Chapter 1 - Imaging

Question 1

Define wave-front.

Answer 1

A wave-front is an imaginery line or surface that moves along with a wave. All points on the same wave-front have the same phase, and the distance between two wave-fronts is a wave length. E.g. crest of a ripple in water.

Question 2

What is the curvature of a circle with radius r?

Answer 2

1/r

Question 3

Define plane wave-front.

Answer 3

A wave-front that has zero curvature because it is so distant from its source. This happens because as r –> infinity, 1/r –> 0.

Question 4

Describe the connection between light as a wave-front and light as a ray.

Answer 4

A ray of light points along the direction of motion of the wave-front and is always at right-angles to the wave-front.

Question 5

What is a converging lens?

Answer 5

A lens that focuses light on a point behind it. It does this by ‘adding curvature to the waves’ or ‘bending the rays,’ bringing them to a focus. E.g. burning glass.

Question 6

Define focal length.

Answer 6

The distance between the centre of the lens and the focal point.

Question 7

Define focal point (focus).

Answer 7

The point where light from a distant object on the axis of the lens is brought to focus by the lens.

Question 8

What is the equation for Lens power?

Answer 8

Lens power (D/dioptres) = 1/focal length f (m)

Question 9

What is the rule for how a lens shapes light? (word form)

Answer 9

Curvature of waves leaving the lens = curvature of waves entering the lens + curvature added by the lens

Question 10

What is the lens equation?

Answer 10

1/v (image distance) = -1/u (object distance) + 1/f

Question 11

Lenses can also be used to magnify the image of an object. Define magnify.

Answer 11

When the size of the image appears larger than the original object.

Question 12

What is the equation for linear magnification in terms of height?

Answer 12

linear magnification (m) = image height (m) / object height (m)

Question 13

What is the equation for linear magnification in terms of distance?

Answer 13

linear magnification (m) = image distance v (m) / object distance u (m)

Question 14

What is a CCD?

Answer 14

A charged-coupled device is a light-sensitive microchip often found in a digital camera. It is a screen covered by millions of pixels. An image from this becomes an array of numbers, which can then be manipulated to edit the image.

Question 15

Define pixel.

Answer 15

A single ‘picture element’. In a digital camera, a lens is used to form a real image on a chip with an array of the order of a million very small light-sensitive detectors. Each detector corresponds to one pixel in the final image.

Question 16

Explain a binary system.

Answer 16

Where numbers in computers, such as numbers representing the image in a digital camera, are stored as on or off values. the two values can be thought of as two digits, 1 and 0 respectively.

Question 17

Define a bit.

Answer 17

A binary digit is the smallest unit of digital information, represented as a 0 or a 1 corresponding to low voltage or high voltage in a digital circuit.

Question 18

Define a byte.

Answer 18

A byte is a sequence of eight bits coded to represent one of 256 alternatives.

Question 19

What is the equation for the number of arrangements of bits N?

Answer 19

N = 2^b or b = log 2 (N), where b is the number of bits available.

Question 20

What is the equation for the amount of information in an image?

Answer 20

Amount of information in an image = no. of pixels * bits per pixel

Question 21

Define resolution.

Answer 21

The scale of the smallest detail that can be distinguished.

Question 22

What is the equation for resolution?

Answer 22

Resolution = width of the object in the image / number of pixels across the object

Question 23

What is image processing and what are the four types?

Answer 23

When a value of a bit in a pixel is changed to change the appearance of the image. This can be done by Changing brightness, Removing noise, Edge detection, and Changing contrast.

Question 24

Describe each form of image processing.

Answer 24

‘Refer to pg 28 of textbook’

Question 25

What equation links wave speed, wavelength and frequency?

Answer 25

speed v (ms^-1) = frequency f (Hz) * wavelength (m)

Question 26

Define time period.

Answer 26

Time period T is the time taken for a wave to complete one full oscillation.

Question 27

What is the equation for time period?

Answer 27

Time period T = 1/frequency f

Question 28

Name all the waves in the electromagnetic spectrum, from largest wavelength to smallest.

Answer 28

Radio waves, microwaves, infrared, visible, ultraviolet, x-rays, and gamma rays.

Question 29

What is a transverse wave?

Answer 29

A moving wave that is made up of oscillations happening perpendicular to the direction of energy transfer.

Question 30

Define polarisation.

Answer 30

Transverse waves are linearly polarised if they vibrate in one plane only. Unpolarised transverse waves vibrate in a randomly changing plane. Longitudinal waves can’t be polarised.

Question 31

What are some examples of real life application of polarisation?

Answer 31

1. Some sunglasses and snow goggles are polarised to dramatically cut down glare or bright days which can help people see the ground curvatures etc. more clearly.
2. Microwaves between a transmitter and a receiver can be polarised using a metal grille.