5.7 The diffraction grating

Question 1

What is a diffraction grating?

Answer 1

» Consists of a plate with many close spaced parallel slits ruled on it

Question 2

Why is light transmitted in certain directions only in diffraction grating?

Answer 2

» Light passing through each slit is diffracted
» Diffract light waves from adjacent slits reinforce each other in certain directions only

Question 3

What is the zero order beam?

Answer 3

» The central beam

Question 4

How can the angle of diffraciton increase between the transmitted beam and the central beam?

Answer 4

» Light of a longer wavelength is used
» A grating with closer slits is used

Question 5

What is the diffraction grating formula?

Answer 5

» n x lambda = d x sintheta
» n = the order number
» d = slit spacing
» Theta - angle from centre to that maximum

Question 6

What is the formula for calculating the slit spacing?

Answer 6

» D = 1/n
» Where N is the number of slits per mm

Question 7

How can you find the maximum number of orders produced?

Answer 7

» Put theta as 90 degrees
» Then n = d / lambda

Question 8

What is important to remember when finding the maximum number of orders?

Answer 8

» N must be an integer, so always round down to the nearest whole nymber

Question 9

Why, when monochromatic light is passed through the diffraciton grating, it is much sharper and brighter then the Young’s double slit?

Answer 9

» More rays of light reinforcing the pattern

Question 10

What is the continous spectra?

Answer 10

» From 350nm to deep red about 650nm
» By measuring the wavelnegth of the brighest part of continous spectrum, we can measure the temperature of the light source

Question 11

What is the line absorption spectra?

Answer 11

» Continous spectrum with dark narrow lines at certain wavelengths
» Dark narrow lines, due to elements absorbing the same wavelength

Question 12

What are 3 uses of diffraction grating?

Answer 12

» Split up starts to get line absorption spectra
» X-ray crystallography - x-rays direction at a thin crystal sheet to form a diffraction pattern
» Spectral analysis of stars

Question 13

Describe the necessary procedures to obtain an accurate and reliable value for the wavelength?

4 Marks

Answer 13

» Use formula 1/n, where n is the number of lines per mm, to calculate value of D
» Find angle of diffraction between first order beam and zero order beam - Describe how to use trigonometry
» Repeat measurements 3 times to get a mean
» Use formula Lambda = Dsintheta/n to get value of lambda
» Repeat with different orders to find the angle of diffraciton, therefore finding the wavelength again