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Unit of Wavelength - Definition, SI Unit, FAQs

Unit of Wavelength - Definition, SI Unit, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 05:00 PM IST

Wavelength is the length of one cycle of a wave, which is the distance. The SI unit of wavelength is meter (m), and the wavelength symbol is λ. In this article we will learn in detail about waves, types of waves, wavelength units, what the unit of wavelength is, and how you measure wavelength.

What is a wave?

A wave is the propagation of disturbance from one point to another in a regular and organized manner. In a scientific manner, a wave is a transfer of energy in a medium. When energy moves through any matter like solid, liquid, or gas, then the particles of these matters get disturbed, which results in the formation of waves.

This Story also Contains
  1. What is a wave?
  2. Types of waves
  3. What is wavelength, and what is the unit of wavelength?
  4. Units of wavelength
  5. Wavelengths of different waves in meters
  6. How to measure wavelength?
Unit of Wavelength - Definition, SI Unit, FAQs
Unit of Wavelength - Definition, SI Unit, FAQs

Some energies need a medium to travel, while some energies do not need a medium to travel from one point to another. Waves are divided into two types: longitudinal waves and transverse waves.

Longitudinal waves: The waves that oscillate in parallel with respect to the propagation of waves. They can travel through solids, liquids, and gases as well.

Transverse waves: The waves that oscillate perpendicular with respect to the propagation of waves. These travel through the surface of liquids and gases, but they can not travel through gases.

Types of waves

There are 3 types of waves:

  • Mechanical waves
  • Electromagnetic waves
  • Matter waves
Background wave

Each of these is defined briefly in the section given below:

Mechanical waves

Mechanical waves are defined as the waves that require a medium to travel. They transfer energy through the medium by oscillating the particles. For example,

  • Sound waves
  • Seismic waves
  • Water waves

Electromagnetic waves

Electromagnetic waves are named so because they are constituted of electric and magnetic fields. These waves propagate in the space as oscillating electric and magnetic fields. As these waves do not need any medium to travel, they can travel through a vacuum. The electromagnetic waves travels with the speed of light, i.e.,

c=1μ0ε03.00×108 ms1.

For example,

  • Radio waves
  • Infrared waves
  • UV radiation
  • Gamma rays
  • Visible waves
  • Microwaves
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Matter waves

The matter waves, also known as de Broglie waves, are the waves that follow the quantum mechanical statement that all matter shows behavior like waves. This leads to the concept of the dual behavior of particles.

For example, particles like electrons and photons show dual nature.

What is wavelength, and what is the unit of wavelength?

Let us define wavelength in physics: a wave is a theoretical representation of energy propagating in a medium or vacuum. This kind of energy is also called a disturbance that propagates in a certain medium, whether it is a solid, liquid, or gas. For example, sound energy. Only a small amount of energy does not require the medium to move, so it can move in a vacuum. For example, light energy.

We know transverse waves travel in consecutive successive crests and troughs, while longitudinal waves travel in successive compressions and rarefactions.

In the case of a transverse wave, the length of successive crests and troughs is called its wavelength, while in the case of a longitudinal wave, the length of successive compressions and rarefactions is called its wavelength.

Wavelength is the length of a wave from the maximum point of a peak to the maximum point of the adjacent peak or the lowermost point of a valley to the lowermost point of the adjacent valley.

It is denoted by a symbol called lambda (λ), and the unit of lambda is the meter. According to the definition given, we know that wavelength is length, so the SI unit of wavelength should be equal to the SI unit of length. We know that the SI unit of length is meters, so the SI unit of wavelength is also meters.

wavelength is a length of one cycle of a wave.

Light is a form of electromagnetic radiation, a field associated with light energy. Light is a very general term because it can be anything from a simple light bulb to a microwave oven. Some characteristics of light include wavelength and frequency. Frequency (usually measured in Hertz) is the number of waves in a given time. The wavelength (usually measured in nanometers) is the distance between two points in a wave. Frequency and wavelength have a positive and negative relationship. For example, if two waves are moving at the same speed, they are negatively correlated. Waves of shorter wavelengths have higher frequencies, and longer wavelengths have lower frequencies. Frequency and wavelength may be related to the speed of light. Light moves at a speed of 3.00 x 108 meters per second. The speed of light, frequency, and wavelength can all be expressed by an equation c=λv, where c is the speed of light, v stands for frequency, and the wavelength sign is λ. As mentioned earlier, this is an inverse relationship, because if one value goes up, the other goes down. By means of this elementary equation, you can also solve for wavelength and frequency to obtain their equations.

