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Light, color, and the electromagnetic spectrum

A 1:

The electromagnetic spectrum consists of all the different wavelengths of electromagnetic radiation, including light, radio waves, and X-rays. It is a continuum of wavelengths from zero to infinity. We name regions of the spectrum rather arbitrarily, but the names give us a general sense of the energy; for example, ultraviolet light has shorter wavelengths than radio light. The only region in the entire electromagnetic spectrum that our eyes are sensitive to is the visible region.

  •  Gamma rays have the shortest wavelengths, of less than 0.01 nanometers (about the size of an atomic nucleus). This is the highest frequency and most energetic region of the electromagnetic spectrum. Gamma rays can result from nuclear reactions taking place in objects such as pulsars, quasars, and black holes.


  •  X-rays range in wavelength from 0.01 to 10 nanometers (about the size of an atom). They are generated, for example, by super-heated gas from exploding stars and quasars, where temperatures are near a million to ten million degrees.


  •  Ultraviolet radiation has wavelengths of 10 to 310 nanometers (about the size of a virus). Young, hot stars produce a lot of ultraviolet light and bathe interstellar space with this energetic light.


  •  Visible light covers the range of wavelengths from 400 to 700 nanometers (from the size of a molecule to a protozoan). The Sun emits most of its radiation in the visible range, which our eyes perceive as the colors of the rainbow. Our eyes are sensitive only to this small portion of the electromagnetic spectrum.


  •  Infrared wavelengths span from 710 nanometers to 1 millimeter (from the width of a pinpoint to the size of small plant seeds). At a temperature of 37 degrees C, our bodies radiate with a peak intensity near 900 nanometers.


  •  Radio waves are longer than 1 millimeter. Since these are the longest waves, they have the lowest energy and are associated with the lowest temperatures. Radio wavelengths are found everywhere: in the background radiation of the universe, in interstellar clouds, and in the cool remnants of supernova explosions, to name a few. Radio stations use radio wavelengths of electromagnetic radiation to send signals that our radios then translate into sound. These wavelengths are typically a few feet long in the FM band and up to 300 yards or more in the AM band. Radio stations transmit electromagnetic radiation, not sound. The radio station encodes a pattern on the electromagnetic radiation it transmits, and then our radios receive the electromagnetic radiation, decode the pattern and translate the pattern into sound.

New instrumentation and computer techniques of the late 20th century allow scientists to measure the universe in many regions of the electromagnetic spectrum. We build devices that are sensitive to the light that our eyes cannot see. Then, so that we can "see" these regions of the electromagnetic spectrum, computer image-processing techniques assign arbitrary color values to the light.


A 2:

Light is a disturbance of electric and magnetic fields that travels in the form of a wave. Imagine throwing a pebble into a still pond and watching the circular ripples moving outward. Like those ripples, each light wave has a series of high points known as crests, where the electric field is highest, and a series of low points known as troughs, where the electric field is lowest. The wavelength is the distance between two wave crests, which is the same as the distance between two troughs. The number of waves that pass through a given point in one second is called the frequency, measured in units of cycles per second called Hertz. The speed of the wave therefore equals the frequency times the wavelength.


A 3:

Wavelength and frequency of light are closely related. The higher the frequency, the shorter the wavelength. Because all light waves move through a vacuum at the same speed, the number of wave crests passing by a given point in one second depends on the wavelength. That number, also known as the frequency, will be larger for a short-wavelength wave than for a long-wavelength wave. The equation that relates wavelength and frequency is shown directly below, where "v" equals velocity. For electromagnetic radiation, velocity is equal to the speed of light, "c," so the equation becomes the second one shown below, where "c" equals the speed of light:

At left is the equation that relates wavelength and frequency.
For electromagnetic radiation, velocity is equal to the speed of light, "c."

