Waves - Lenses

Question 1

what is a convex lens

Answer 1

a lens which bulges outwards. causes rays of light that are parallel to converge at the principal focus

Question 2

what is the principle focus of a convex lens

Answer 2

where the rays hitting the lens parallel to the axis all meet

Question 3

what is the focal length

Answer 3

distance from the centre of the lens to the principal focus

Question 4

there are principal focuses on each side of a convex lens. which focus will rays focus on

Answer 4

the one to the far side of the lens in relation to where the rays are coming from

Question 5

an incident ray that passes through the principal focus before meeting the lens will refract how

Answer 5

refracts through the lens are travels parallel to the axis

Question 6

what is a concave lens

Answer 6

causes parallel rays of light to diverge

Question 7

what is the principal focus point of a concave lens

Answer 7

the point where rays hitting the lens parallel to the axis appear to all come from. So if you trace back the refracted rays its the point where they all meet

Question 8

what happens if an incident ray passes through the centre of a concave lens

Answer 8

carriers on in same direction

Question 9

what happens if an incident ray is traveling towards the far side principal focus point of a concave lens

Answer 9

refract to travel in parallel to the axis

Question 10

how is a real image formed

Answer 10

light rays from a point on an object come together to form an image. The light rays pass through the same point. real images can be captured on a scree.

Question 11

how are virtual images formed

Answer 11

light rays from a point on an object are diverging after they have left the lens. so light from the point on the object appears to be coming from a different place. Rays don’t pass through that point, they just appear to.

Question 12

what 3 things do you have to say when you are describing an image

Answer 12

how big it is compared to the object
whether its upright or inverted
whether its real or inverted

Question 13

what is a ray diagram

Answer 13

a diagram that shows the path taken by light rays through a lens

Question 14

how are convex and concave lens shown on a ray diagram

Answer 14

convex is a double sided arrow

concave is a double sided arrow with the arrow head going the other way

Question 15

what is the first thing you need to do when drawing a ray diagram for convex

Answer 15

draw a line from the top of the object that passes through the middle of the lens without changing direction

Question 16

after you have drawn a line from the top of the object going through the middle of the lens what do you need to do in your ray diagram for convex

Answer 16

draw a second ray from the top of the object that travels parallel to the axis. The ray should refract at the lens so it passes through the principle focus point.

If the 2 rays don’t meet you need to extend the refracted rays back on the near side of the lens as virtual rays ( dotted lines)

Question 17

what are the 3ed and 4th rays you draw in a ray diagram for convex

Answer 17

you do the same as the first 2 rays but from the bottom of the object

Question 18

where do you draw your image on a ray diagram for convex

Answer 18

between the 2 meeting points of the four rays

if the bottom of your object is on the axis then the bottom of your image will be too

Question 19

what does the type of image produced by a convex lens depend on

Answer 19

where the object is placed in relation to the priciple focus point

Question 20

what is beyond 2F

Answer 20

an object beyond 2F will produce a real, inverted image that is is smaller than the object. The image will sit between f and 2f on the far side of the lens

the point 2F is the length of 2 principle focus points behind the lens

Question 21

what is at 2F

Answer 21

an object that is placed at 2F will produce a real inverted image that is the same size as the object. The image will sit at 2F on the far side to the lens

Question 22

Between F and 2F

Answer 22

an object placed between F and 2F will produce a real inverted image thats bigger than the object. It will sit beyond 2F on the far side of the lens

Question 23

between f and the lens

Answer 23

virtual image that is the right way up but bigger than the object. It will be on the same side of the lens

Question 24

how do you draw a ray diagram for a concave lens

Answer 24

draw ray from the top of the object that passes straight through centre of the lens. Then draw a ray from top of object parallel to axis. This ray should refract from the lens as if it has come straight from the principle focus.

draw a dotted ( virtual ray) from the focus to this point
repeat the process at the bottom of the object.

draw the image. The top is where the ray that passes through the middle of the lens meets the virtual ray going to the top. The same principle applies to the bottom of the object

Question 25

what are the properties of an image produced by a concave lens

Answer 25

always virtual, right way up, smaller than the object and on the same side of the lens as the object+

Question 26

what images do magnifying glasses create

Answer 26

virtual, upright image that is larger than object and on the same side of the lens as the object.

for this to happen the object must be closer to the lens than the focal length

Question 27

what is the equation for magnification

Answer 27

image height/ object height

no units cus magnification is a ratio