Lens

Question 1

The two types of lens

Answer 1

Convex (converging)
Concave (diverging)

Question 2

What does a convex lens do

Answer 2

Causes rays of light which are parallel to the axis of the lens to converge at the principal focus

Question 3

Where is the principal focus of a convex lens

Answer 3

Where rays hitting the lens parallel to the axis meet

Question 4

Rules for convex lenses

Answer 4

1. An incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side
2. An incident ray passing through the centre of the lens carries on in the same direction
3. An incident ray passing through the principal focus before meeting the lens refracts through the lens and travels parallel to the axis

Question 5

What does Concave lens do

Answer 5

Causes parallel rays of light to diverge

Question 6

Where is the principal focus of a concave lens

Answer 6

Where the rays hitting the lens parallel to the axis appear to come from

Question 7

What are virtual rays

Answer 7

Rays that aren’t actually there but show the path that it looks like the light has taken

Question 8

What is the focal length

Answer 8

The distance between the centre of the lens and the principal focus

Question 9

3 rules of concave lenses

Answer 9

1. An incident ray travelling parallel to the axis refracts through the lens and travels in line with the near side principal focus (so it appears to have come from the principal focus)
2. An incident ray passing through the centre of the lens carries on in the same direction
3. An incident ray passing through the lens towards the far side principal focus refracts through the lens and travels parallel to the axis

Question 10

What kind of images are formed from lenses

Answer 10

Virtual and real

Question 11

Real images

Answer 11

Formed when the light rays from a point on an object come together to form an image -> they pass through the same point

Question 12

Where does the screen have to be placed

Answer 12

Exactly where the rays meet otherwise it’ll be out of focus

Question 13

Virtual images

Answer 13

Formed when the light rays from a point in an object are diverging after they have left the lens -> so the light from the point on the object appears to be coming from a different place

• the light rays don’t actually pass through that point they jsut appear to

Question 14

What to mention when describing an image

Answer 14

• how big it is compared to the object
• whether it’s upright or inverted
• whether it’s real or virtual

Question 15

Images formed by concave lenses

Answer 15

• virtual
• upright
• smaller than the object

Question 16

Convex at different points

Answer 16

>2F: inverted, real, smaller
At 2F: inverted, real, same size
Between 2F and F: inverted, real, bigger
Between F and lens: upright, virtual, bigger + on the same side

Question 17

What lenses do magnifying glasses use

Answer 17

Convex lenses

Question 18

How do magnifying glasses work

Answer 18

By creating a virtual, upright image that is bigger than the object and on the same side as the object

Question 19

What has to happen for an object to be magnified

Answer 19

The object being magnified must be closer to the lens that the focal point