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Thomas Young
- Thomas Young postulated that light is a wave and is subject to the superposition principle; his great experimental achievement was to demonstrate the constructive and destructive interference of light (c. 1801).
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The Hubble Space Telescope can detect a portion of infrared and ultraviolet wavelengths as well as visible light. The light we can see with our eyes is part of a range of radiation known as the electromagnetic spectrum. Shorter wavelengths of light are higher energy, and longer wavelengths of light are lower energy.
- Overview
- Introduction to electromagnetic waves
- Basic properties of waves: Amplitude, wavelength, and frequency
- Example: Calculating the wavelength of a light wave
- Period
- The electromagnetic spectrum
- Quantization of energy and the dual nature of light
- The photon
- Example: Calculating the energy of a photon
- Conclusion
Properties of electromagnetic radiation and photons
Electromagnetic radiation is one of the many ways that energy travels through space. The heat from a burning fire, the light from the sun, the X-rays used by your doctor, as well as the energy used to cook food in a microwave are all forms of electromagnetic radiation. While these forms of energy might seem quite different from one another, they are related in that they all exhibit wavelike properties.
If you’ve ever gone swimming in the ocean, you are already familiar with waves. Waves are simply disturbances in a particular physical medium or a field, resulting in a vibration or oscillation. The swell of a wave in the ocean, and the subsequent dip that follows, is simply a vibration or oscillation of the water at the ocean’s surface. Electromagnetic waves are similar, but they are also distinct in that they actually consist of 2 waves oscillating perpendicular to one another. One of the waves is an oscillating magnetic field; the other is an oscillating electric field. This can be visualized as follows:
As you might already know, a wave has a trough (lowest point) and a crest (highest point). The vertical distance between the tip of a crest and the wave’s central axis is known as its amplitude. This is the property associated with the brightness, or intensity, of the wave. The horizontal distance between two consecutive troughs or crests is known as the wavelength of the wave. These lengths can be visualized as follows:
Keep in mind that some waves (including electromagnetic waves) also oscillate in space, and therefore they are oscillating at a given position as time passes. The quantity known as the wave’s frequency refers to the number of full wavelengths that pass by a given point in space every second; the SI unit for frequency is Hertz (Hz) , which is equivalent to “per seconds” ( written as 1s or s−1) . As you might imagine, wavelength and frequency are inversely proportional: that is, the shorter the wavelength, the higher the frequency, and vice versa. This relationship is given by the following equation:
c=λν
where λ (the Greek lambda) is the wavelength (in meters, m ) and ν (the Greek nu) is the frequency (in Hertz, Hz ). Their product is the constant c , the speed of light, which is equal to 3.00×108 m/s . This relationship reflects an important fact: all electromagnetic radiation, regardless of wavelength or frequency, travels at the speed of light.
A particular wave of electromagnetic radiation has a frequency of 1.5×1014 Hz .
What is the wavelength of this wave?
We can start with our equation that relates frequency, wavelength, and the speed of light.
c=λν
Next, we rearrange the equation to solve for wavelength.
λ=cν
The last quantity we will consider is the period of a wave. A wave’s period is the length of time it takes for one wavelength to pass by a given point in space. Mathematically, the period (T ) is simply the reciprocal of the wave’s frequency (f ):
T=1f
The units of period are seconds (s ).
Now that we have an understanding of some basic properties of waves, we’ll look at the different types of electromagnetic radiation.
Electromagnetic waves can be classified and arranged according to their various wavelengths/frequencies; this classification is known as the electromagnetic spectrum. The following table shows us this spectrum, which consists of all the types of electromagnetic radiation that exist in our universe.
As we can see, the visible spectrum—that is, light that we can see with our eyes—makes up only a small fraction of the different types of radiation that exist. To the right of the visible spectrum, we find the types of energy that are lower in frequency (and thus longer in wavelength) than visible light. These types of energy include infrared (IR) rays (heat waves given off by thermal bodies), microwaves, and radio waves. These types of radiation surround us constantly, and are not harmful, because their frequencies are so low. As we will see in the section, “the photon,” lower frequency waves are lower in energy, and thus are not dangerous to our health.
To the left of the visible spectrum, we have ultraviolet (UV) rays, X-rays, and gamma rays. These types of radiation are harmful to living organisms, due to their extremely high frequencies (and thus, high energies). It is for this reason that we wear suntan lotion at the beach (to block the UV rays from the sun) and why an X-ray technician will place a lead shield over us, in order to prevent the X-rays from penetrating anything other than the area of our body being imaged. Gamma rays, being the highest in frequency and energy, are the most damaging. Luckily though, our atmosphere absorbs gamma rays from outer space, thereby protecting us from harm.
Next, we will talk about the relationship between a wave’s frequency and its energy.
