# 1: Imaging_checked Flashcards

Define Frequency

The number of whole vibrations (or oscillations), f, passing a given point each second.

Frequency is measured in Hertz, Hz

The frequency is the inverse of the period

f = 1/T

Define Period

The time, T, taken for a whole vibration (oscillation)

Unit of Period is ‘second’, s

The frequency is the inverse of the period

f = 1/T

Define Phase Difference

The amount by which one wave lags behind another wave.

Phase difference is measured in degrees or radians

What do lenses do?

**Lenses** change the curvature of the incident wavefront by **refraction.**

A lens **adds curvature** to waves as they pass through it. If waves are uncurved before passing through the lens, and parallel to the lens axis, they will be given spherical curvature, centred on the **focus (or focal point)** of the lens.

Explain how a converging lens curves wavefronts by changing the speed of the wave

A converging lens curves the wavefronts by **slowing down** the part of the wave (or light) travelling through the middle of the lens more than parts of wave (or light) at the lens edges.

All points on a wavefront take the **same amount of time** to get to the focus point.

The **more powerful** (thicker) the lens, the more it will **curve** the wavefronts that travel through it - so the **shorter** its **focal length**

What is the focal length?

The focal length, f, is the distance between the **lens axis** and the **focus**

What is the lens power equation (D)?

lens power = 1 / f (focal length in metres)

Lens power is measured in Dioptres

What is the curvature of wave equation

**Curvature = 1 / r**

**(radius or distance in metres)**

What is the lensmaker equation words and symbols?

The lens equation states: curvature of waves leaving lens = curvature of waves before entering lens + curvature added by lens

**1/v = 1/u + 1/f**

- v = image distance (lens to image formed)
- u = object distance (lens to source)
- f = focal length

why is object distance (1/u) negative

You always measure **distance**s from the **lens axis**, and count distances to the **right as poitive**; and distances to the **left as negative**

(Just like drawing graphs)

Describe the wavefronts of a distant light source. What curvature will a converging lens give them

If you’ve got a **distant light source**, the wavefronts approaching the converging lens will be **flat (1/u = 0)**

The converging lens will then give them a curvature of 1/f

Describe the wavefronts if the source is at the focus of the lens

If the source is at the focus of the lens, the wavefronts will start off **curved** w/ **negative curvature.**

This is because u is measured as a negative distance.

This -ve curvature is then cancelled out by the +ve curvature added by the converging lens - so the wavefronts will be made **flat**

What are the linear magnification equations for height and distance

linear magnification =

image height (m) / object height (m)

image distance (m) / object distance (m)

v/u

What is a bit?

A single binary digit {i.e. 0 or 1}

What is a byte?

A byte is a group of 8 bits

What is a pixel?

A pixel is the smallest unit of a digital image or graphic that can be displayed and represented on a digital display device.

What is the equation for number of arrangements [or alternatives] of bits(b)?

N = 2^{b}

- N = number of levels or alternatives ot arrangements
- b = number of bits

How do you calculate number of bits, b, from number of arrangements N?

**b = log _{2}N**

- b = number of bits
- N = number of levels or alternatives or arrangements

What is the equation to calculate image resolution?

Resolution = width of image / number of pixels across object

Resolution is measured in metres per pixel [mpixel^{-1}]

What is the equation to calculate amount of information in an image?

Amount of information = total number of pixels * bits per pixel

In processing a digital image, what does adding a fixed number to each value of a pixel do?

Adding a fixed number to each value of pixel increases the brightness (if the fixed number is +ve) - each pixel has a higher number therefore has a lighter colour.