends on the wavelength of the light as well as the properties of the material, and as a result, different wavelengths, or colors of light, are bent by different amounts and therefore follow slightly different paths through the prism. The violet light is bent more than the red.

Figure \(\PageIndex{4}\): Prism Refraction. When white sunlight passes through a prism, the light is broken into a rainbow-colored band. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{4}\).

If the light leaving the prism is focused on a screen, the different wavelengths or colors that make up white light are lined up side by side just like a rainbow, Figure \(\PageIndex{5}\). Because this array of colors is a spectrum of light, the instrument used to form the spectrum is called a spectrometer. The spectrum of white light, ranging from roughly 400 to 700 nanometers (nm) is called a continuous spectrum.

Figure \(\PageIndex{5}\): Continuous Spectrum. When white light passes through a prism, it is dispersed and forms a continuous spectrum of all the colors from violet to red. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{5}\).

The Spectrum of the Sun

When Newton described the laws of refraction and dispersion in optics, and observed the solar spectrum, all he could see was a continuous band of colors. If the spectrum of the white light from the Sun and stars were simply a continuous rainbow of colors, astronomers would have little interest in the detailed study of a star’s spectrum once they had learned its average surface temperature. In 1802, however, William Wollaston built an improved spectrometer that included a lens to focus the Sun’s spectrum on a screen. With this device, Wollaston saw that the colors were not spread out uniformly, but instead, some ranges of color were missing, appearing as dark bands in the solar spectrum. He mistakenly attributed these lines to natural boundaries between the colors. In 1815, German physicist Joseph von Fraunhofer, upon a more careful examination of the solar spectrum, found about 600 such dark lines, or missing colors, which led scientists to rule out the boundary hypothesis (Figure \(\PageIndex{6}\)). To determine how these gaps were made, scientists had to learn more about how light interacts with matter at the atomic scale.

Figure \(\PageIndex{6}\): Visible Spectrum of the Sun. This simulation of the spectrum of the Sun as Wollaston and von Fraunhofer would have seen it in the early 19th century, appears similar to a continuous spectrum, except for the many narrow, dark gaps. (CC0; Phrood~commonswiki, et al. via Wikimedia Commons). Alternative description of Figure \(\PageIndex{6}\).