Just as wavelength and frequency are associated with light, they are also associated with energy. The tinier the wavelength, the greater the frequency and the more the energy. The lengthier the wavelength, the lesser the frequency and the lesser the energy. The energy equation is E = hv, where E signifies energy, h signifies Planck's constant (6.62607015×1034 m2 kg/s), and v signifies frequency. The energy equation is a direct relationship between frequency and energy, because the higher the frequency, the higher the energy. This is likely since h is a constant.

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Units of wavelength

As mentioned earlier, the SI unit of wavelength is meters. Some wavelengths are very short, and some wavelengths are very long. Solving various numerical problems also requires larger and smaller unit lengths. Therefore, we use an exponent of 10 to measure large properties, while a negative exponent is used to measure shorter wavelengths.

The following are all units of wavelength:

  • Kilometer, abbreviated as km, is equivalent to 103 m.
  • Megameter, abbreviated as Mm, is equivalent to 1000000 m or 106 m.
  • Gigameter, abbreviated as Gm, is equivalent to 1000000000 m or 109 m.
  • Terameter, abbreviated as Tm, is equivalent to 1012 m.
  • Petametre, abbreviated as Pm, is equivalent to 1015 m.
  • Decimetre, abbreviated as dm, is equivalent to 101 m.
  • Centimetre, abbreviated as cm, is equivalent to 102 m.
  • Millimetre, abbreviated as mm, is equivalent to 103 m.
  • Micrometer, abbreviated as μm, is equivalent to 106 m.
  • Nanometre, abbreviated as nm, is equivalent to 109 m.

Wavelengths of different waves in meters

Light is a type of energy that travels in waves; it is made when matter is heated up or gains energy. Excess energy is released in part as light; this energy is called electromagnetic radiation. When we talk about light, we usually mean visible light, which is the light we can see with our eyes, but there are more types of electromagnetic radiation that are invisible to us, including radio waves, microwaves, x-rays, and gamma rays. Scientists can detect and measure invisible radiation by using special tools. Collectively with visible light, all these kinds of radiation are called the electromagnetic spectrum. All electromagnetic radiation moves in waves, but unlike types have unlike wavelengths. The wavelength of electromagnetic emission or light is associated with how much energy it has.

The wavelength of electromagnetic emission or light is associated to how ample of energy it has.

Now let us take a look at the wavelength of these electromagnetic radiations here. Wavelength is measured in m and nm.

Gamma rays: Gamma rays have the shortest wavelength. It is less than 0.001 nanometers or 1012 meters.

X-ray: The wavelength range of X-rays is 0.001-10 nm.

Ultraviolet: The wavelength range of ultraviolet is 10–400 nm.

Visible light: The wavelength of visible light is within the range of 400–700 nm.

Infrared: The infrared wavelength range is 700 nm to 1 mm.

Radio waves: Radio waves have the lengthiest wavelength. Its length exceeds 1 mm or 0.001 meters.

Also Read:

How to measure wavelength?

To understand wavelength measurement and to know clearly about how to measure wavelength, let us take an example.

Calculate the wavelength of radiation with a frequency of 5 X 1014 Hz.

We know,

The frequency of the radiation is v=5×1014 Hz.

The speed of light in a vacuum is c=3×108 m/s.

λ=cv = speed of light/frequency

λ=3×108 m/s5×1014 Hz

λ=6×107 m

Also check-

NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. When the K.E. of an electron is greater than before, the wavelength of the related wave will?

The wavelength will decrease.

2. Light with frequency of 7.22 X 1014 Hz lies in the violet region of visible spectrum. What is the wavelength of this light?

 λ=c/v =speed of light/frequency

= 3 X 108/ 7.22 X 101014= 4.13 X 108m

3. What is the relation amongst wavelength and frequency?

Wavelength and frequency of a wave are inversely proportional. As wavelength increases, the frequency decreases.

4. State the SI units of wavelength and frequency

Meters and Hertz are the SI unit of wavelength and frequency respectively.

5. What is the wavelength range of x-ray?

The wavelength range of X-ray is 0.001-10 nm.

6. What does the wavelength of the matter wave depend on?

The wavelength of matter wave depends upon the mass, velocity, and momentum of the particle.

7. Which color have the longest wavelength?

The red color has the longest wavelength of about 700 nm in the visible range.