We have already described how light travels through space as a wave. This has been well-known for quite some time; in fact, the Dutch physicist Christiaan Huygens first described the wave nature of light as far back as the late seventeenth century. For about 200 years after Huygens, physicists assumed that light waves and matter were quite distinct from one another. According to classical physics, matter was composed of particles that had mass, and whose position in space could be known; light waves, on the other hand, were considered to have zero mass, and their position in space could not be determined. Because they were considered to be in different categories, scientists did not have a good understanding of how light and matter interacted. This all changed in 1900 , however, when the physicist Max Planck began studying blackbodies – bodies heated until they began to glow.
Planck found that the electromagnetic radiation emitted by blackbodies could not be explained by classical physics, which postulated that matter could absorb or emit any quantity of electromagnetic radiation. Planck observed that matter actually absorbed or emitted energy only in whole-number multiples of the value hν , where h is Planck’s constant, 6.626×10−34 J⋅s , and ν is the frequency of the light absorbed or emitted. This was a shocking discovery, because it challenged the idea that energy was continuous, and could be transferred in any amount. The reality, which Planck discovered, is that energy is not continuous but quantized—meaning that it can only be transferred in individual “packets” (or particles) of the size hν . Each of these energy packets is known as a quantum (plural: quanta).
While this might sound confusing, we are actually already very familiar with quantized systems. The money we use daily, for example, is quantized. For instance, when you go into a store, you will not see anything on sale for a price of one dollar and two and a half cents ($1.025) . This is because the smallest possible monetary unit is the penny—it is impossible to transfer money in any smaller amount than this. Just as we cannot pay the cashier at the store half of a cent, energy cannot be transferred in anything less than a single quantum. We can think of quanta as being like “pennies” of electromagnetic energy—the smallest possible units by which such energy can be transferred.
Planck’s discovery that electromagnetic radiation is quantized forever changed the idea that light behaves purely as a wave. In actuality, light seemed to have both wavelike and particle-like properties.
Planck’s discoveries paved the way for the discovery of the photon. A photon is the elementary particle, or quantum, of light. As we will soon see, photons can be absorbed or emitted by atoms and molecules. When a photon is absorbed, its energy is transferred to that atom or molecule. Because energy is quantized, the photon’s entire energy is transferred (remember that we cannot transfer fractions of quanta, which are the smallest possible individual “energy packets”). The reverse of this process is also true. When an atom or molecule loses energy, it emits a photon that carries an energy exactly equal to the loss in energy of the atom or molecule. This change in energy is directly proportional to the frequency of photon emitted or absorbed. This relationship is given by Planck’s famous equation:
E=hν
A photon has a frequency of 2.0×1024 Hz .
What is the energy of this photon?
First, we can apply Planck's equation.
E=hν
Next, we plug in our given value for the frequency, as well as the value for Planck's constant, h , and solve.
E=(6.626×10−34 J⋅s)×(2.0×1024 s−1)=1.3×10−9 J
Electromagnetic radiation can be described by its amplitude (brightness), wavelength, frequency, and period. By the equation E=hν , we have seen how the frequency of a light wave is proportional to its energy. At the beginning of the twentieth century, the discovery that energy is quantized led to the revelation that light is not only a wave, but can also be described as a collection of particles known as photons. Photons carry discrete amounts of energy called quanta. This energy can be transferred to atoms and molecules when photons are absorbed. Atoms and molecules can also lose energy by emitting photons.
[Attributions and references]
- Reflection. Reflection is when incident light (incoming light) hits an object and bounces off. Very smooth surfaces such as mirrors reflect almost all incident light.
- Absorption. Absorption occurs when photons from incident light hit atoms and molecules and cause them to vibrate. The more an object's molecules move and vibrate, the hotter it becomes.
- Diffraction. Diffraction is the bending and spreading of waves around an obstacle. It is most pronounced when a light wave strikes an object with a size comparable to its own wavelength.
- Scatter. Scattering occurs when light bounces off an object in a variety of directions. The amount of scattering that takes place depends on the wavelength of the light and the size and structure of the object.
Explain the evidence for Maxwell’s electromagnetic model of light. Describe the relationship between wavelength, frequency, and speed of light. Discuss the particle model of light and the definition of photon. Explain how and why the amount of light we see from an object depends upon its distance.
Mar 14, 2012 · 1678 – Wave theory. Dutch physicist Christiaan Huygens argues that light consists of waves and uses this theory to explain double refraction. Thomas Young’s experiments (1801) support Huygens’s wave theory.
May 24, 2024 · We know that light is a wave based on how it behaves – it exhibits the same properties of other waves we have examined – it interferes with itself, it follows an inverse-square law for intensity (brightness), and so on.
Jan 25, 2005 · An individual photon may be thought of as a packet of waves cork-screwing rapidly through space. Each packet is an oscillation along two perpendicular axes of force – the electrical and the...
