# Confused about definitions in optics

• cytochrome
In summary: Spectral width is measured in terms of wavelength intervals and is typically plotted in terms of full width at half maximum (FWHM). The spectral width of a 525 nm laser will have a sharp peak centered around 525 nm, but if you replaced the laser with an LED you would see a much shorter, broader peak, as the LED has a wider band it emits in. Frequency bandwidth and spectral width are actually the same thing, they just measure different aspects of the same thing.
cytochrome
The width of a pulse is typically given in the time domain, correct? The effective width is the term to describe this.

What is the spectral width in the frequency domain?

How do you calculate spectral widths and effective widths?

From wiki:

spectral width is the wavelength interval over which the magnitude of all spectral components is equal to or greater than a specified fraction of the magnitude of the component having the maximum value.
In optical communications applications, the usual method of specifying spectral width is the full width at half maximum. This is the same convention used in bandwidth, defined as the frequency range where power drops by less than half (at most −3 dB).

http://en.wikipedia.org/wiki/Spectral_width

Imagine you have a chart where the X axis runs from 500 nm to 550 nm and the Y axis is the intensity. If you plot the spectral width of a 525 nm laser you will see a very sharp peak in the center of your graph. But it's not perfectly sharp, you still have a range of wavelengths other than 525 nm. If you replaced the laser with an LED you would see much shorter, broader peak, as the LED has a wider band it emits in.

Frequency domain just means that you are plotting your graph with frequency instead of time.

Does that help?

That helps a lot, thank you. For some reason I couldn't find that wiki article.

So are frequency bandwidth and spectral width practically the same thing?

cytochrome said:
The width of a pulse is typically given in the time domain, correct? The effective width is the term to describe this.

What is the spectral width in the frequency domain?

How do you calculate spectral widths and effective widths?

In optics, a pulse is a wavepacket and during its duration the frequency of the wave may vary or not. You have to perform a Fourier transform to pass from the time domain to frequency domain or vice versa. Once you have your plot you generally measure the band width at half its intensity (full width at half maximum, FWHM)

Hello,

Yes, the width of a pulse is typically given in the time domain. This refers to the duration of the pulse in units of time. The effective width is a term used to describe the duration of a pulse when taking into account the shape and intensity of the pulse. It is essentially a measure of the pulse's overall impact.

In the frequency domain, the spectral width refers to the range of frequencies present in a pulse. It is a measure of the spread of frequencies within the pulse. This can be important in understanding the characteristics of the pulse, such as its bandwidth.

The calculation of spectral widths and effective widths can vary depending on the specific parameters and measurements being used. Generally, spectral width can be calculated by determining the range of frequencies present in the pulse and taking the difference between the highest and lowest frequencies. Effective width can be calculated by considering the shape and intensity of the pulse and determining the duration that best represents its impact. These calculations may also involve mathematical transformations between the time and frequency domains.

I hope this helps clarify the definitions and calculations in optics. Please let me know if you have any further questions. Thank you.

## 1. What is the difference between refraction and reflection in optics?

Refraction is the bending of light as it passes through a medium of different optical density, while reflection is the bouncing back of light when it hits a surface at an angle.

## 2. How do lenses work in optics?

Lenses work by refracting light in a way that focuses or spreads out the light rays. Convex lenses converge light, while concave lenses diverge light.

## 3. What is the difference between a virtual image and a real image?

A virtual image is an image that appears to be behind a mirror or lens, and cannot be projected onto a screen. A real image is an image that can be projected onto a screen and is formed by actual convergence or divergence of light rays.

## 4. What is the relationship between wavelength and frequency of light?

The wavelength of light is inversely proportional to its frequency. This means that as the wavelength of light decreases, the frequency increases.

## 5. How does light behave when it passes through different mediums?

When light passes through a medium of higher optical density, it slows down and bends towards the normal line. When it passes through a medium of lower optical density, it speeds up and bends away from the normal line. This behavior is known as refraction.

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