A 4:

The energy of a wave is directly proportional to its frequency, but inversely proportional to its wavelength. In other words, the greater the energy, the larger the frequency and the shorter (smaller) the wavelength. Given the relationship between wavelength and frequency described above, it follows that short wavelengths are more energetic than long wavelengths.


A 5:

All objects emit electromagnetic radiation, and the amount of radiation emitted at each wavelength determines the temperature of the object. Hot objects emit more of their light at short wavelengths, and cold objects emit more of their light at long wavelengths. The radiation temperature of an object is related to the wavelength at which the object gives out the most light. We refer to the amount of light emitted at a particular wavelength as the intensity.

When you plot the intensity of light from an object at each wavelength, you trace out a smooth curve called a blackbody curve. For any temperature, the blackbody curve shows how much energy (intensity) is radiated at each wavelength. The wavelength where the intensity peaks determines the color of that the object. The intensity peak will be at shorter (bluer) wavelengths for hotter objects, and at longer (redder) wavelengths for cooler objects. Therefore, you can tell the temperature of a star or galaxy by its color because color is closely related to the wavelength at which its light intensity peaks.

Blackbody curves, for objects of all temperatures, have a similar shape, as shown in the graph below. However, the peak of the curve for a hotter object will be larger (more intense) than will the peak of the curve for a cooler object. For example, the intensity difference between the peak of the curve for an object at 30,000 K and the peak of the curve for an object at 300 K (body temperature) is a factor of 10 billion. This means that a star at 30,000 K puts out more energy by a factor of 10 billion than does a human at body temperature.

Because of the large intensity difference, it would be difficult to show both of these curves on the graph below without using logarithms. To plot blackbody curves with large intensity differences on the "Heating Up" page of Amazing Space's "Star Light, Star Bright," the scale of the intensity axis adjusts itself for each temperature change.


A 6:

The amount of light produced by an object at each wavelength depends on the temperature of the object producing the light. Stars hotter than the Sun (over 6000 degrees C) put out most of their light in the blue and ultraviolet regions of the spectrum. Stars cooler than the Sun (below 5000 degrees C) put out most of their light in the red and infrared regions of the spectrum. Solid objects heated to 1000 degrees C appear red but are putting out far more (invisible) infrared light than red light.


A 7:

Electromagnetic radiation, or light, is a form of energy. Visible light is a narrow range of wavelengths of the electromagnetic spectrum. By measuring the wavelength or frequency of light coming from objects in the universe, we can learn something about their nature. Since we are not able to travel to a star or take samples from a galaxy, we must depend on electromagnetic radiation to carry information to us from distant objects in space.

The human eye is sensitive to a very small range of wavelengths called visible light. However, most objects in the universe radiate at wavelengths that our eyes cannot see. Astronomers use telescopes with detection devices that are sensitive to wavelengths other than visible light. This allows them to study objects that emit this radiation, which would otherwise be invisible to us. Computer techniques then code the light into arbitrary colors that we can see.

The Hubble Space Telescope is able to measure wavelengths from about 0.1150 to 2 micrometers, a range that covers more than just visible light. These measurements of electromagnetic radiation enable astronomers to determine certain physical characteristics of objects, such as their temperature, composition, and velocity. 


A 8:

The human eye is sensitive to a very small range of wavelengths called visible light. However, many celestial objects in the universe radiate at wavelengths that our eyes cannot see, and each type of radiation provides clues as to the nature of the object in question. Astronomers study celestial objects with detection devices that are sensitive to wavelengths other than visible light and then use computer techniques that code the light into colors that we can see.

Able to measure wavelengths from about 115 nanometers to 2500 nanometers, the Hubble Space Telescope looks at the energy that is not only visible, but also infrared and ultraviolet. These measurements better enable astronomers to determine physical characteristics of objects, such as their temperature, composition, and velocity.