8. How frequency and energy depends on the wavelength of any wave?

The lower wavelength waves have the higher frequency and the higher energy as well, while the waves  with higher wavelength hav the lower frequency and lower energy by the relation E=hv and E=hcλ.

9. What is the SI unit of wavelength?
The SI unit of wavelength is the meter (m). However, depending on the type and scale of the wave, other units like nanometers (nm), micrometers (μm), or kilometers (km) may be used.
10. How is wavelength related to frequency?
Wavelength and frequency are inversely related. As wavelength increases, frequency decreases, and vice versa. This relationship is described by the equation: wave speed = frequency × wavelength.
11. How does wavelength affect the energy of a wave?
Wavelength is inversely proportional to energy. Shorter wavelengths correspond to higher energy waves, while longer wavelengths correspond to lower energy waves.
12. How does changing the medium affect a wave's wavelength?
When a wave enters a new medium, its speed and wavelength change, but its frequency remains constant. If the wave slows down in the new medium, its wavelength decreases proportionally.
13. What is the relationship between wavelength and diffraction?
Diffraction is more pronounced when the wavelength of a wave is comparable to or larger than the size of an obstacle or opening. Longer wavelengths diffract more than shorter wavelengths.
14. What is wavelength?
Wavelength is the distance between two consecutive crests or troughs in a wave. It represents the spatial period of a wave, or how far the wave travels before it repeats its shape.
15. Can wavelength be negative?
No, wavelength cannot be negative. It is always a positive quantity as it represents a physical distance between wave peaks or troughs.
16. What's the difference between wavelength and amplitude?
Wavelength is the distance between two consecutive crests or troughs, while amplitude is the maximum displacement of the wave from its equilibrium position. Wavelength affects the wave's energy, while amplitude affects its intensity.
17. What is a standing wave, and how does it relate to wavelength?
A standing wave is a wave that remains in a constant position, formed by the interference of two waves traveling in opposite directions. The distance between two adjacent nodes in a standing wave is half a wavelength.
18. What is the de Broglie wavelength?
The de Broglie wavelength is the wavelength associated with a particle's wave-like nature in quantum mechanics. It is calculated by dividing Planck's constant by the particle's momentum.
19. How does wavelength relate to the Doppler effect?
The Doppler effect causes an apparent change in wavelength (and frequency) when there is relative motion between the wave source and the observer. As the source moves towards the observer, the wavelength appears shorter, and vice versa.
20. How does wavelength affect the refraction of light?
Different wavelengths of light refract (bend) at slightly different angles when passing from one medium to another. This phenomenon causes dispersion, which is responsible for the separation of white light into a spectrum of colors in a prism.
21. What is the relationship between wavelength and the energy levels in atoms?
The wavelengths of light emitted or absorbed by an atom correspond to specific energy transitions between electron energy levels. Each element has a unique set of spectral lines related to these transitions.
22. How does wavelength affect the behavior of waves in optical fibers?
In optical fibers, different wavelengths of light travel at slightly different speeds, leading to dispersion. This effect is crucial in designing fiber optic communication systems and choosing appropriate wavelengths for data transmission.
23. What is coherence length, and how is it related to wavelength?
Coherence length is the distance over which a wave maintains a predictable phase relationship. It is related to the spectral width of the source and is typically many times the wavelength for laser light.
24. What is the concept of group velocity, and how does it relate to wavelength?
Group velocity is the velocity at which the overall shape of a wave's amplitudes propagates through space. It can differ from the phase velocity (speed of individual wave crests) and depends on how the wave's speed varies with wavelength.
25. What is the relationship between wavelength and the uncertainty principle?
The uncertainty principle in quantum mechanics relates the uncertainties in position and momentum. For waves, this principle manifests as a relationship between the uncertainty in wavelength and the uncertainty in position.
26. How does wavelength affect the design of optical coatings?
Optical coatings, such as anti-reflection coatings, are designed with specific thicknesses related to the wavelengths of light they are intended to manipulate. The thickness is often a quarter or half of the target wavelength.
27. What is the significance of the Compton wavelength?
The Compton wavelength is a quantum mechanical property of a particle, equal to the wavelength of a photon whose energy is the same as the rest mass energy of the particle. It plays a role in quantum electrodynamics and particle physics.
28. What is the relationship between wavelength and the refractive index of a material?
The refractive index of a material often varies with wavelength, a phenomenon called dispersion. This relationship is crucial in optics and explains effects like the splitting of white light into colors by a prism.
