P3.1.3 Lenses

Question 1

Define refraction.

Answer 1

Refraction is the change of direction of light as it passes from one medium to another with a change in density.

Question 2

What happens to light when it travels from a less dense medium into a denser medium?

Answer 2

It slows down and bends towards the normal.

Angle of incidence > Angle of refraction

Question 3

What happens to light when it travels from a more dense medium into a less dense medium?

Answer 3

It speeds up and bends away from the normal.

Angle of incidence < Angle of refraction

Question 4

What is the normal?

Answer 4

An imaginary line perpendicular to the surface of the medium / the boundary of the two mediums.

Question 5

What does the amount of refraction depend on?

Answer 5

The angle at which the light hits the boundary between the substances.

The difference in relative densities, between the two media.

Question 6

What is a refractive index and how do you calculate it?

Answer 6

The degree to which a material slows the speed of light compared to its speed in a vacuum.

sin( i )
_____ sin(i)/sin(r)
sin ( r )

Question 7

What is the angle of incidence ( i )?

Answer 7

The angle between the normal and the light ray traveling into the medium.

Question 8

What is the angle of refraction ( r )?

Answer 8

The angle between the normal and the refracted light ray in the new medium.

Question 9

Name 2 types of lenses.

Answer 9

Converging lens

Diverging lens

Question 10

Define a lens.

Answer 10

A transparent block that causes light to refract to form an image.

Question 11

Describe a converging lens.

Answer 11

A converging lens is curved outwards on both sides.

/ \
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Question 12

Describe a diverging lens.

Answer 12

A diverging lens is curved inwards on both sides.

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/ \

Question 13

How does a converging lens work?

Answer 13

Rays from a single point on a distant object arrive at the lens parallel to one another. Converging lenses refract these parallel rays so that they are come together at a point called the principal focus.

Question 14

How does a diverging lens work?

Answer 14

Diverging lenses refract the parallel rays of light so that they spread apart from one another. This means that they form a virtual image which appears to be behind the lens and that cannot be projected onto a screen.

Question 15

Define the focal length.

Answer 15

The distance between the centre of the lens and the image.

Question 16

What is the principal axis?

Answer 16

An imaginary line drawn through the centre of the lens at 90° to it in a ray diagram.

Question 17

How do you describe an image that is larger than the real object?

Answer 17

Magnified.

Question 18

How do you describe an image that is smaller than the real object?

Answer 18

Diminished.

Question 19

How do you describe an image that is upside down compared to the real object?

Answer 19

Inverted

Question 20

What type of image is created by a converging lens?

Answer 20

Real.

Question 21

What type of image is created by a diverging lens?

Answer 21

Virtual.

Question 22

How do you calculate magnification?

Answer 22

Magnification = Image height / object height.

Image height = Magnification x object height
Object height = Image height / magnification

Question 23

Describe how a ray diagram is drawn for a diverging lens.

Answer 23

A ray parallel to the principal axis - refracted away from the principal axis in line with the focal point on the other side of the lens.

A ray directed at the principal focus after the lens - refracted to run parallel to the principal focus.

A ray travelling through the centre of the lens - continues in the same direction.

The rays leaving the lens are traced backwards in straight lines until they reach a point at which they cross. This is where the virtual image appears to come from.

Question 24

Describe how a ray diagram is drawn for a converging lens.

Answer 24

A ray parallel to the principle axis - refracted through the focal point on the other side.

A ray travelling through the focal point on the way to the lens - refracted to travel parallel to the principal axis.

A ray travelling through the centre of the lens - continues in the same direction.

Question 25

How do you calculate optical power?

Answer 25

1 / focal length