# Chapter One- Imaging (Advancing Physics AS)

These cards are intended to help with Chapter One of the Physics In Action exam for the OCR Physics B course, normally taken in January. They may be helpful to any AS Physics student looking to revise Imaging.

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## The Wave Equation

A WAVE with FREQUENCY, f and WAVELENGTH, λ, will be moving at SPEED, v according to:

v (in m/s) = f (in Hz) λ ;(in m)

This is in your formula book, so you don't have to remember it. However, you DO need to know how to rearrange and what units you're using.

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## Images and Information

IMAGES can be FORMED with many different types of SIGNALS, including ULTRASOUND (an ultrasound scanner builds up an image from the strength and time delay of reflected pulses) and all parts of the ELECTROMAGNETIC SPECTRUM (Radio waves, microwaves, infra-red, visible light, ultra-violet, x-rays & gamma rays).

IMAGES can be recorded ELECTRONICALLY by MICROSENSORS; for example a CHARGE-COUPLED DEVICE (CCD) used in cameras, webcams, TV cameras, portable videos and mobile telephones.

IMAGES on the ATOMIC SCALE can recorded by SCANNING METHODS, for example the SCANNING TUNNELLING MICROSCOPE.

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## Images and Information

IMAGES can be STORED as an ARRAY OF PIXELS, each defined by a BINARY NUMBER (Binary is a number system that only uses two digits 1 or a 0 (sometimes referred to as on or off)). The VALUE of the number gives the COLOUR (or SHADE of GREY) of the CORRESPONDING PIXEL.

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## Images and Information

Because digital images are essentially numbers, they can be manipulated in different ways to produce different effects.

IMAGES can be SMOOTHED (reducing contrast) by SUITABLE AVERAGING (i.e by taking the MEAN).

IMAGES can be SHARPENED by LOCATING EDGES using the LAPLACE RULE.

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## Images and Information

Other image modifying techniques include;

MULTIPLYING by a FIXED VALUE to IMPROVE CONTRAST.

ADDING FALSE COLOUR, by REPLACING all PIXELS of a certain VALUE (that corresponds to a certain COLOUR or SHADE) with a different value, to HIGLIGHT FEATURES.

REPLACING PIXELS with the MEDIAN of their immediate neighbours to REDUCE NOISE.

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## Logarithms

One BIT (binary digit) of information has 2 ALTERNATIVES (0 or 1). 1 BYTE of information is equal to 8 BITS (256 alternatives).

Number of alternatives = 2 ^ [^ means to the power of]

In symbols: N=2^I

Number of bits= log to the base 2 (number of alternatives)

In symbols: I=log to the base 2 (N)

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## Binary

Binary is a different way of writing numbers that only requires two symbols; a one or a zero, sometimes referred to as "on" or "off". Our normal number system works using columns with the headings that increase in powers of 10; "units" "tens" "hundreds" "thousands" and so on. Binary works using headings that increase in powers of 2; "ones", "twos" "fours" "eights" "sixteens" etc. Here's an example:

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## Eyes and Lenses

A CONVERGING LENS brings light to a FOCUS. It ADDS a CONSTANT CURVATURE to the light. The amount of curvature added is the POWER of the lens; a more powerful lens brings light to a focus at a shorter distance and hence adds more curvature.

Power of the lens (in dioptres, D) = 1 / Focal Length (m)

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