1.11 How Lenses Work

Question 1

What are the two different types of lenses?

Answer 1

• Convex
• Concave

Question 2

Describe concave lenses (4)

Answer 2

• A concave lens caves inwards. It causes parallel rays of light to diverge (spread out).
• When viewing the rays from the side of the lens from which the rays emerge, it will look like the rays have come from somewhere else. We show this using a virtual ray - a ray that isn’t actually there. Virtual rays are drawn by tracing the emerging rays back.
• The principal focus of a concave lens is the point where rays hitting the lens parallel to the axis appear to come from.
• For any lens, the distance from the centre of the lens to the principal focus is called the focal length. There’s a principal focus on each side of any lens.

Question 3

Describe convex lenses (3)

Answer 3

• A convex (converging) lens bulges outwards. It causes parallel rays of light to converge (be brought together).
• The principal focus of a convex lens is where rays hitting the lens parallel to the axis all meet.
• For any lens, the distance from the centre of the lens to the principal focus is called the focal length. There’s a principal focus on each side of any lens.

Question 4

The shorter the focal length, the more ___________ the lens.

Answer 4

powerful

Question 5

What are the two ways in which we can make lenses more powerful? (2)

Answer 5

• Make it more curved
• Use a different material that naturally refracts light more strongly

Question 6

How are images formed?

Answer 6

Images are formed at points where all the light rays from a certain point on an object come together or appear to come together.

Question 7

What are the two different types of images that can be formed by lenses?

Answer 7

• Real
• Virtual

Question 8

How is a real image formed?

Answer 8

A real image is formed when the light rays actually come together to form the image.

Question 9

How is a virtual image formed?

Answer 9

A virtual image is when the light rays from the object appear to be coming from a completely different place to where they’re actually coming from.

Question 10

What is an example of a virtual image in real life?

Answer 10

A mirror

Question 11

Another name for a convex lens is a:

Answer 11

Converging lens

Question 12

The distance between the principle focus and the centre of the lens is called the ‘_____ _______’

Answer 12

focal length

Question 13

Describe how to draw a ray diagram for an image through a convex lens (6)

Answer 13

1) Pick a point on the top of the object. Draw a ray going from the object to the lens parallel to the axis of the lens.
2) Draw another ray from the top of the object going right through the middle of the lens.
3) The incident ray that’s parallel to the axis is refracted through the principal focus (F). Draw a refracted ray passing through F.
4) The ray passing through the middle of the lens doesn’t bend.
5) Mark where the rays meet. That’s the top of the image.
6) Repeat the process for a point on the bottom of the object. When the bottom of the object is on the axis, the bottom of the image is also on the axis.

Question 14

Describe how the distance from the lens to the object affects the size and position of the image in ray diagrams (3)

Answer 14

1) An object 2F (two focal lengths) from the lens produces a real, inverted (upside down) image the same size as the object and at 2F on the other side of the lens.
2) An object between F and 2F will make a real, inverted image bigger than the object and beyond 2F.
3) An object nearer than F will make a virtual image the right way up, bigger than the object and on the same side of the lens

Question 15

Describe how to draw a ray diagram for an image through a concave lens (5)

Answer 15

1) Pick a point on the top of the object. Draw a ray going from the object to the lens parallel to the axis of the lens.
2) Draw another ray from the top of the object going right through the middle of the lens.
3) The incident ray that’s parallel to the axis is refracted so it appears to have come from the principal focus (F). Draw a ray from the principal focus. Make it dotted before it reaches the lens (as it’s virtual here).
4) The ray passing through the middle of the lens doesn’t bend.
5) Mark where this ray meets the virtual ray. That’s the top of the image.

Question 16

In ray diagrams, why do images through a concave lens always appear the same?

Answer 16

A concave lens always produce a virtual image. The image is the right way up, smaller than the object and on the same side of the lens as the object - no matter where the object is.

Question 17

Describe the similarities and differences between convex and concave lenses in their images

Answer 17

• Differences: A concave lens always produces a virtual image. The image is the right way up, smaller than the object and on the same side of the lens as the object - no matter where the object is. Whereas, convex lenses have a lot of variety in their image type. They can produce both real and virtual images, upright and inverted, on either side of the lens, not to mention images of any size.
• Similarities: Convex and concave lenses both have a principal focus along the axis on each side. Also, rays passing through the centre of either lens are never refracted.