Lenses

Question 1

what do lenses do in order to focus light

Answer 1

they refract it

Question 2

what is a converging lens built like and why

Answer 2

• the surface of the lens protrudes outwards

- so parallel rays of light passing through the lens converge at a single point

Question 3

what is a diverging lens built like and why

Answer 3

• the surface of the lens inclines inwards

- so parallel rays of light passing through the lens diverge from a single point

Question 4

what is the focal point of a lens

Answer 4

• the point where parallel rays of light will converge
• or the point where the rays of light appear to be diverging from
• after passing through the given lens

Question 5

what is the difference between he focal points of converging (convex) and diverging (concave) lenses

Answer 5

• converging lenses have a real focal point in front of the lens
• whereas diverging lenses have a virtual focal point behind the lens

Question 6

what is the power of a lens

Answer 6

its ability to deviate rays of light through large angles

Question 7

how would the focal length of a lens change as it gets more powerful and why

Answer 7

• the focal length will decrease
• because the rays of light are being refracted at larger angles so they meet closer to the lens than they normally would
• therefore the distance between this focal point and the optical centre of the lens is decreased (focal length)

Question 8

what is the equation for the power of a lens

Answer 8

• power = 1 / focal length

- P = 1 / f

Question 9

what are the units for power in this case

Answer 9

• dioptre

- m^-1

Question 10

why isnt this power dimensionally consistent to other units of power

Answer 10

because it is not related to the rate of conservation of energy

Question 11

what would the power of a lens with a focal length of 0.25m be

Answer 11

1 / 0.25 = 4

Question 12

what is the main difference about the powers between converging and diverging lenses

Answer 12

• converging lenses have positive power values

- whereas diverging lenses have negative powers

Question 13

how would you work out the overall power of a combination of two or more lenses

Answer 13

• add them up

- Pc = P1 + P2 + P3…

Question 14

what is formed when light from an object passes through a lens

Answer 14

an image of the object

Question 15

what two types of images can be formed

Answer 15

real or vitrual

Question 16

when is a real image formed

Answer 16

• when the rays of light actually intersect

- so an image can be projected on a screen

Question 17

when can a virtual image be seen

Answer 17

• when you look through the lens

- and there appears to be a point where the rays originate

Question 18

what are ray diagrams used for

Answer 18

finding out the positions, size and nature of images produced by lenses

Question 19

on a ray diagram for a converging lens, there is a ray passing horizontally, one passing at an angle downwards to the origin and one starting from the focal point behind the lens and travelling at and angle downwards. how would each ray react after passing through the lens

Answer 19

• the horizontal one would be refracted to the middle (y = 0) to the focal point
• the second one would continue to move at the same angle and direction, making a straight line
• the third ray would begin moving horizontally

Question 20

on a ray diagram for a diverging lens, there is a ray passing horizontally, one passing at an angle downwards to the origin and one travelling downwards at an angle but at a trajectory so that it meets the y axis above the origin. how would each ray react after passing through the lens

Answer 20

• the first ray would be refracted outwards (upwards at an angle) with the focal point being behind the lens
• the second ray would continue to move at the same angle and direction making a straight line
• the third ray would begin travelling horizontally with the focal point being in front of the lens

Question 21

what is the principle axis

Answer 21

• the line that passes normally through the optical centre of the lens
• aka the ‘x axis’

Question 22

what is the optical centre of a lens

Answer 22

• the point which rays of light will pass through the lens without deviation
• aka the ‘origin’

Question 23

when labeling a ray diagram, what would u, v, h0, h1 and f be representing

Answer 23

• u = the distance between the object and the optical centre
• v = the distance between the image and the optical centre
• h0 = the height of the object
• h1 = the height of the image
• f = the distance between the optical centre and the focal point (focal length)

Question 24

using the idea of similar triangles, what ratios can be deduced

Answer 24

• h0 / u = h1 / v

- h0 / f = h1 / v - f

Question 25

if the linear magnification of the image, m, is the ratio of the size of the image to the size of the object, what equation can be written

Answer 25

m = h1 / h0 = v / u = v - f / f

Question 26

when you rearrange that equation what is the name of the equation you would get

Answer 26

the lens equation

Question 27

what is the lens equation

Answer 27

(1 / u) + (1 / v) = 1 / f

Question 28

what can the lens equation be used to do

Answer 28

determine the image position for an type of lens

Question 29

what condition must be met in order for the equation to be used

Answer 29

the real positive sign convention must be used

Question 30

what is the difference between the focal lengths and distances of converging and diverging lenses

Answer 30

• they are positive for converging lenses

- and negative for diverging lenses