29. What is the relationship between wavelength and the resolving power of a diffraction grating?
The resolving power of a diffraction grating is its ability to separate closely spaced wavelengths. It is directly proportional to the number of grooves in the grating and the order of diffraction, and inversely proportional to the wavelength.
30. How do scientists measure the wavelength of light?
Scientists use various methods to measure light wavelengths, including spectroscopy, interferometry, and diffraction gratings. These techniques allow for precise measurements of wavelengths across the electromagnetic spectrum.
31. What is the wavelength range of visible light?
The wavelength range of visible light is approximately 380-700 nanometers (nm). Different wavelengths within this range correspond to different colors that humans can perceive.
32. How does wavelength relate to the photoelectric effect?
In the photoelectric effect, the energy of incident light must exceed a threshold to eject electrons from a material. This threshold depends on the light's frequency (and thus wavelength), not its intensity.
33. What is the relationship between wavelength and wave speed in different media?
The relationship between wavelength (λ), frequency (f), and wave speed (v) is given by the equation v = fλ. As the wave speed changes in different media, the wavelength changes proportionally while the frequency remains constant.
34. How does wavelength affect the resolution of optical instruments?
The resolution of optical instruments, such as microscopes and telescopes, is limited by the wavelength of light used. Shorter wavelengths allow for higher resolution and the ability to distinguish finer details.
35. What is a wavefront, and how is it related to wavelength?
A wavefront is a surface of constant phase in a wave. The distance between two adjacent wavefronts is equal to one wavelength. Wavefronts are perpendicular to the direction of wave propagation.
36. How does wavelength affect the penetration of electromagnetic waves through materials?
Generally, longer wavelengths penetrate materials more easily than shorter wavelengths. This is why radio waves (long wavelength) can pass through buildings, while visible light (shorter wavelength) cannot.
37. What is the wavelength of sound waves in air?
The wavelength of sound waves in air varies depending on the frequency. For audible sounds (20 Hz to 20 kHz), wavelengths range from about 17 m to 1.7 cm. Lower frequency sounds have longer wavelengths.
38. How does wavelength relate to the wave equation?
The wave equation describes how waves propagate in space and time. Wavelength is a key parameter in this equation, relating to the spatial periodicity of the wave.
39. What is the difference between wavelength and wavenumber?
Wavelength is the distance between wave peaks, while wavenumber is the spatial frequency of a wave, typically expressed as the number of waves per unit distance. Wavenumber is the reciprocal of wavelength.
40. How does wavelength affect the scattering of light?
The amount of scattering depends on the ratio of the wavelength to the size of the scattering particles. Shorter wavelengths scatter more than longer wavelengths, which is why the sky appears blue (as blue light scatters more in the atmosphere).
41. How does wavelength affect the design of antennas?
The size and design of antennas are often related to the wavelength of the electromagnetic waves they are intended to transmit or receive. For example, the length of a simple dipole antenna is typically half the wavelength of the signal.
42. How does wavelength affect the efficiency of solar cells?
Solar cells are designed to absorb light of specific wavelengths efficiently. The bandgap of the semiconductor material in a solar cell determines which wavelengths of light can be effectively converted into electricity.
43. What is the concept of characteristic wavelength in X-ray diffraction?
In X-ray diffraction, the characteristic wavelength is the wavelength of X-rays produced by electrons transitioning between specific energy levels in atoms. These wavelengths are used to probe the structure of crystals.
44. How does wavelength relate to the concept of phase velocity?
Phase velocity is the speed at which the phase of a wave propagates in space. It is related to wavelength and frequency by the equation: phase velocity = wavelength × frequency.
45. How does wavelength affect the design of metamaterials?
Metamaterials are engineered to have properties not found in nature, often by creating structures smaller than the wavelength of the waves they interact with. The size and arrangement of these structures are carefully designed relative to the target wavelengths.
46. How does wavelength relate to the concept of wave packets?
A wave packet is a localized disturbance consisting of a superposition of waves with different wavelengths. The spread of wavelengths in a wave packet determines its spatial extent and how it evolves over time.
47. What is the significance of the Planck length in relation to wavelength?
The Planck length is the scale at which quantum effects of gravity become significant. It represents a theoretical lower limit on the measurement of length and thus on the shortest possible wavelength of any wave.
48. How does wavelength affect the design of photonic crystals?
Photonic crystals are designed with periodic structures on the scale of the wavelength of light. This periodicity creates photonic band gaps that prohibit the propagation of certain wavelengths of light in specific directions.

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