Option C: Optics

Question 1

What is meant by chromatic aberration? Explain how it is formed. (3)

Answer 1

Image formed by a convex lens is multi-colored. (1)
Different coloured light has different wavelengths (or frequencies) so that the refractive indices in the glass are also different. (1)
As a result, their focal points on the principal axis are also different. (1)

Question 2

Describe how the effects of chromatic aberration may be reduced. (1)

Answer 2

Use a combination of lenses to reduce the effect.

Question 3

Outline why reducing the size of the aperture will reduce the effects of spherical aberration. (2)

Answer 3

Since the light rays passing through the edge and near the centre of the lens converge at different focal points, (1)
If the size of the aperture is reduced, the light rays become closer to one focus (or more concentrated) . (1)

Question 4

Define principal axis. (1)

Answer 4

Principal axis is the line joining the two foci of a lens.

Question 5

Define focal point.

Answer 5

Focal point is the point on the principal axis (1) that
light rays parallel to the principal axis passing through a convex lens will converge at. (1)

Question 6

Define the terms near point and far point. (2)

Answer 6

Near point: Position of nearest object that an unaided eye can focus clearly on

Far point: Position of furthest object that an unaided eye can focus clearly on.

Question 7

Explain why gel is applied when using ultrasound scanning. [2]

Answer 7

The impedance of the gel is similar to that of the skin/body tissue. [1]
(or the gel replaces the layer of air which has impedance smaller than that of the skin/body tissue.)
More ultrasound can penetrate through the skin. [1]

Question 8

Outline the advantage of having the image positioned at the near point of the eye. (1)

Answer 8

It gives the maximum angular magnification without straining the eye.

Question 9

Define acoustic impedance of a medium. [1]

Answer 9

The product of the speed of sound and the density of the medium

Question 10

Derive the angular magnification of a magnifying glass when it is in normal use. (3)

Answer 10

https://drive.google.com/file/d/16EZEIANPPkXzG4zU4Ib_iRK_zBUcHQU9/view?usp=sharing
(For normal use, angular magnification = linear magnification!)

Question 11

Derive the angular magnification of telescope when it is in normal adjustment

Answer 11

https://drive.google.com/file/d/1XPM4TzUVBAflwEYK8UOduweleyMysqtA/view?usp=sharing

Question 12

Derive the angular magnification of a microscope when it is in normal adjustment.

Answer 12

https://drive.google.com/file/d/1cPUh8p7T7xaslnbEbgoJs9K3CBZHi_m5/view?usp=sharing

Question 13

Define the angular magnification of a magnifying glass.

Answer 13

Angular magnification is the ratio of angle subtended by image at eye to the angle subtended by object at the near point.

Question 14

Outline how ultrasound is generated for medical diagnostic purposes. [2]

Answer 14

When an a.c. voltage is applied to a piezoelectric crystal, [1] the crystal changes its shape (or vibrate) to emit ultrasound. [1]

Question 15

State two differences between ultrasound A-scan and B-scan. [3]

Answer 15

1. A-scan only shows the amplitude of intensity of the ultrasound against time. [1]
   B-scan shows a 2-D image of cross section through a patient. [1]
2. The ultrasound probe of A-scan is fixed in position while that of B-scan is moved by the operator [1]
   and the computer records the echo.

Question 16

State one advantage and one disadvantage of using ultrasound of frequency 1 MHz, rather than 3 MHz, in medical diagnosis. [2]

Answer 16

Advantage: Using ultrasound of lower frequency (1MHz) produces scans of greater penetration / depth
Disadvantage: Using ultrasound of lower frequency (1MHz) produces scans of lower resolution

Question 17

Define attenuation coefficient. [1]

Answer 17

The probability of a (X-ray) photon to be absorbed in 1 m of the medium

Question 18

Suggest why an X-ray scan does not allow for the differentiation between muscle and blood. [2]

Answer 18

The difference in densities between muscle and blood is too small. [1] It cannot produce X-ray images with sufficient contrast. [1]

Question 19

State two advantages of NMR imaging over computed tomography (CT) imaging. [2]

Answer 19

1. NMR imaging does not produce ionizing radiation.
2. NMR produces images with higher quality / contrast.

Question 20

Explain how fluorescent emitters are used to enhance the image formed on a photographic X-ray plate. [3]

Answer 20

Fluorescent emitters are placed on each side of the photographic X-ray film. [1]
After the fluorescent emitters absorb the X-rays, it emits visible light which causes additional exposure. [1]
It produces better images with higher contrast. [1]

Question 21

State what is meant by coherence of two light waves. (2)

Answer 21

Two light waves are coherent when they have a constant phase difference (1) and the same frequency (2) .

