Flashcards in 9.2 Diffraction Deck (16):

1

## What is diffraction?

### The spreading of a wave as it foes through an aperture or past an obstacle.

2

## What is the path difference between two light rays of identical wavelength?

### (b/2)sin@ where b is the width of the slit. If the angle is very small this is approximately equal to b/2@ if the angle is expressed in radians.

3

## What happens if the path difference is a half wavelength?

###
There will be destructive interference when the rays meet at the screen which is assumed to be at a long distance compared with the slit width. The same condition holds for a pair of rays from just below the top and just below the middle of the opening or indeed for any other such pair of rays at an angle @. The result will be a dark band at the screen at the diffraction angle

@D = lambda/b

4

## What will happen to rays leaving the slit in a direction along the centre line where @=0?

### They will arrive in phase and so there will be constructive interference and a bright band at the centre of the diffraction pattern.

5

## What is the intensity of the first secondary maximum compared to the central maximum?

### Around 4.5% of that of the central maximum.

6

## What happens to the diffraction patter as the wavelength increases?

### The width of the diffraction pattern increases.

7

## If white light is incident on the slit what happens to the diffraction pattern?

### Each constituent colour will have its own characteristic pattern and their combination will be a pattern which is white at the centre but coloured to the sides.

8

## What does a detailed analysis of a diffraction pattern for a circular aperture diameter b show?

### That the first diffraction minimum is observed at a diffraction angle of @D = 1.22(lambda/b)

9

## What is resolution?

### a measure of the ability of a detection device to distinguish two objects - to see them as separate objects.

10

## What is the angular separation of light from two point sources that diffracts when they pass through an aperture separated by a distance s and distance d to the aperture?

### @A = s/d

11

## What happens if their separation is small enough?

### The diffraction patterns of the two sources will overlap and the two sources may appear as one.

12

## What is the Rayleigh Criterion?

### Two sources are said to be just resolved if the central maximum of the diffraction pattern of one source falls on the first minimum of the other.

13

## To satisfy the Rayleigh criterion what must the angular separation of the two sources be?

###
@A = @D = 1.22(lambda/b)

where b is the diameter of the circular aperture used to collect the light from the sources. So to know whether to sources are resolved we compare the angular separation @A and the diffraction angle.

14

## What happens if the sources are not resolved?

###
@A < @D = 1.22(lambda/b)

15

## What happens if the sources are just resolved?

###
The central maximum of one coincides with the first minimum of the other

@A = @D = 1.22(lambda/b)

16