4.6.2.5 Lenses (physics only)

Question 1

How does a lens form an image?

Answer 1

a lens forms an image by refracting light

Question 2

What do all lenses do?

Answer 2

all lenses refract light

Question 3

What does where the lines meet on convex/concave lens ray diagram show?

Answer 3

where the lines meets shows position of image

Question 4

Description of convex lens:

Answer 4

thicker at centre than at the edges

Question 5

Principal axis:

Answer 5

the central ray passes through the lens without being refracted as this ray is passing directly along the normal

Question 6

Principal focus:

Answer 6

all of the other rays refract and are focused on a point - principal focus (F)

Question 7

Focal length:

Answer 7

the distance from the centre of the lens to the principal focus is called the focal length

Question 8

How do convex lenses vary?

Answer 8

• different convex lenses have different focal length depending on the strength of the lens

Question 9

Features of image of object 2 or more focal lengths away from convex lens:

Answer 9

1. Image is diminished (smaller than the object)
2. Image is inverted (upside down)
3. This is a real image (rays meet at a point - if we placed a screen there we would see the image on the screen)

Question 10

Features of image of object less than 2 focal lengths away from convex lens:

Answer 10

1. Image is magnified
2. Image is inverted
3. Image is real

Question 11

What do the properties of the image depend on?

Answer 11

properties of the image depend on the distance between the object and the lens

Question 12

Description of concave lens:

Answer 12

a concave lens is thicker at the edges than at the centre

Question 13

Refraction in concave lenses:

Answer 13

• concave lenses make light rays diverge

Question 14

Principal focus (concave lens):

Answer 14

• if you plot lines behind the lens they come to a point - principal focus (F)
• however, concave lens not actually focusing light at principal focus - light rays only appear to be coming from principal focus

Question 15

How to draw ray diagram for concave lens:

Answer 15

1. Draw ray from top of object passing through centre of lens without changing direction
2. Draw another ray from top of object. This passes through the lens and is refracted - ray must appear to have come from principal focus so draw dotted line to show this

Question 16

3 key features of image from concave lens:

Answer 16

1. Image is diminished
2. Image is right way up (not inverted)
3. Image is virtual (as rays do not actually cross at focal point only appear to cross at focal point - if we placed a screen where the image appears the image would not appear on the screen

Question 17

Difference between real and virtual images:

Answer 17

• real images are formed where light rays intersect and can projected on to a screen
• virtual images cannot be projected on to a screen as no light actually intersects at the point they form