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Visual Optics > 2.2 - Thin lenses and refraction > Flashcards

2.2 - Thin lenses and refraction Flashcards

1
Q

What happens during thin lens approximation

A
  • 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
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2
Q

Definition of thin lens approximation

A

Thin lens takes surface powers and adds them together to find single power of single thin lens

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3
Q

What does thin lens approximation depend on

A

Whether light changes or not in its vergence between surfaces

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4
Q

Examples of where thin lens approximation happens

A

Spectacle lens

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5
Q

Is it true that physically thin doesn’t always mean thin lens works well

A

YES

Physically thin doesn’t always mean thin lens works well

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6
Q

What is thin lens approximation not good for

A

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

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7
Q

What is classed as distant object

A

5/6m = infinite

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8
Q

What happens in a distant object

A

Consider how diverging the light rays are as a point object is moved farther away …

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9
Q

How is light represented in infinitely distant object

A

Light from an infinitely distant object is represented by parallel rays. L = 0D

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10
Q

What is object vergence for infinitely distant object

A

L = 0D

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11
Q

What happens to value of vergence further away from object

A

The further away an object is from lens, the lower magnitude of vergence on the scale i.e. gets closer to 0

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12
Q

What is front focal point

A

F

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13
Q

What happens when object placed at F ( front focal point )

A

Image forms at infinity

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14
Q

What is front focal length

A

f

Tells us where front focal point is with respect to lens

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15
Q

What is back focal point

A

F’

2nd focal point

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16
Q

What happens when object is at infinity

A

Forms image at 2nd focal point i.e. F’ ( back focal point )

17
Q

What is back focal length

A

f’

It is a property of the lens; its surface curvatures and refractive index

18
Q

Which side is F’

A

Image

= BACK focal point

19
Q

Which side is F

A

Object

= FRONT focal point

20
Q

Where are distances measured from

A

From surface/lens refracting the light

21
Q

Where are heights measured from

A

From optical axis

22
Q

What is unit for power of lens

A

Dioptres, D

23
Q

What is power of thin lens related to

A

Focal lengths

24
Q

What is equation for Front focal point i.e. 1st focal point

A

F = -n / f

25
Q

What is equation for back focal point i.e. 2nd focal point

A

F’ = n’ / f’

26
Q

What can power of thin lens be used to work out

A

Back and front focal lengths provided we know refractive indices on either side of lens

27
Q

What is refractive indices if lens is in air

A

They are same on both side

n = 1

28
Q

What is back focal length in human eye

A

22m

29
Q

Is back focal length or front focal length longer in eye

A

Back focal length

30
Q

Do eyes or air have higher refractive index

A

Eyes

31
Q

Why is back focal length longer in eyes

A

Refractive index ( n’ in eye is 4/3 ) but outside the eye its 1

32
Q

What is paraxial surface power

A

Defined in terms of refractive index values either side of surface and radius of curvature

33
Q

What does surface power depend on

A

On radius of curvature and difference in refractive index

34
Q

True of false…eyes have powerful optical surfaces

A

True

Radius of curvature steep = increases power

35
Q

What is equation for paraxial surface power

A

F = n’ - n / r

n’ - n = difference in refractive index either side of surface
r = radius is curvature of surface

36
Q

What is definition of refraction equation

A

Relates the vergence of the light striking a surface/lens to that leaving it

37
Q

What is refraction equation

A

L’ = L + F

L' = image vergence 
L = object vergence 
F = power of surface
38
Q

What is equation for object vergence

A

L = n/l

L = object vergence 
n = refractive index of object space 
l = object distance
39
Q

What is equation for image vergence

A

L = n’/l’

L' = image vergence 
n' = refractive index of image space 
l' = image distance

Visual Optics (18 decks)

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