Question 22

State what is meant by attenuation.

Answer 22

Attenuation is the loss of intensity (or power) in the transmission of a signal through a medium.

Question 23

State two causes of attenuation and two causes of dispersion in an optical fibre.

Answer 23

Attenuation:
- Impurities in glass core
- Energy of transmission signal is scattered by the glass

Causes of dispersion:
- Light of of different wavelengths have different refractive indices , they refract to different degrees.
- Light with the same wavelength but different-order modes travel in different paths, reaching the end of the fibre at different times

(abagi註: textbk writes “different-order modes” instead of “different modes”, so i followed textbk)

Question 24

Barium has an attenuation coefficient that is much larger than that for human tissue. Explain why a patient is asked to drink a liquid barium meal to help produce an X-ray image of the digestive system. [3]

Answer 24

Stomach tissue has similar attenuation coefficient to surrounding tissue. [1]
The difference in contrast between stomach and the surrounding tissue is not great enough to produce a clear image.
Since barium absorbs X-rays well, [1] it allows stomach walls to show contrast clearly after drinking barium meal. [1]

Question 25

State three advantages of optic fibres over coaxial cables for communication. (3)

Answer 25

1. The data transfer rate through optical fibres is higher.
2. The signal travels at longer distance without amplification.
3. It is more difficult to interfere the signal in optical fibres.

Question 26

Communication signals are transmitted through optic fibres using infrared radiation. Suggest why infrared radiation rather than visible light is used in these transmissions. (1)

Answer 26

Infrared radiation suffers lower attenuation

Question 27

State and explain why it is an advantage for the core of an optic fibre to be extremely thin. (1)

Answer 27

In a thin core, rays of different signals travel along (almost) the same path so waveguide dispersion can be reduced.

Question 28

State the main physical difference between step-index and graded-index fibres. (1)

Answer 28

The refractive index of the core of step-index fibres is constant while that of graded-index fibres decreases away from the axis.

Question 29

Explain how the graded-index optic fibre reduces waveguide dispersion. (4)

Answer 29

Waveguide dispersion occurs when rays not parallel to the central axis take longer to transmit. (1)
In a graded-index fibre, the refractive index of the outer core is smaller than that near the centre of core. (1)
Hence, rays travel faster at the outer core. (1)
Rays travelling along different path arrive at nearly the same time. (1)

Question 30

Light travels from one optical medium to another. State the conditions necessary for total internal reflection to occur at the boundary between the two media. (2)

Answer 30

Light must travel from optically denser medium to optically less dense medium. (1)
The angle of incidence must be greater than the critical angle. (1)

Question 31

State and explain why lasers are used as light sources for optical fibres. (2)

Answer 31

Laser is monochromatic so that all light has the same speed in optical fibres. (1)
Laser can be switched very rapidly so more information can be carried. (1)

Question 32

Outline the 3 advantages of using parabolic mirrors over spherical mirrors. (3)

Answer 32

Parabolic mirrors do not suffer from spherical aberration but spherical mirrors do. (1)
Parabolic mirrors reflect parallel light rays through one point (1), but spherical mirrors reflect parallel light rays through different points. (1)

Question 33

Outline why X-rays are not suitable to scan an organ such as liver. [2]

Answer 33

Attenuation coefficients of liver and the surrounding tissue are similar. (1)
The difference in contrast between tissue and liver is not great enough to show a clear image. (1)

Question 34

Define half-value thickness

Answer 34

It is the distance travelled by an X-ray such that its intensity reduces to the half of the incident intensity.

