2.2 - Thin lenses and refraction

Question 1

What happens during thin lens approximation

Answer 1

• 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

Question 2

Definition of thin lens approximation

Answer 2

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

Question 3

What does thin lens approximation depend on

Answer 3

Whether light changes or not in its vergence between surfaces

Question 4

Examples of where thin lens approximation happens

Answer 4

Spectacle lens

Question 5

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

Answer 5

YES

Physically thin doesn’t always mean thin lens works well

Question 6

What is thin lens approximation not good for

Answer 6

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

Question 7

What is classed as distant object

Answer 7

5/6m = infinite

Question 8

What happens in a distant object

Answer 8

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

Question 9

How is light represented in infinitely distant object

Answer 9

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

Question 10

What is object vergence for infinitely distant object

Answer 10

L = 0D

Question 11

What happens to value of vergence further away from object

Answer 11

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

Question 12

What is front focal point

Answer 12

F

Question 13

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

Answer 13

Image forms at infinity

Question 14

What is front focal length

Answer 14

f

Tells us where front focal point is with respect to lens

Question 15

What is back focal point

Answer 15

F’

2nd focal point

Question 16

What happens when object is at infinity

Answer 16

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

Question 17

What is back focal length

Answer 17

f’

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

Question 18

Which side is F’

Answer 18

Image

= BACK focal point

Question 19

Which side is F

Answer 19

Object

= FRONT focal point

Question 20

Where are distances measured from

Answer 20

From surface/lens refracting the light

Question 21

Where are heights measured from

Answer 21

From optical axis

Question 22

What is unit for power of lens

Answer 22

Dioptres, D

Question 23

What is power of thin lens related to

Answer 23

Focal lengths

Question 24

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

Answer 24

F = -n / f

Question 25

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

Answer 25

F' = n' / f'

Question 26

What can power of thin lens be used to work out

Answer 26

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

Question 27

What is refractive indices if lens is in air

Answer 27

They are same on both side | n = 1

Question 28

What is back focal length in human eye

Answer 28

22m

Question 29

Is back focal length or front focal length longer in eye

Answer 29

Back focal length

Question 30

Do eyes or air have higher refractive index

Answer 30

Eyes

Question 31

Why is back focal length longer in eyes

Answer 31

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

Question 32

What is paraxial surface power

Answer 32

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

Question 33

What does surface power depend on

Answer 33

On radius of curvature and difference in refractive index

Question 34

True of false...eyes have powerful optical surfaces

Answer 34

True | Radius of curvature steep = increases power

Question 35

What is equation for paraxial surface power

Answer 35

F = n' - n / r n' - n = difference in refractive index either side of surface r = radius is curvature of surface

Question 36

What is definition of refraction equation

Answer 36

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

Question 37

What is refraction equation

Answer 37

L' = L + F ``` L' = image vergence L = object vergence F = power of surface ```

Question 38

What is equation for object vergence

Answer 38

L = n/l ``` L = object vergence n = refractive index of object space l = object distance ```

Question 39

What is equation for image vergence

Answer 39

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