Chapter 1 - Lenses

Question 1

What is the shape of a converging lens?

Answer 1

Convex (thicker in the middle)

Question 2

What is the shape of a diverging lens?

Answer 2

Concave (thinner in the middle)

Question 3

What does a converging lens do?

Answer 3

Causes incoming parallel light rays to converge at a single point

Question 4

What does a diverging lens do?

Answer 4

Causes incoming parallel light rays to diverge away from each other

Question 5

Define “focal length” (f)

Answer 5

The distance between its optical centre and its principal focus

Question 6

Define “object distance” (u)

Answer 6

The distance from the object to the optical centre of the lens

Question 7

Define “image distance” (v)

Answer 7

The distance from the optical centre of the lens to the image

Question 8

Define “optical centre”

Answer 8

A point in a lens where light rays go through without refraction

Question 9

Define “principal axis”

Answer 9

A line that passes through the optical centre and is perpendicular to the lens

Question 10

Define “principal focus” / “focal point”

Answer 10

A point where rays of light parallel to the principal axis meet after going through the lens

Question 11

Define “focal plane”

Answer 11

A plane that passes through the principal focus and is perpendicular to the principal axis
(All parallel beams of light meet at a point on the focal plane after going through a lens)

Question 12

What happens to light rays passing through the optical centre?

Answer 12

Rays are not deviated

Question 13

What happens to light rays parallel to the principal axis?

Answer 13

Rays will be refracted by the lens to pass through the focal point on the other side of the lens

Question 14

What happens to light rays passing through the focal point?

Answer 14

Rays will be refracted by the lens to emerge parallel to the principal axis on the other side of the lens

Question 15

What happens to light rays parallel to each other?

Answer 15

Rays will converge to a point on the focal plane

Question 16

What happens to all light rays coming from the same point on the object (real image)?

Answer 16

Rays will meet at the corresponding point on the real image after passing through the lens

Question 17

What happens to all light rays coming from the same point on the object (virtual image)?

Answer 17

Rays will appear to originate from the corresponding point on the virtual image after passing through the lens

Question 22

Formula to calculate linear magnification

Answer 22

m = hi / ho = v/u

[ linear magnification = image height / object height = image distance / object distance ]

Question 23

u = ∞

Characteristics of image

Answer 23

Real, inverted, diminished

Question 24

u = ∞

Location of image

Answer 24

v = f

Question 25

u = ∞

Uses

Answer 25

Objective lens of telescope

Question 26

2f < u < ∞

Characteristics of image

Answer 26

Real, inverted, diminished

Question 27

2f < u < ∞

Location of image

Answer 27

f < v < 2f

Question 28

2f < u < ∞

Uses

Answer 28

Camera

Question 29

u = 2f

Characteristics of image

Answer 29

Real, inverted, same size of object

Question 30

u = 2f

Location of image

Answer 30

v = 2f

Question 31

u = 2f

Uses

Answer 31

Photocopier

Question 32

f < u < 2f

Characteristics of image

Answer 32

Real, inverted, magnified

Question 33

f < u < 2f

Location of image

Answer 33

2f < v < ∞

Question 34

f < u < 2f

Uses

Answer 34

Projective lens of microscope

Question 35

u = f

Characteristics of image

Answer 35

Depends on usage

Question 36

u = f

Location of image

Answer 36

v = ∞

Question 37

u = f

Uses

Answer 37

Spotlight

Question 38

u < f

Characteristics of image

Answer 38

Virtual, upright, magnified

Question 39

u < f

Location of image

Answer 39

Same side as object

Question 40

u < f

Uses

Answer 40

Magnifying glass

Question 41

Lens formula

Answer 41

1/f = 1/u + 1/v

Question 42

Lens formula: sign convention

Answer 42

• Real image: u and v are positive

- Virtual image: u is positive and v is negative (image on same side of lens as object)

Question 43

A person with perfect vision will have blurred vision under water: Explain why

Answer 43

The bending effect for light travelling from water into the eye is different from light travelling from air into the eye. The rays of light no longer meet on the retina of the eye.

Question 44

A person with perfect vision will have blurred vision under water: How can the person overcome this?

Answer 44

Wear goggles so that the light is still entering the eye from air

Question 45

A person with a particular degree of short-sightedness will have perfect vision under water: Explain why

Answer 45

People with short-sightedness have the light focused in front of the retina. The difference in refractive indices between water and the eye being less than that between air and the eye, will mean that the bending effect on the rays entering from water is weaker. So the rays will meet further away (i.e. on the retina) and the person sees a sharp image.

Question 46

A converging lens with a focal length of 20.0cm is used to create an image of the sun on a paper screen.
How far from the lens must the paper be placed to produce a clear image? Why?

Answer 46

The lens must be at a distance of f (20.0cm).
Rays of distant objects (considered to be parallel) will converge on a point along the focal plane on the opposite side of the lens.