Wavelength of Light

Wavelength of Light - Relationship Between Frequency and Wavelength

Light can be defined as any electromagnetic radiation occurring within a specific portion of the electromagnetic spectrum. The word light usually refers to visible light, which is the visible and distinguishable to the naked human eye and is responsible for the sense of sight. Visible light is known to have wavelengths in the range of 400 to 700 nanometres (nm), or 4.00 × 10-7 to 7.00 × 10-7 m, which is between infrared which has longer wavelengths and ultraviolet which has shorter wavelengths. The wavelength of visible light implies it has a frequency of roughly 430 to 750 terahertz (THz). The speed of light in a vacuum is experimentally found to be 299,792,458 meters per second, which is approximately 3 × 108 meters per second. Visible light, just like all the other forms of electromagnetic radiations, is found to move at this speed in a vacuum.

In physics, sometimes the term light refers to electromagnetic radiation of any wavelength, irrespective of whether it is visible or not. In this sense, forms of radiations like gamma rays, radio waves, X rays, and microwaves are all forms of light. Light is known to exhibit both wave nature and particle nature and this occurrence is identified as the dual nature of light. Light exists as a particle while it propagates as a wave. The study of light is known as optics, and optics is an important area of study in physics.

Wavelength:


We know that light can be understood both as a particle and a wave. Photons are the light particles which exist in the form of "packets" of electromagnetic energy. On the other hand, waves are the form of energy where electromagnetic radiation takes on when it is propagating.

Light does not travel in a straight light line. It travels in the form of a transverse wave. A wave which consists of oscillation while moving which occur perpendicular to the direction of transfer of energy is called as transverse waves. Wavelength is the distance between two consecutive crests or two consecutive troughs in a transverse wave. Wavelength also represents a repeating pattern of any traveling energies, such as light or sound. Wavelength is usually expressed by the units of nanometres (nm) or micrometres (µm). It is represented by the symbol λ which is read as lambda. 






The relationship between frequency and wavelength:


Wavelength and frequency are closely related to each other, especially with respect to light. Wavelength is the distance between two consecutive crests or two consecutive troughs, while frequency is the number of waves that pass through a single given point in a specified amount of time. Frequency and wavelength are inversely proportional, which means longer the wavelength, lower the frequency. All forms of light waves move through the vacuum at the same speed of 3 × 108 meters per second. Therefore, the number of waves passing through a specific point, that is frequency, depends solely on the wavelength. Frequency will be higher when the wavelength is short, as more crests or troughs pass through a specific point when the wavelength is short. Conversely, the frequency will be lower when the wavelength is longer as fewer crests or troughs pass through a specific point when the wavelength is longer. The equation which relates frequency and wavelength of light or any other electromagnetic wave, when λ is the wavelength, f is the frequency and c is the speed of light is 

λf = c.

This implies,
        Frequency, f = c/λ
        Wavelength, λ = c/f

Light is made up of multiple different wavelengths, and each wavelength corresponds to a different colour. The colour we see is a result of a wavelength being reflected from the object we are viewing. The visible spectrum is the spectrum of light which is visible and distinguishable to the naked eye, and this spectrum ranges from dark red at 700 nm to violet at 400 nm.

Visible light waves are the only electromagnetic waves we can physically detect or see without any external assistance. These lights are viewed as the colours of the rainbow, that is violet, indigo, blue, green, yellow, orange and red. This also includes the colour white, which is made up of all the seven colours mentioned above. White light is thus called polychromatic light. Light from the sun is polychromatic light, which means it contains all the seven colours. Lasers form monochromatic lights, which mean it produces a single colour. Different objects around us absorb and reflect different wavelengths of light. Hence, they appear in different colours. The colours we view objects as are the wavelengths that they reflect or transmit.

For example, a piece of red cloth appears red because the dye molecules in that piece of cloth have absorbed the wavelengths of all other colours of the spectrum and reflect only the wavelength corresponding to the colour red. If a colour at the other end of the spectrum, like violet or blue, falls on this piece of red cloth, it will appear black, because there is no red light to reflect, and the colours like blue or violet get absorbed. Black objects appear so because they absorb all colours. Conversely, white objects appear so as they reflect all colours.

We can detect colours because the retina of our eyes contains two types of photoreceptors, namely rods and cones. The rods only let us see black, white and grey, while the cones let us see colour. Our cones only work when the available light is bright enough. This is the reason we cannot easily distinguish colours when the light is very dim.

There is a total of three types of cones in the human eye. They are sensitive to short(S), medium(M) and long(L) wavelengths of light in the visible spectrum. These cones have also been known as blue sensitive cones, green-sensitive cones, and red sensitive cones traditionally. But as each cone represents a wide range of wavelengths, the labels S, M and L are more accepted now.

Table of the wavelengths of various colours, and their frequencies:


Colour Wavelength in nmFrequency in THz
Red750 – 610480 – 405
Orange610 – 590510 – 480
Yellow590 – 570530 – 510
Green570 – 500580 – 530
Blue500 – 450670 – 600
Indigo450 – 425600 – 700
Violet425 – 400700 – 790


White light: White light's wavelength extends from 400 to 750 nm. When the white colour is passed through the prism, the light spectrum is formed due refraction of different wavelengths through different angles.

Ultraviolet Light: Ultraviolet light extends from the end of the visible region and the X-ray region in the electromagnetic spectrum. It gets its name as it is the light closest to the violet portion of the visible light and is in the range of 10 to 400 nm. 

Infrared Light: Infrared radiation has a longer wavelength than visible light and is close to the red portion of the visible spectrum of light. It extends from 750 nm to 1 mm. Infrared radiation cannot be seen but can be felt in the form of heat.

Red Light and Orange Light: Red light and orange lights whose wavelength lies between 750 to 610 nm and 610 to 590 nm respectively are best viewed naturally during sunrise and sunset. This is because the associated wavelengths of red and orange from sunlight are not properly scattered by the atmosphere during these times.

Yellow Light: Yellow light has a wavelength between 590 and 570 nm. Yellow light is emitted by low-pressure sodium lamps.

Green Light: Green colour, whose wavelength extends from 570 to 500 nm, can be prominently seen in grass and leaves. Grass reflects green wavelength and absorbs all other wavelengths and thus appears green.

Blue Light: Blue light has a wavelength ranging from 500 to 450 nm. The atmosphere scatters shorter wavelengths efficiently and thus the wavelength corresponding to the colour blue is scattered efficiently by the atmosphere. That’s why the sky appears blue when we look up at it.

Indigo Light Violet Light: With a wavelength between 450 and 425 nm, indigo is a colour which is between the primary colour blue and the colour violet in the colour wheel. Violet with a wavelength of 425 to 400 nm is the visible light with the shortest wavelength. It has a shorter wavelength and is hence scattered more effectively by the atmosphere. But since our eyes are sensitive to blue colour, the sky appears blue rather than indigo or violet colour.