Lecture 4 - Diffraction and Interference Flashcards
Concave lens
Negative
Diverging
Convex lens
Positive
Converging
Drawing ray diagrams
- Start with lens ( positive or negative )
- Always draw optical axis ( dashed line horizontally in middle )
- Label focal points
- Draw rays
- Object and where image forms
What are the 3 rays when drawing a ray diagram
- Parallel to optical axis, refracts through second focal point
- Undeviated ray - through centre of optical axis, without changing angles - diagonal
- Through 1st focal point, then straight
Where is image formed on ray diagram
Where image rays converge
What is the left space of ray diagram
Object space
What is the right space of ray diagram
Image space
Whats the difference between ray diagram for positive and negative lens
Negative lens - focal points switched around….
1st is on right and 2nd is on left
Positive lens - 1st focal point on left, 2nd focal point on right
What is the ray part of after reflection
Image space
What is the ray part of before reflection
Object space
What is interference
Monochromatic light split into 2 paths by a beamsplitter, with half the amount of light going in each path
Mirrors are then used to bring the 2 paths back together and amount of light reaching a screen is observed
What is constructive interference
double amplitude
two points on two different peaks meet at same point
What is destructive interference
2 lights cancel out - no light
a’s peak meets b’s trough
What happens when waves are between 0 and pie
Amplitudes vary between maximum and zero
How does anti - reflection coating work
Some light transmitted and reflected off surface of glass
Meet on retina - cancel out
Some light reflected
2 rays meet eye - cancel each other out = no reflection
Problems with anti - reflection coating
Doesn’t cancel all reflections = not 100%
If white - different wavelengths = difficult ( if monochromatic = fine )
What is diffraction
When light goes through aperture - deforms slightly - bending of edges
Bending of extreme wavefronts
Waves spread out as they go through a narrow gap or round obstacles
What is an aperture
A gap/hole in lens through which light passes
Diffraction and rays
Approaching hole ( aperture ) - go through or dont
Diffraction and wavefronts
Portion of light goes through, other portion doesn’t
How does aperture size affect difftaction
The LARGER the aperture size - the LOWER the amount of diffraction - more unaffected wavefront
The SMALLER the aperture size - the HIGHER the amount of diffraction
What happens when light approaches aperture
Light approaches aperture
Defracts along edges
Wavefront not spherical
What does amount of diffraction depend on
Wavelength of wave compared to size of gap
