LENSES (chapter one)

Question 1

Describe the ray point of view

Answer 1

• Light travels out in straight lines from a small source

- Light in a parallel beam/from a very distant source has rays approximately parallel to each other

Question 2

Describe the wave point of view

Answer 2

• Light spreads out in spherical waves from a small source(like ripples)
• Wave fronts from a parallel beam/distant source have zero curvature (do not say they are parallel)

Question 3

What does a lens do in the ray point of view?

Answer 3

Bends the rays to bring them to a focus

Question 4

What does a lens do in the wave point of view?

Answer 4

Adds curvature to the wave, centering it on the focus

Question 5

How does a lens add curvature to a wave?

Answer 5

The middle is slowed down more than the edges as the middle of the lens is thicker than the edges. This ‘bends’ the wave.

Question 6

What is the distance from lens to focus called?

Answer 6

focal length, f

Question 7

Does a powerful lens have a small or large focal length?

Answer 7

small

Question 8

What is the power of a lens?

Answer 8

How much curvature it has added

=1/focal length

Question 9

What is the power of a lens measured in?

Answer 9

Dioptre

no s

Question 10

What is v?

Answer 10

image distance

distance of image formed from lens

Question 11

What is u?

Answer 11

Object distance

distance of original object from lens

Question 12

What is the rule for curvature and lenses?

Answer 12

1/v=1/u+1/f

Curvature of wave after lens=curvature before lens+ curvature added by lens

Question 13

What is the cartesian sign convention?

Answer 13

All distances to the left of the lens are negative (u) and all to the right are positive (v)

Question 14

What does a negative magnification mean?

Answer 14

The image will be inverted

Question 15

What does a magnification of less than one mean?

Answer 15

The image will become smaller

Question 16

What is the formula for linear magnification?

Answer 16

image height/object height

m=v/u

Question 17

What causes light to slow down when travelling through a lens?

Answer 17

Refraction

Question 18

What is the formula for refractive index?

Answer 18

speed of light in free space/speed of light in that medium

Question 19

What happens when an object is placed at the focal length?

Answer 19

No image will be formed
The rays radiate from the source with curvature, the lens will curve them more to parallel, meaning no focus point will ever be reached

Question 20

When will an image be formed at the focal length?

Answer 20

When the source is distant

so that wavefronts are parallel

Question 21

What happens when light is from a nearby source?

Answer 21

The wave still has some curvature when it goes through the lens, meaning the image will be formed FURTHER than the focal point

Question 22

What happens when the source is closer than the focal length?

Answer 22

The image distance will be negative and so a virtual image will be formed on the left side of the lens

Question 23

What is a virtual image?

Answer 23

An image which forms on the same side of the lens as the source (eg. in a magnifying glass)

Question 24

When is f positive?

Answer 24

In a converging lens