2.2 - Thin lenses and refraction Flashcards
What happens during thin lens approximation
- Light doesn’t converge or diverge ( can quantify in terms of vergence ) significantly between the surfaces = no significant change in value of vergence as light moves between 2 surfaces
- The refraction effects of both surfaces can be added – possible to represent lens as one surface, refracting the light once - reduced the 2 individual surface refraction down to 1 single refraction of single thin lens
Definition of thin lens approximation
Thin lens takes surface powers and adds them together to find single power of single thin lens
What does thin lens approximation depend on
Whether light changes or not in its vergence between surfaces
Examples of where thin lens approximation happens
Spectacle lens
Is it true that physically thin doesn’t always mean thin lens works well
YES
Physically thin doesn’t always mean thin lens works well
What is thin lens approximation not good for
Contact lenses
They are very thin ( 0.1mm )
Not always good for thin lens approximation as surfaces steeply curved = change in vergence between 2 surfaces even though gap between 2 surfaces is small
What is classed as distant object
5/6m = infinite
What happens in a distant object
Consider how diverging the light rays are as a point object is moved farther away …
How is light represented in infinitely distant object
Light from an infinitely distant object is represented by parallel rays. L = 0D
What is object vergence for infinitely distant object
L = 0D
What happens to value of vergence further away from object
The further away an object is from lens, the lower magnitude of vergence on the scale i.e. gets closer to 0
What is front focal point
F
What happens when object placed at F ( front focal point )
Image forms at infinity
What is front focal length
f
Tells us where front focal point is with respect to lens
What is back focal point
F’
2nd focal point
What happens when object is at infinity
Forms image at 2nd focal point i.e. F’ ( back focal point )
What is back focal length
f’
It is a property of the lens; its surface curvatures and refractive index
Which side is F’
Image
= BACK focal point
Which side is F
Object
= FRONT focal point
Where are distances measured from
From surface/lens refracting the light
Where are heights measured from
From optical axis
What is unit for power of lens
Dioptres, D
What is power of thin lens related to
Focal lengths
What is equation for Front focal point i.e. 1st focal point
F = -n / f
What is equation for back focal point i.e. 2nd focal point
F’ = n’ / f’
What can power of thin lens be used to work out
Back and front focal lengths provided we know refractive indices on either side of lens
What is refractive indices if lens is in air
They are same on both side
n = 1
What is back focal length in human eye
22m
Is back focal length or front focal length longer in eye
Back focal length
Do eyes or air have higher refractive index
Eyes
Why is back focal length longer in eyes
Refractive index ( n’ in eye is 4/3 ) but outside the eye its 1
What is paraxial surface power
Defined in terms of refractive index values either side of surface and radius of curvature
What does surface power depend on
On radius of curvature and difference in refractive index
True of false…eyes have powerful optical surfaces
True
Radius of curvature steep = increases power
What is equation for paraxial surface power
F = n’ - n / r
n’ - n = difference in refractive index either side of surface
r = radius is curvature of surface
What is definition of refraction equation
Relates the vergence of the light striking a surface/lens to that leaving it
What is refraction equation
L’ = L + F
L' = image vergence L = object vergence F = power of surface
What is equation for object vergence
L = n/l
L = object vergence n = refractive index of object space l = object distance
What is equation for image vergence
L = n’/l’
L' = image vergence n' = refractive index of image space l' = image distance
