Lenses and power

Question 1

lenses

Answer 1

pieces of glass that bend light in order to bring it to a focus

they make use of refraction to bend light

Question 2

What determines how much power a lens has?

Answer 2

the amount of refraction that occurs

more refraction means more power

Question 3

two types of lens

Answer 3

converging lens/convex

diverging lens/concave

Question 4

converging lens

What is it also known as?

Answer 4

bends rays of light towards one another, bringing them to a point

thicker in the middle

convex lens

Question 5

diverging lens

What is it also known as?

Answer 5

bends rays of light away from each other

thinner in the middle

concave lens

Question 6

symbols used for convex and concave lenses in ray diagrams

Answer 6

arrows point outwards for a convex lens

arrows point inwards for a diverging lens

Question 7

How does the shape of the lens affect the power?

Answer 7

a thicker lens has more sharply curved faces, so it can refract more and is more powerful

Question 8

How does the power of a lens influence the focal point?

Answer 8

a more powerful lens has a smaller focal length

distance from middle of lens to focal point

Question 9

What is the focal point?

How is it different in a convex and concave lens?

Answer 9

the point where the rays of light cross over

convex:
rays that enter parallel to the principal axis will leave and cross the focal point

concave:
rays that enter parallel leave as if they entered from the focal point
(they bend away, but if the line is traced back they meet on the entry side at the focal point)

Question 10

three rules of refraction for convex lenses

Answer 10

1) any incident ray that passes through the centre of the lens will continue in the same direction when it exits the lens
2) any incident ray travelling parallel to the principal axis will travel through the focal point on the other side
3) any incident ray travelling through the focal point on the way to the lens will leave parallel to the principal axis

Question 11

three rules of refraction for concave lenses

Answer 11

1) any incident ray that passes through the centre of the lens will continue in the same direction when it exits the lens
2) any incident ray that enters parallel to the principal axis will leave as if it entered from the focal point (on the entry side)
3) any incident ray that enters as if heading towards the focal point on the opposite side will leave parallel to the principal axis

Question 12

steps for drawing a ray diagram

Answer 12

• label the object on the entry side
• accurately draw 2 rays of light that follow different rules entering and leaving the lens
• use arrows to indicate the direction of travel
• the point where the rays intersect on the other side of the lens is the image

Question 13

How do you draw a ray diagram if the object is in front of the focal point?

Answer 13

draw a dotted line between the focal point and the top of the object

Question 14

What happens if the object is at the focal point of a converging lens?

Answer 14

no image can be formed

Question 15

What is the line in the middle of a ray diagram called?

Answer 15

principal axis

Question 16

How do you calculate the power of a lens?

Answer 16

the reciprocal of the focal length

power (D) = 1/focal length (m)

Question 17

What is the power of a lens measured in?

Answer 17

dioptres (D)

Question 18

Explain how the power of a lens is related to its shape [2]

Answer 18

the more curved the glass surface is, the greater the power of the lens

Question 19

What is the relationship between the power of a lens and its focal length? [1]

Answer 19

as the power increases, the focal length decreases

Question 20

For a long-sighted person, light rays from a point near to the eye are brought to a focus at a point beyond the retina.

Suggest how the person’s vision could be corrected. [2]

Answer 20

light rays need to be refracted so that they are more parallel and do not need to be refracted as much by the eye

using a converging lens refracts the light rays so that they are parallel and will then be focused on the retina