5.3 - Refraction, Reflection, Polarisation and Lenses Flashcards
How does reflection work?
Reflection occurs when a wave hits a boundary between 2 media and doesn’t go through
As the 2 different media are too different to each other
The surface also has to be very flat, this is why we don’t see refection from a wall
What is the angle proportionality during reflection?
Angle of incidence = Angle of reflection
θ(i)=θ(r)
What is the normal?
The normal is the line perpendicular to the medium change
What’s the angle of incidence?
The angle from the incident ray (the ray going towards new media) to the normal
Where are all the angles measured from?
All angles used are measured from the normal to their ray
- θ incidence
- θ reflection
- θ refraction
What surfaces reflect light best?
Flat surfaces are the best
- they are the smoothest
Opaque surfaces are good
- the light is absorbed by electrons and will re-emit as a reflected wave
Rough surfaces are the worst
- won’t reflect as light is scattered in all different directions
What is refraction?
When light hits a similar, but different median, light is refracted as it slows down or speeds up
Slows down when:
- faster median to slower median
- angle decreases
- θ(i) > θ(r)
Speeds up when:
- slower median to faster median
- angle increases
- θ(i) < θ(r)
What determines the speed light goes through the median?
The density of the median
The more dense, to slower light will travel
What is the refractive index, n?
The refractive index, n measures how much light slows down when passing through the median compared to speed of light in a vacuum, c
n=c/v
where v=speed of light in median
What does the number of the refractive index, n tell us?
The smaller n, the closer v is to the speed of light
so smaller n, the faster v
The higher n, the slower v
n will always be > 1
- as nothing is faster than c
What is Snell’s Law?
Snell’s Law relates the angle of incidence to the angle of refraction by using their refractive index’s
n(1)sin θ(1)=n(2)sinθ(2)
where θ(1) = θ(i)
What’s the critical angle?
The angle of incidence in an (optically) denser medium which the angle of refraction (in the less dense medium) is 90°
The critical angle is when light goes straight through the median wall
So the critical angle will be the angle of incidence
And the angle of refraction =90 deg
θ(2) = 90
This means sinθ(2)=1
and n(2)=1 as it’s air
so…
nsin(C)=1x1
sin(C)=1/n
What is Total Internal Reflection (TIR)
TIR is when the ray totally reflects
so all of the light is reflected in TIR
As it’s reflection:
- θ(i)=θ(r)
TIR happens on 2 conditions:
- θ(i) > θ(C)
- going from a more dense median to a less dense one, so n(1)>n(2)
What’s a converging (convex) lens?
A converging lens brings light onto one point (principal focus)
‘converges’ the light rays
The more curved the lens, the smaller the focal length
Creates a real image
Image is created inverted
What is a diverging (concave) lens?
The rays diverge away
Unlike converging lenses, a virtual is created as the image isn’t actually there, it just a appears like it is
- in reality, it’s further away
What is the principal focus, F?
The point where all the rays go together
In a converging lens, it’s on the right
In a diverging lens, it’s on the left, but dotted lines are drawn to it
What is the focal length, f?
The length between the middle of the lens and the principal focus
How do you draw a converging lens diagram?
Draw a line down the middle of the converging lens
Draw a line from the object to the centre of the lens on the principle axis and a line horizontal to the lens middle
Then carry on the line from the floor all the way, and connect the other one to the principal focus, F and follow it down to connect the lines
This will create the image
How do you draw a diverging lens diagram?
Draw a horizontal and line to centre of lens
Then draw a dotted line from the horizontal line, at the centre of lens to the principle focus, which is on the same side
Where the 2 lines meet is the top of the image
What’s the principle axis
The line that doesn’t get deviated (converged or diverged)
The middle of the lens vertically
What is the power of a lens?
The more powerful a lens, the more the rays will curve
The more curved the lens the more power
The more power means that the image will appear closer
so the more powerful the lens, the shorter the focal length
Power (dioptures, D)=1/f(m)
P=1/f
The shorter the length, the more powerful the lens
What happens to the power in a diverging lens?
In a diverging lens, the focal length is negative, at it’s on the other side
So. the power is also negative
How do you find the total power of a lens?
You add all the powers together
p=p(1)+p(2)+p(3)…
When do images become enlarged with a converging lens?
When the object is closer than 2f, the object will enlarge
When it’s exactly f, or less, the image is too big and is infinite
When do images stay the same size with a converging lens?
When the focal length =2f
When do images become smaller with a converging lens?
When the object is further than 2f, the image will be smaller
What’s the lens equation?
We can find the actual position of an image through ray diagrams, or the lens equation
1/f=1/u+1/v
when f= focus length
when u= object distance
when v= image distance
This equation only works for thin converging or diverging lenses
v and f are negative in diverging lenses
What 2 medians are used in our core practicals?
Air
Perspex block
How do you find the magnification of a lens?
M=image size/object size
M=h(i)/h(o)
or M=v/u
What is polarisation?
Occurs when particles are only allowed to oscillate in one of the directions perpendicular to the direction of wave propagation
Waves only pass in one wave angle
How, using the photoelectric effect, can you tell light behaves as a wave?
In the wave theory, intensity of a wave is directly proportional to amplitude^2 (A^2)
If the light intensity increases, more energy is being delivered to the metal surface per second
So the wave theory would predict more electrons can absorb enough energy per second to escape
- so higher emission rate