Question 35

Outline why X-ray dose needs to be kept to a minimum. (2)

Answer 35

X-ray causes ionizing radiation (1) which
could cause cell damage / increase risk of cancer. (1)

Question 36

Describe one possible enhancement technique for X-rays. (2)

Answer 36

An fluorescent emitters are placed on each side of the photographic film. (1)
After X-ray is absorbed by the screen, visible light is emitted which enhances the contrast of the X-ray image. (1)

Question 37

Discuss one extra procedure to get an appropriate image of the intestine. (2)

Answer 37

Take a barium meal (1) to increase the contrast between inside and outside the intestine. (1)

Question 38

Outline how an image is produced using computed tomography (CT). (5)

Answer 38

When a patient is moved into the CT scanner,
X-ray images of cross section are taken (1)
at many different angles. (1)
A series of successive images (1)
are combined by computer (1)
to form a 3-D image of the patient. (1)

Question 39

Compare standard X-ray imaging to computed tomography (CT). (3)

Answer 39

X-ray produces 2-D images but CT produces 3-D images. (1)
X-ray image is produced by a single exposure of X-ray but CT image is produced by many exposures from different angles. (1)

The time of taking X-ray images is much shorter than that of CT scans. Patients are under some dose of radiation when taking an X-ray image but that is much greater when taking a CT scan. (1)

Question 40

Outline the physical principles of NMR imaging. (5)

Answer 40

When a magnetic field is applied to a patient,
the spin of the protons in the patient’s body (1)
aligns to the external magnetic field. (1)
When the proton returns to the spin-up/lower energy state, a radio frequency pulse is emitted. (1)
The frequency depends on the energy difference between the spin-up and spin down states. (1)
The position of the proton is located by detecting the emission of the RF pulse. (1)

Question 41

Explain the role of these the uniform magnetic field and the gradient field in nuclear magnetic resonance imaging.

Answer 41

The uniform field aligns the magnetic moment of the protons in the patient’s body. (1)
Radio frequency pulse is emitted when the proton returns to the lower energy state.
The frequency depends on the external magnetic field. (1)
The gradient field superimposes with the uniform field so that the resultant magnetic field at every point in the body is different. (1)
Since different parts of the body correspond to different frequencies, the emission of the RF pulse can be located. (1)

Question 42

State one advantage and one disadvantage of using ultrasound imaging in medicine compared to using X-ray imaging. [2]

Answer 42

Advantage: Ultrasound imaging does not produce ionizing radiation
Other possible answers:
- relatively harmless
- can be performed in a doctor’s office
- can be used to measure blood flow rate
- video imagine possible, e.g. heart, foetus

Disadvantage:
- Limited resolution
- Difficulty in imaging lungs or gastrointestinal system
- Difficulty in imaging any body part with a gas in it

Source: M19 TZ2 HL P3 Q17ai

Question 43

In nuclear magnetic resonance imaging (NMR) a patient is exposed to a strong external magnetic field so that the spin of the protons in the body align parallel or antiparallel to the magnetic field. A pulse of a radio frequency (RF) electromagnetic wave is then directed at the patient.
(a) Describe the effect of the RF signal on the protons in the body.
(b) Outline the measurement that needs to be made after the RF signal is turned off.
(c) Describe how the measurement in (b) provides diagnostic information for the doctor.

Answer 43

(a) Proton spin direction/energy state changes
(b) Relaxation time «of signal/proton spin»
& Location/time delay of the emitted RF signal
(c) Different types of tissue show different relaxation times, thus relaxation time gives information on identifying tissue type/density/health.
Information about the locations of the emitted RF signals helps produce (a series of photon patterns that can be combined, as with CAT, into) a three-dimensional computer image.

Question 44

Explain why optical fibres have a higher data transfer rate than coaxial cables. [2]

Answer 44

Fibres have broader bandwidth than cables so they can carry multiple signals simultaneously.

Question 45

State three disadvantages of earthbounde telescopes over satellite-borne telescopes.

Answer 45

• Light pollution (excess light in the atmosphere). This can be partly overcome by locating telescopes in remote areas, far from large cities.
• Atmospheric turbulence (mainly due to convection currents and temperature diff erences). This makes air move unpredictably, making the positions of stars appear to vary. It can be partly overcome by locating telescopes on high mountains, where the atmosphere tends to
  be more stable.
• Absorption of various wavelengths by the atmosphere. This makes observation at these wavelengths impossible. This is especially true for X-ray and ultraviolet wavelengths, which are almost completely absorbed by the atmosphere.
  These problems do not exist for satellite-based telescopes in orbit around the Earth.