Diffraction Gratings

How to create an interference pattern using a diffraction grating

Pass a beam of monochromatic light through slits

Diffraction grating

A material with slits equally spaced apart

Explain how an interference pattern is created when shining light through a diffraction grating

At each slit, light diffracts, producing coherent sources of light. As they are coherent and in phase, they produce an interference pattern.

What does the number of slits on a diffraction grating affect

The sharpness

Explain how a diffraction grating with more slits affects the interference pattern

More sources are created, so there are more beams reinforcing an interference pattern, so it is sharper

What does it mean for an interference pattern to be sharper

The bright bands are brighter and narrower and the dark bands are darker

Why may a sharper interference pattern be needed

They can be used for more precise measurements

State the order of orders in a 3rd order interference pattern

3rd order

2nd order

1st order

Zero order

1st order

2nd order

3rd order

What are the maxima like in an interference pattern for monochromatic light

Sharp

Fringe width

The distance between the maxima

Maximum in an interference pattern

Areas of constructive interference

Brightest maximum in an interference pattern

Zero order

Experiment to find the wavelength of light using a diffraction grating

1- Shine a monochromatic light source through a diffraction grating so that an interference pattern is created on a screen

2- Measure the distance D between the grating and screen and the mean distance x between the zero order and 1st order

3- Use the small angle approximations such that θ = x/D

4- Taking n = 1 and the value of d for the grating, calculate λ using d sinθ = nλ

5- Repeat for other orders, then repeat for diffraction gratings of different values of d

Diffraction grating equation

nλ = d sinθ

Should θ be in radians or degrees for the diffraction grating equation

Radians

What is θ in the diffraction grating equation

The angle between the zero and nth order line

What is d in the diffraction grating equation

Distance between two slits in a diffraction grating

What is n in the diffraction grating equation

The order line being used for θ

Explain how the wavelength affects how spread out the interference pattern is

If λ is larger then sinθ is larger so θ is larger, spreading out the pattern more

What does it mean for a greeting to be coarser

Slits are further apart

Explain how a coarser diffraction grating affects how spread out the interference pattern is

A larger d gives a smaller sinθ, giving a smaller θ, so the pattern will be less spread out

What does it mean if the value of sinθ is greater than 1 for a diffraction grating

The order n used in the equation does not exist

Explain why white light splits into different colours in a diffraction grating

White light is made up of many wavelengths of light and different wavelengths are diffracted at different values of θ

What does a non-zero order on the interference pattern of white light look like

A spectrum, with red furthest from the zero order and violet closest

Why is red on the outside of the orders on the interference patterns of white light

It has the largest wavelength, so diffracts at the greatest angle

What does the zero order on the interference pattern of white light look like

Why

Explain the colour of the zero order of white light

All wavelengths pass straight through, so it is white

Why are diffraction gratings used for spectra rather than prisms

They’re more accurate

What are diffraction gratings used for

Astronomers and chemists identifying elements

Explain the colourful pattern on DVDs

They are made up of grooves etched into a reflective surface which act like a diffraction grating, creating an interference pattern in your eyes