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light sources
alternative light sources
| Front | Back |
|---|---|
| What is light | electromagnetic radiation detected by human eye radiation occurs over a wide range of wavelengths e.g. gamma to radio waves transports energy at the speed of light |
| white light | most common, seen daily, naturally emited from sun and white light bulbs made up of all the colours of the spectrum - contrast of 0 broad wavelength from 400nm to 700nm (due to mixture of colours) |
| prism effect (isaac newton) | as a ray of white incident light enters one side of the prism, the emerged ray is bent by the angle of deviation which is dependent on the angle the incident light enters the prism (so each colour of the visible spectrum shows) |
| prism effect | visible spectrum of coloured light is seperated due to difference in wavelength size, meaning refraction index is not constant for each colour as the wavelength decrease from red to violet |
| colour absorpiton and reflection | colour characteristics can be described as hue, saturation and brightness hue - actual colour seen saturation - depth of colour brightness - intensity of light reflecting |
| subtractive theory | when white light meets a surface light bounces back off it chromophores absorb some of the wavelengths making up white light, whist reflecting others |
| subtractive theory and filters | filters allow specific wavelengths to pass through e.g. red filter will allow long wavelengths from red to pass while absorbing short wavelengths from blue and green |
| photoluminescence | occurs when a molecule absorbs energy from a photon causing an electron to transfer to a higher energy state which radiates a photon as it returns back to ground state light is then re-emitted from the molecule (fluorescence or phosporescence) |
| fluorescence | Substance absorbs short-wavelength light (excitation wavelength). Electrons move to a higher-energy excited state. Electrons return to the ground state. Energy released as longer-wavelength light (emission wavelength). |
| Role of barrier filters | Blocks reflected/excitation light from the source. Allows fluorescent light to be seen more clearly. Works by absorbing specific wavelengths that would overpower fluorescence. Fluorescence has a longer wavelength than the incident light. |
| use of ALS at crime scene | Used to detect biological materials on evidence (e.g. clothing, furniture). One of the main presumptive tests for body fluid stains. Helps maximise evidence recovery. Widely researched, but effectiveness and best use still being investigated. |
| blood | Blood stains are seen because blood absorbs light, creating contrast against the background. Different wavelengths/filters may be used depending on substrate colour. Blood serum and plasma do fluoresce under UV light at 365 nm. |
| saliva | stains tend to be invisible to naked eye low fluorescence intensity, so doesnt tend to be easily detected absorbency of the surface acts as a hindrance to fluorescence |
| semen | naturally fluoresces between 350-500nm |
| limitations for body fluid screening using ALS | lots of things fluoresce e.g. washing detergents, food residues, waxes, cosmetics, soaps, cleaning products |
| smartwater - DeterTech | smartwater forensic liquid products have a successful record in reducing crime contains unique elements invisible to naked eye but glows yellow-green under UV light |
| infrared - gun shot residue | GSR composition varies depending on gun and ammunition used. IR light is effective for visualising GSR on fabrics. Many fabrics reflect IR light. GSR particles absorb IR light and appear darker, increasing contrast. |
| whats examined in questioned documents | Forensic examination of suspected/counterfeit documents. Includes analysis of: inks (handwritten & printed), security features, paper Investigates alterations such as: tears folds holes obliterations erasures indentations |
| What is Diamond Nucleic Acid Dye used for | Nucleic acid dye used as an alternative to ethidium bromide. Binds to single- and double-stranded DNA & RNA. Mainly used in gel electrophoresis. Binds to DNA backbone Specific to eukaryotic DNA Used to visualise trace DNA. |
Created by:
Mihaela07