5.7 The Diffraction Grating

Question 1

What does a diffraction grating consist of

Answer 1

A plate with many closely spaced parallel slits ruled on it

Question 2

What happens when a parallel beam of monochromatic light is directed normally at a diffraction grating

Answer 2

Light is transmitted by the grating in certain directions only as:

• The light passing through each slit is diffracted
• The diffracted light waves from adjacent slits reinforce each other in certain directions only including the incident light direction and cancel out in all other directions

Question 3

What is the central beam referred to as

Answer 3

The zero order beam

Question 4

What is the zero order beam in the same direction as

Answer 4

The incident beam

Question 5

The angle of diffraction between each transmitted beam and the central beam increases if:

Answer 5

Light of a longer wavelength is used (eg. by replacing a blue filter with a red filter)
A grating with closer slits is used

Question 6

Diffraction grating: As each diffracted wavefront emerges from a slit, it _______ a wavefront from a slit _______ to it

Answer 6

Reinforces

Adjacent

Question 7

Diffraction grating equation

Answer 7

d Sin ϴ = nλ

Question 8

The number of slits per metre on the grating N =

Answer 8

1/d

Where d is the grating spacing

Question 9

For a given order and wavelength, the ______ the value of d, the ______ the angle of diffraction. In other words, the ______ the number of slits per metre, the ______ the angle of diffraction

Answer 9

Smaller
Greater
Larger
Bigger

Question 10

Fractions of a degree are usually expressed either as a decimal or in minutes (abbreviated ‘) where 1° =

Answer 10

60’

Question 11

To find the max number of orders produced, substitute ϴ=90° in the…

Answer 11

…grating equation and calculate n using n = d/λ

Question 12

The max number of orders is given by the value of…

Answer 12

… d/λ rounded down to the nearest whole number

Question 13

How can diffraction gratings be made (glass)

Answer 13

By cutting parallel grooves very close together on a smooth glass plate
Each transmits some incident light and reflects or scatters some so the grooves act as coherent emitters of waves just as if they were slits
However effective slit width needs to be much smaller than grating spacing so that diffracted waves spread out widely
Otherwise higher order beams much less intense than lower order

Question 14

Diffraction grating in a spectrometer

Answer 14

To study the spectrum of light from any light source and to measure light wavelengths very accurately
Designed to measure angles to within 1 arc minute which is a sixtieth of a degree
Angle can be measured accurately using light of known wavelength

Question 15

What do most industrial and research laboratories ow use

Answer 15

A spectrum analyser

Question 16

What is a spectrum analyser

Answer 16

An electronic spectrometer linked to a computer that gives a visual display of the variation of intensity with wavelength

Question 17

A minimum in the single slit diffraction will…

Answer 17

…supress a diffracted order from the grating and there will be a missing order in the pattern as a result

Question 18

The number of maxima observed is…

Answer 18

2n + 1

Where n is the greatest order

Question 19

What are the types of spectra

Answer 19

Continuous, line emission, line absorption

Question 20

Example of continuous spectra

Answer 20

The spectrum of light from a filament lamp

Question 21

CONTINUOUS SPECTRA: What does the most intense part of the spectrum depend on

Answer 21

The temperature of the light source

The hotter the light source, the shorter the wavelength of the brightest part of the spectrum

Question 22

CONTINUOUS SPECTRA: How can you measure the temperature of the light source

Answer 22

By measuring the wavelength of the brightest part of a continuous spectrum

Question 23

Example of line emission spectra

Answer 23

A glowing gas in a vapour lamp or discharge tube

Emits light at specific wavelengths so its spectrum consists of narrow vertical lines of different colours

Question 24

LINE EMISSION SPECTRA: What are the wavelengths of the lines characteristic of?

Answer 24

The chemical element that produced the light

Question 25

LINE EMISSION SPECTRA: How can you identify elements?

Answer 25

If the glowing gas contains more than one element, the elements can be identified by observing its line spectrum

Question 26

What is a line absorption spectrum

Answer 26

A continuous spectrum with narrow dark lines at certain wavelengths

Question 27

LINE ABSORPTION SPECTRUM: Example of the spectrum of light from a filament lamp

Answer 27

Observed after passing it through a glowing gas

Thin dark vertical lines are observed superimposed on the continuous spectrum

Question 28

LINE ABSORPTION SPECTRUM: What is the pattern of the dark lines caused by?

Answer 28

The elements in the glowing gas
These elements absorb light of the same wavelengths they can emit at so the transmitted light is missing these wavelengths.
Atoms of glowing gas that absorb light then emit light subsequently but not necessarily in same direction as transmitted light