P3 Part A

Question 1

Why are X-rays used in hospitals?

Answer 1

They are high frequency, short wavelength electromagnetic waves) wavelength is roughly the same diameter of an atom. They are transmitted by healthy tissue but are absorbed by denser materials, such as bone and metal. They affect photographic material in the same way as light, which means they can be used to take photographs.

Question 2

What are X-ray photographs used for?

Answer 2

They can be used to diagnose medical conditions like bone fractures or dental problems.

Question 3

How are X-ray images formed?

Answer 3

X-ray images can be formed electronically using charge-coupled devices (CCDs). CCDs are silicon chops that are divided up into a grid of millions of identical pixels. CCDs detect X-rays and produce electronic signals that are used to form high resolution images.

Question 4

How does a CT (computerised axial tomography) scan work? (4 main stages)

Answer 4

1. Patient is put inside a cylindrical scanner and an X-ray beam is fired through the body from an X-ray tube and picked up by detectors on the opposite side.
2. The X-ray tube and detectors are rotated during the scan.
3. A computer interprets the signals from the detectors to form an image of a two dimensional slice through the body.
4. Multiple two dimensional CT scans can be put together to make a 3D image of the inside of the body.

Question 5

Why are X-rays used to treat cancer?

Answer 5

They cause ionisation, which means high does will kill living cells and can therefore be focused to kill cancer cells.

Question 6

Outline the 3 steps in using X-rays to treat cancer

Answer 6

1. The X rays are focused on the tumour using a wide beam
2. The beam is rotated around the patient with the tumour at the centre
3. This minimises the exposure of normal cells to radiation, and so reduces the chances of damaging the rest of the body

Question 7

What material is used to shield areas of a patient’s body that aren’t being treated/scanned by X-rays? What other precaution is taken?

Answer 7

Lead

Exposure time to X-rays is also kept to a minimum.

Question 8

What is ultrasound and how is it created?

Answer 8

Ultrasound is sound with a higher frequency than humans are able to hear.

Electrical systems can be made which produce electrial oscillations of any frequency. These can easily be converted into mechanical vibrations to produce a high frequency.

Question 9

When an ultrasound wave passes from one medium into another, some of the wave is reflected/transmitted/refracted off the boundary. What is this process called and how is it used?

Answer 9

This process is known as partial reflection. It means wherever there are boundaries between materials, ultrasound gets reflected back and the time it takes for these reflections to reach a detector can be used to measure how far away the boundary is.

This is how ultrasound imaging works.

Question 10

How does ultrasound break down kidney stones?

What are two benefits of this method?

Answer 10

Kidney stones are hard masses that block the urinary tract. An ultrasound beam concentrates high-energy waves at the kidney stone and turns it into sand-like particles that can pass out of the body in urine.

• Doesn’t need surgery and is relatively painless

Question 11

Other than breaking down kidney stones, how else can ultrasound be used in medicine?

Answer 11

In the pre-natal scanning of a foetus

The exact timing and distribution of ultrasound echoed back to the detector is processed by a computer to produce a video image.

Question 12

How can oscilloscope traces be used to find boundaries?

Answer 12

Given the ‘seconds per division’ setting of the oscilloscope, you can calculate the time between pulses by measuring on the screen. If you know the speed of sound in the medium you can work out the distance between boundaries using this formula:

distance (m) = speed (m/s) x time (s)

s = v x t

Question 13

Once you find the distance travelled by the ultrasound, what must you do?

Answer 13

The reflected pulses have travelled tehre and back so the distance you calculate willl be twice the distance between boundaries, meaning your answer to s = v x t must be divided by 2.

Question 14

Are ultrasound, X rays and CT scans safe?

Answer 14

Ultrasound is non-ionising so yes.

X-rays are ionising are ionising and can cause cancer if a person is exposed to a too high dosage.

CT scans use lots of X-ray radiation so they aren’t taken unless they are really needed because they can put the patient at risk.

