Topic 5 - Waves Pt2

Question 1

What’s an interface?

Answer 1

A boundary between 2 different media

Question 2

Explain what happens to waves at interfaces?

Answer 2

• When a wave passes from 1 medium to another, some of its energy is reflected & some is transmitted
• the proportion of energy reflected or transmitted depends on the 2 media involved
• if they have very different densities most of the energy is reflected
• if they have quite similar densities most of the energy is transmitted

Question 3

What’s refraction?

Answer 3

• It’s when a wave changes direction as it enters a different medium
• This is a result of the wave slowing down or speeding up (you can tell which of the two it is if the wave bends towards or away from the normal)
• the speed changes because the wavelengths the wave is changing & the frequency stays constant ( v = f Y)

Question 4

What does it mean if the light Ray bends towards the normal?

Answer 4

• it’s slowing down ( wavelength decreased frequency stays the same)
• the Ray is going from a less optically dense material to a more optically dense material

Question 5

What does it mean if the light Ray bends away the normal?

Answer 5

• the wave is speeding up ( wavelength increases frequency stays the same)
• it’s going from an optically denser material to a less optically dense material

Question 6

What’s the refractive index?

Answer 6

• it measures how much the material slows down light
• light goes fastest in a vacuum. It slows down in other materials because it interacts with the particles in them, the more optically dense the more light slows down

Question 7

What’s the refractive index of air?

Answer 7

1

Question 8

How would you find the refractive index of a material?

Answer 8

n = c / v
n = refractive index (ratio so no units)
c = speed of light in a vacuum (c = 3 x 10*8 m/s)
v = speed of light in that material

Question 9

Explain snells law

Answer 9

n1 sin ø1 = n2 sin ø2

• ø1 = is the angle the incoming light makes to the normal, angle of incidence
• ø2 = is the angle the refracted ray makes with the normal, angle of refraction
• the light is crossing a boundary, going from a medium with refractive index n1 to a medium with refractive index n2

Question 10

What happens when the angle of refraction is a right angle?

Answer 10

• the angle of incidence is critical
• when ø1 = critical angle ø2 = 90 degrees and light is refracted along the boundary
• it happens to light going from a dense material to a less dense material

Question 11

Snells law can simplify to what?

Answer 11

Sin C = 1 / n
n air =1 and sin(90) =1
n is the refractive index of the material

Question 12

What’s total internal reflection?

Answer 12

At angles of incidence greater than C, refraction is impossible, so all the light is reflected back into the material

Question 13

What’s the focal length of a converging lens?

Answer 13

• It’s the distance between the lens axis and focal plane

- f is positive for a converging lens because it’s in front of the lens

Question 14

What’s the focal length of a diverging lens?

Answer 14

• it’s the distance between the labs axis & the principal focus
• f is negative for a diverging lens because it’s behind the lens

Question 15

Explain the properties of a converging lens?

Answer 15

• lens change the direction of light rays by refraction
• converging lenses bulge outwards
• rays parallel to the principal axis of the lens converge onto a point called the principal focus

Question 16

Explain the properties of a diverging lens?

Answer 16

• diverging lenses cave inwards
• the principal focus of a diverging lens is at a point behind the lens
• the principal focus is the point that rays from a distant object, parallel to the profila la axis appear to come from

Question 17

What’s the power of a lens?

What equation can you use to find the power of a lens?

Answer 17

-power tells you the lens ability to bend light. The higher the power, the more the lens will refract light
- p = 1/f
f= focal length (m)
P= power units: dioptres (D)

Question 18

Explain what happens to the power when you have lots of thin lens’ in a line with their principal axis Lined up?

Answer 18

If the lenses are touching or are very close the total power of the lenses is found by just adding all the powers together
P = p1+ p2 + p3

Question 19

What are the 2 ways to calculate magnification?

Answer 19

M = image height / object height
M= image distance / object distance (M = v/u)
Magnification has no units as it’s a ratio

Question 20

What’s a real image?

Answer 20

• it’s when light rays from a point on an object are made to pass through another point in space
• the image can be captured on a screen

Question 21

What’s a virtual image?

Answer 21

• it’s formed when light rays from a point on an object appear to have come from another point in space.
• the light lights rays aren’t actually where the image appears to be
• so it can’t be captured on a screen

Question 22

What kind of image does a converging lens form?

Answer 22

• both real and virtual images, depending where the object is
• if the object is further than the focal length away from the lens, the Image is real
• if the objects closer, the image is virtual

Question 23

What kind of image does a diverging lens form?

Answer 23

It will always form a virtual image - the position of the object in relation to the focal length will not affect the type of image formed

Question 24

What’s the thin lens equation?

Answer 24

1/f = 1/u + 1/v
f = focal length 
U = object distance 
V = image distance

Question 25

Describe an experiment to measure the refractive index of a glass block

Answer 25

1. draw round the block on a piece of paper 2. shine a beam of light from a Ray box into the glass box 3. trace the path of the incoming & outgoing beams on either side of the block 4. Remove block & join up lines 5. Mesure angles of incidence & refraction where light enters & exits 6. Rearrange snells law to make the refractive index of the material the subject and substitute n = 1 for air and the value of ø1 & ø2 (Use a narrower beam to be more precise as the uncertainty in the position of the beam will be lower)

Question 26

What’s ultrasound?

Answer 26

- any sound wave produced above the human range of hearing (20 000 Hz) - it can used to measure distances, because some of it is reflected when there’s a change in medium - bats & dolphins send out ultrasound & wait for the echo to navigate in the dark

Question 27

Why is it better to use pulses than continuous sound waves?

Answer 27

To stop the reflected wave interfering with the original wave

Question 28

What are the 3 rules for drawing Ray diagrams?

Answer 28

1. If parallel to the principal focus it’s refracted through the focus 2. A Ray to the centre of the lens passes through undeflected 3. A Ray through the focus if refracted parallel to the axis

Question 29

What are 3 ways to describe an image?

Answer 29

1. Magnified or diminished 2. Upright or inverted 3. Real or virtual

Question 30

What kind of defect of vision would be corrected using a diverging lens?

Answer 30

Shortsightedness