Lenses - Paper 1

Question 1

Name the two different types of lens

Answer 1

Concave (diverging)
Convex (converging)

Concave is like a cave - it curves inwards
Therefore convex is the opposite

Question 2

What would make a ray not refrect when passing through a lens?

Answer 2

If it enters directly along the normal, along the principle axis

Question 3

Are convex lenses thicker at the centre or at the edges?

Answer 3

At the centre

Remember - No caves

Question 4

Is the main purpose of a lense to reflect or refract light?

Answer 4

Refract

Question 5

When entering a lens, does a light ray bend towards or away from the normal?

Answer 5

Towards the normal

Question 6

When leaving a lens, does a light ray bend towards or away from the normal?

Answer 6

Away from the normal

Question 7

What is the principle axis?

Answer 7

An imaginary horizontal line through the centre of a lens

It a ray of light passes through this, it will run along the normal and not refract

Question 8

What is the focal length?

Answer 8

The distance from the centre of the lens to the principle focus

Question 9

What name would be given to the distance from the centre of the lens to the principle focus?

Answer 9

The focal length

Question 10

What is the principle focus?

Answer 10

The point where light rays appear to converge or diverge from

Question 11

What is the symbol for the principle focus?

Answer 11

F
(capital F)

Question 12

In a convex lens, what would you call the point at which light rays converge?

Convex lenses are also called converging lenses

Answer 12

The principle focus

Question 13

Give another name for principle focus

Answer 13

Focal point

Question 14

Give another name for a diverging lens

Answer 14

Concave lens

Question 15

Give another name for a converging lens

Answer 15

Convex lens

Question 16

What is the focal point of a diverging lens?

Answer 16

The point where rays of light appear to originate from

Question 17

What does the focal length of a lens depend on?

Answer 17

Its strength

Question 18

The focal length is the distance between which two points?

Answer 18

The focal point (principle focus)
and the centre of the lens

Question 19

What is the more scientific word to describe an image which is smaller than the object?

Answer 19

Diminished

Question 20

When describing an image on a ray diagram, how many points do you need to make? And what are they?

Answer 20

3
Magnified or Diminished
Inverted or Upright
Real or Virtual

Question 21

With a convex lens, if an object is more than 2 focal lengths away, how would you describe the image?

Answer 21

Diminished
Inverted
Real

Question 22

With a convex lens, if an object is between 1 and 2 focal lengths away, how would you describe the image?

Answer 22

Magnified
Inverted
Real

Question 23

With a convex lens, how far away would an object need to be for the image to be diminished?

Answer 23

More than 2 focal lengths away

Question 24

With a convex lens, how far away would an object need to be for the image to be exactly the same size?

Answer 24

Exactly 2 focal lengths

It would still be inverted

Question 25

With a convex lens, how far away would an object need to be for the image to be magnified?

Answer 25

Less than 2 focal lengths ## Footnote If between 1 and 2 focal lengths, it will be real, inverted and magnified But if under 1 focal length away from lens, it will be virtual, upright and magnified

Question 26

What do convex lenses do?

Answer 26

Refract parallel light rays inwards to a single point (the principle focus/focal point)

Question 27

What do concave lenses?

Answer 27

Refract parallel light rays outwards (disperse light)

Question 28

Which lens disperses light?

Answer 28

Concave (Diverging lens) ## Footnote Disperses and Diverging both begin with D

Question 29

Where can principle focuses be found?

Answer 29

On **both sides** of **all lenses**

Question 30

Having a shorter focal means what for the strength of the lens?

Answer 30

It is stronger more powerful

Question 31

Give two ways to make a lens more powerful

Answer 31

Increase curvature Use a material which refracts light more strongly

Question 32

In lenses, when is an image formed?

Answer 32

When all light rays (from a particular point on an object) appear to come together

Question 33

When is a real image formed?

Answer 33

When light rays actually do come together to form the image

Question 34

Which lens always forms virtual images?

Answer 34

Diverging (Concave)

Question 35

Give three characteristics of images formed by concave/diverging lenses

Answer 35

Diminished Upright Virtual

Question 36

When is a virtual image formed?

Answer 36

When the light rays don't come together where the image appears to be

Question 37

How do you draw virtual rays?

Answer 37

With **a dashed line** traced back to the focal point from the points where light rays leave the lens

Question 38

Give an example of virtual rays

Answer 38

An image in a mirror

Question 39

Is a concave lens wider in the middle or at its ends?

Answer 39

At its ends

Question 40

When drawing a ray diagram, where do you have to draw the rays to and from?

Answer 40

From a point at the top of the object: one parallel to the axis which changes direction when it meets the lens and one straight through the centre of the lens (that therefore isn't refracted) and then, if the bottom of the image is not on the axis, repeat this with a point at the bottom of the object

Question 41

How do you draw incident light rays on a ray diagram?

Answer 41

Both starting from the same point, one goes parallel to the axis and the other goes to the centre of the lens (on the axis) ## Footnote The ray which goes to the centre of the lens will not refract so, when drawing this in real life, you can just continue it in a straight line because this means it will not refract.

Question 42

With a convex lens, if an object is less than 1 focal length away, how would you describe the image?

Answer 42

Virtual Upright Magnified ## Footnote The refracted rays won't ever meet

Question 43

How far from a convex lens would an object have to be for a virtual image to be formed?

Answer 43

Less than one focal length away ## Footnote Focal length - the distance from the centre of the lens to the principle focus (focal point)