Question 15

What is the picture quality of ultrasound, X-ray and CT imagery like?

Answer 15

Ultrasound is fuzzy. X-rays produce clear images of bones and metal but not much else. CT scans produce detailed images and can be used to diagnose complex illnesses/in the planning of complicated surgery.

Question 16

What is refraction?

Answer 16

Refraction is when waves change direction as they enter a different medium. This is caused by the change in density, which changes the speed of the waves. When waves slow down, they bend towards the ‘normal’.

Question 17

When light enters glass or plastic it slows down to about 2/3rds of it’s speed in air - true or false?

Answer 17

True

Question 18

When a ……. hits a boundary at ….. (ie. along the normal), it will not change direction but it will still ……. ……….

Answer 18

When a wave hits a boundary at 90° (ie. along the normal), it will not change direction but it will still slow down

Question 19

When light hits a ……. ……. (e.g. plastic or glass) some of the light will …… …….. the new medium but some of it will be …… - it all depends on the angle of ………..

Answer 19

When light hits a new medium (eg. plastic or glass) some of the light will pass through but some of it will be reflected - it all depends on the angle of incidence.

Question 20

Draw and label a diagram of light passing through a perspex block.

Give the equation for calculating the refractive index

Answer 20

refractive index (n) = sin i ÷ sin r

Question 21

What is the refractive index?

Answer 21

It is a ratio of speed of light in a vacuum to speed of light in that medium

Question 22

What is the refractive index of a substance if the angle of incidence for a light beam travelling through is 42° and the angle of refraction is 35°?

Answer 22

Refractive index = sin i ÷ sin r

= sin42 ÷ sin35

= 1.18

Question 23

How do lenses form images and what are the two main types of lens?

Answer 23

Lenses form images by refracting light and changing its direction. The two main types are converging and diverging.

Question 24

Describe a converge lens and what it does to rays of light. Where is a converge lens’ principal focus?

Answer 24

A converging lens is convex - it bulges outwards and causes parallel rays of light to converge (move together) at the principal focus.

The principal focus is where rays hitting the lens parallel to the axis all meet.

Question 25

Describe a diverging lens and what it does to rays of light. Where is a diverging lens’ principal focus?

Answer 25

A diverging lens is concave - it caves inwards and caused parallel rays of light to diverge (spread out).

The principal focus is the point where rays hitting the lens parallel to the axis appear to all come from you can trace them all back until they meet up at a point behind the lens.

Question 26

Where is the axis of a lens?

Answer 26

The axis is a line passing through the middle of the lens

Question 27

Draw and label diagrams of converging and diverging lenses.

Answer 27

Question 28

What is the difference between a ‘real’ image and a ‘virtual’ image?

Answer 28

A real image is where light from an object comes together to form an image on a ‘screen’’ - like the image formed on an eye’s retina.

A virtual image is when the rays are diverging, so the light from the object appears to be coming from a totally different place. When you look in a mirror, you see a virtual image of your face (because you appear to be behind the mirror).

Question 29

What three factors must you consider when describing an image

Answer 29

1. How big is it compared to the object
2. Whether it’s upright or inverted
3. Whether it’s real or virtual

Question 30

What are the three rules for refraction in a converging lens

Answer 30

1. An incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side.
2. An incident ray passing through the principal focus refracts through the lens and travels parallel to the axis.
3. An incident ray passing through the centre of the lens carries on in the same direction.

Question 31

What are the three rules for refraction in a diverging lens?

Answer 31

1. An incident ray parallel to the axis refracts through the lens and travels in line with the principal focus (so it appears to have come from there)
2. An incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis.
3. An incident ray passing through the centre of the lens carries on in the same direction.

Question 32

What type of lens do magnifying glasses use, and what type of image do they produce?

Answer 32

Magnifying lenses use a converging lens and create a virtual image, the light rays don’t actually come from the place where the image appears to be.

