Option C: Optics Flashcards
What is meant by chromatic aberration? Explain how it is formed. (3)
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)
Describe how the effects of chromatic aberration may be reduced. (1)
Use a combination of lenses to reduce the effect.
Outline why reducing the size of the aperture will reduce the effects of spherical aberration. (2)
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)
Define principal axis. (1)
Principal axis is the line joining the two foci of a lens.
Define focal point.
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)
Define the terms near point and far point. (2)
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.
Explain why gel is applied when using ultrasound scanning. [2]
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]
Outline the advantage of having the image positioned at the near point of the eye. (1)
It gives the maximum angular magnification without straining the eye.
Define acoustic impedance of a medium. [1]
The product of the speed of sound and the density of the medium
Derive the angular magnification of a magnifying glass when it is in normal use. (3)
https://drive.google.com/file/d/16EZEIANPPkXzG4zU4Ib_iRK_zBUcHQU9/view?usp=sharing
(For normal use, angular magnification = linear magnification!)
Derive the angular magnification of telescope when it is in normal adjustment
https://drive.google.com/file/d/1XPM4TzUVBAflwEYK8UOduweleyMysqtA/view?usp=sharing
Derive the angular magnification of a microscope when it is in normal adjustment.
https://drive.google.com/file/d/1cPUh8p7T7xaslnbEbgoJs9K3CBZHi_m5/view?usp=sharing
Define the angular magnification of a magnifying glass.
Angular magnification is the ratio of angle subtended by image at eye to the angle subtended by object at the near point.
Outline how ultrasound is generated for medical diagnostic purposes. [2]
When an a.c. voltage is applied to a piezoelectric crystal, [1] the crystal changes its shape (or vibrate) to emit ultrasound. [1]
State two differences between ultrasound A-scan and B-scan. [3]
- 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] - 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.
State one advantage and one disadvantage of using ultrasound of frequency 1 MHz, rather than 3 MHz, in medical diagnosis. [2]
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
Define attenuation coefficient. [1]
The probability of a (X-ray) photon to be absorbed in 1 m of the medium
Suggest why an X-ray scan does not allow for the differentiation between muscle and blood. [2]
The difference in densities between muscle and blood is too small. [1] It cannot produce X-ray images with sufficient contrast. [1]
State two advantages of NMR imaging over computed tomography (CT) imaging. [2]
- NMR imaging does not produce ionizing radiation.
- NMR produces images with higher quality / contrast.
Explain how fluorescent emitters are used to enhance the image formed on a photographic X-ray plate. [3]
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]
State what is meant by coherence of two light waves. (2)
Two light waves are coherent when they have a constant phase difference (1) and the same frequency (2) .
State what is meant by attenuation.
Attenuation is the loss of intensity (or power) in the transmission of a signal through a medium.
State two causes of attenuation and two causes of dispersion in an optical fibre.
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)
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]
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]