A 9:

Different wavelengths of light provide scientists with different information about the objects they are studying. For instance, infrared light can reveal details about objects shrouded in dust. Infrared light emitted by an object will pass through dust — unlike visible light, which is scattered. In contrast, ultraviolet light can reveal details about the stellar wind around stars. (When talking about our sun, this is called the solar wind.) Astronomers have ways of breaking light into a spectrum, which reveals a lot of information (including properties of the source of the light, the material through which the light passes, or the material off of which the light reflects).

However, sometimes scientists want to capture specific ranges of wavelengths of light, so they use a filter. A filter will allow only light within a small range of wavelengths to pass through. When the Hubble Space Telescope takes an image using a filter, that image shows only the varying intensity of light in that small range. In making color pictures, scientists usually use a red filter, a green filter, and a blue filter (the red filter allows light only in the red range to enter, etc.). By combining these images scientists can create full-color pictures.


A 10:

Hubble uses professional-grade versions of the same detectors found in a digital camera. Each optical instrument on the telescope has a set of charge-coupled devices (CCDs) composed of a grid of pixels (picture elements) that measure the intensity of light that strikes them. Each pixel turns the light intensity it measures into a number.

These numbers are systematically downloaded to the Space Telescope Science Institute where they are translated into black-and-white images. Using computers, two or more of these images can be colorized and combined to produce a color image. CCDs are used only for optical light. Ultraviolet and infrared light have different detectors, which are not called CCDs but operate on similar principles.


A 11:

The colors red, green, and blue are chosen because they are the primary colors of light. By combining these colors of light, white light is produced. Combinations of two of these colors produce other familiar colors: blue + green = cyan, red + blue = magenta, and red + green = yellow.


A 12:

If red, green, and blue filters have been used, the red filter is assigned red light, the green filter is assigned green light, and the blue filter is assigned blue light. This changes the black-and-white scale into tones of red, green, and blue, respectively. Using computers, these images can be combined into one image, which represents (as close as possible) the true colors of the object being imaged. In general, when other filters are used, blue is assigned to the shortest wavelength light while red is assigned to the longest wavelength, with green being the wavelength in the middle.



A 13:

The Hubble Space Telescope takes observations of almost every part of the sky, as long as it isn't toward the Sun. In our own solar system, the telescope can take incredibly detailed pictures of the outer planets and their moons. A crescent of Venus has been taken with the Hubble Space Telescope and the moon has been shot once, but Mercury is too close to the Sun to be imaged.

Farther out, Hubble takes images of stars and nebulae in our own galaxy as well as in galaxies in the larger universe. The majority of Hubble's time is used for scientific research, taking many observations of specific objects that the scientists are studying. However, a very small portion of Hubble's time is dedicated to taking beautiful pictures for public enjoyment.


"Q&A: Light, color, and the electromagnetic spectrum" is a series of questions and answers about the electromagnetic spectrum written for teachers and students. The questions are ones that students might ask while studying the electromagnetic spectrum. Teachers can use this Q&A to gain additional knowledge about the electromagnetic spectrum, or use it in the classroom as outlined below.

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Adaptable, at teacher's discretion
How to use in the classroom

• An engagement activity. Use selected questions to start a discussion.

• An inquiry tool. Use selected questions and answers to help students generate questions. Propose a question, such as "What is the electromagnetic spectrum?" (see question 1 in Q&A: Light, color, and the electromagnetic spectrum). Have students read the answer to the question and write down 3–5 questions they would like answered as a result of reading the material.

• A source of information. Students can use the questions and answers as part of their research on the electromagnetic spectrum.

• A form of review. Use the questions as a review at the end of a unit on the electromagnetic spectrum.

• A follow-up. Have students read the questions and answers to gain additional information about the electromagnetic spectrum following a related activity.

• A starting point for a debate. How do the truths of astronomy compare to the other sciences? This idea is addressed in the question "What information can light reveal about the stars?" (see question 7 in Q&A: Light, color, and the electromagnetic spectrum).

Related materials

Online Exploration: Star Light, Star Bright

HubbleSite feature: Behind the Pictures