Question 33

What is the magnification formula?

Answer 33

Magnification = image height ÷ object height

Question 34

How is focal length related to the power of a lens?

What is the unit used for power of a lens?

Answer 34

The more powerful the lens, the more strongly it converges rays of light, so the shorter the focal length.

Dioptres is the unit used

Question 35

What is the formula for working out the power of a lens?

Answer 35

Power (dioptres, D) = 1 ÷ focal length

Question 36

What two factors determine focal length?

Answer 36

1. the refractive index of the lens material
2. the curvature of the two surfaces of the lens

Question 37

Draw and label a diagram of a human eye

Answer 37

Question 38

What is the purpose of the cornea?

Answer 38

The cornea is a transparent ‘window’ with a convex shape, and a high refractive index. It does most of the eye’s focusing.

Question 39

What is the iris and what is it’s purpose?

Answer 39

The iris is the coloured part of the eye. It’s made up of muscles that control the size of the pupil - the hole in the middle of the iris. This controls the intensity of light entering the eye.

Question 40

How does the lens in the eye work?

Answer 40

It changes shape to focus light from objects at varying distances. It’s connected to the ciliary muscles by the suspensory ligaments and when the ciliary muscles contract, tension is released and the lens takes on a fat, more spherical shape. When they relax, the suspensory ligaments pull the lens into a flatter, thinner shape.

Question 41

What happens in the retina?

Answer 41

Images are formed on the retina, which is covered in light sensitive cells. These cells detect light and send signals to the brain to be interpreted.

Question 42

When you take a photograph, light from the subject travels to the camera and is refracted by the lens, forming an image on the film. Give 3 features of images taken by a camera.

Answer 42

• The image on the film is a real image because light rays actually meet there
• The image is smaller than the object, because the object is a lot further away than the focal length of the lense.
• The image is inverted.

Question 43

What is the far point and near point of an eye?

Answer 43

The far point is the furthest distance that the eye can focus on comfortably, the near point is the closest distance that the eye can focus on. In adults, the near point tends to be about 25cm.

Question 44

What is short sightedness caused by and how can it be corrected?

Answer 44

Short sighted people can’t focus on distant objects because their eyeball is too long/ the cornea and lens system are too powerful.

A diverging lens can be put in front of the eye, this diverges light before it enters the eye, which means the lens can focus it on the retina.

Question 45

What is long sight caused by and how can it be corrected?

Answer 45

Long sight happens when the cornea and lens are too weak or the eyeball is too short, meaning the images of near objects are brought into focus behind the retina.

This can be corrected by a converging lens with a positive power. Light is refracted and states to converge before it enters the eye and the image can be focused on the retina, where it belongs.

Question 46

How do lasers surgically correct eye problems?

Answer 46

A laser can be used to vaporise some of the cornea to change its shape, and therefore its focusing ability/power.

Question 47

How is light sent along optical fibres?

Answer 47

By total internal reflection.

The waves bounce off the sides of a thin inner core of glass/plastic. (Depends on whether the angle of incidence is bigger than the critical angle)

Question 48

What happens when the angle of incidence is:

a) less than the critical angle
b) equal to the critical angle
c) greater than the critical angle?

Answer 48

a) Most of the light passes out but a little bit is internally reflected.
b) The emerging ray comes out along the surface, there’s quite a bit of internal reflection.
c) No light comes out, it’s all internally reflected (total internal reflection).

Question 49

If a material has a ……. refractive index, it will totally internally reflect more - more …. will be incident at an angle bigger than the critical angle.

Answer 49

If a material has a high refractive index, it will totally internally reflect more - more light will be incident at an angle bigger than the critical angle.

Question 50

How do endoscopes work?

Give one an advantage of this technology

Answer 50

Endoscopes contain two bundles of optical fibres, one to carry light to the area of interest and one to carry an image back so it can be viewed.

• doctors can now perform operations by only cutting tiny holes (keyhole surgery)