Light and Vision

Light

• A transverse electromagnetic wave

• A continuum of the electromagnetic spectrum from least energetic (radio waves) to most energetic (y rays)

Wavelength determines…

Type of wave

Visible light is made up of waves ranging from…

400-700nm

Light travels at…

299,800 km/s

Transverse waves

A wave in which the medium vibrates at right angles to the direction of the propagation

Wave length

Distance between two crests or dips on successive waves

Frequency

Number of complete waves that pass a particular point in one second

Velocity

How fast the wave is moving forwards. Dependant on the medium through which the light is passing through

Properties of light

• Travels in straight lines

• Travels very fast, ~300,000 km/s

• Faster than sound e.g. can see fireworks but not heat

Radio waves

• Longest waves in the electromagnetic spectrum (1mm-1000 km/s)

• primary means of communication - carry signals for TV, radio, cellphones, garage door opener, GPS

• Can pass through air, water and building material but not metal

Magnetic Resonance Imaging

• Strong radio waves and magnetic fields are used to generate images of organs in the body due to hydrogen atoms absorbing and emitting radio waves in a magnetic field

• The changes in atom alignment create images of the body

• Generates images of body that are hard to see with other imaging modalities

Microwaves

• Slightly shorter wavelength than radio waves (10-0.1cm)

• Certain bands of microwave light interact strongly with water and oxygen

• Microwaves (the machine) uses a wavelength which particularly interacts with water molecules - this gets them excited and moving, this motion turns light energy into heat

• Process called radiation

Microwaves in medicine

• Imaging of cancer cells

• Mammography

• Microwave imaging

Best used in combination

Imaging of cancer cells

dielectric properties (ability to conduct electricity or sustain electric field)

Mammography

X-ray, provides good spatial resolution, can identify tumours can’t tell if they are cancerous - biopsy needed

Microwave imaging

• creates 3d image of breast tissue

• no compression

• can locate and differentiate cancerous and normal tissue

• more specific than mammogram but lower resolution

Infra-red light

Used to monitor health of planets vegetation, temperature of ocean, weather tracking, short-range communications

Near infrared

Closer on the spectrum to visible light used for communication

Far infrared

Closer on the spectrum to microwaves, transmits heat

Infra-red light uses

1. Fibre optics

2. Infrared imaging

Fibre optics

Endoscopies

Infrared imaging

• Ophthalmology

• Optometry

Visible light

Spectrum of light perceivable to the human eye, 400-700nm

Wavelength of light determines ________

the colour that we see

White light is the _______ of _________

combination of all wavelengths, visible light

Luminosity

Produce/emit light

Reflectivity

Do not produce light but reflect light from luminous objects

Visible light can either be _______, ________, or ________

Reflected, absorbed, transmitted

When light is reflected, it is perceived as colour depending on the _______

wavelength reflected

If all light is reflected it is seen as…

white

If all light is absorbed, it is seen as…

black

An object is seen as red if…

all light is absorbed except the red wavelength, which is reflected

Refraction

The bending of light as it changes from one medium to another to another

The light beam ______ and bends away from normal when it enters a _________ medium

accelerates, less dense

An example of refraction is…

seeing fish under water

• we perceive fish in an apparent position as our brains are used to light travelling in straight line

Polarisation

a property of transverse waves which specifies the geometrical orientation of the oscillations

Unpolarised light

Electromagnetic fields oscillate in all possible directions

Polarised light

Electric and magnetic fields oscillate in one plane

Polarised sunglasses

Reduce glare by eliminating light not oscillating on the vertical plane

Anatomy of an eye

• Anterior section - front

• Posterior section - back

Anterior section

Conjunctiva, cornea, aqueous humour, iris, lens, ciliary body

Posterior section

Vitreous humour, retina, choroid, sclera, optic nerve

In a relaxed eye, _____ rays from a _____ object fall into focus on the ______

________ rays from a near object will fall into focus ______ the _____

In this case, ______ in the eye change the shape of the _____ to maintain _____ on the retina

parallel, distant, retina

diverging, behind, retina

muscles, lens, focus

Refractive conditions

• Hyperopia

• Myopia

Hyperopia

long sightedness condition when images are focused behind the retina causing blurring or eye strain

Myopia

short sightedness conditions when images are focused in front of the retina causing objects to appear blurred

Hyperopia is corrected with…

convex lenses that cause light rays to converge thus focus them on the retina

Myopia is corrected with…

concave lenses that cause light rays to diverge, thus focus them on the retina. The focal point sirs in front of the lens

Blindness

• Corneal opacity

• Cataract

• Retinal detachment

• Visual tract

Corneal opacity

Opacity or translucency of the cornea caused by trauma, infection or disease

Cataract

Opacity of the lens caused by aging changes or trauma

Retinal detachment

Separation of the retinal layer of the underlying vascular layer, causing hypoxia and neuronal death

Visual tract

Problems with transmission of the visual signal to the visual venture in the occipital lobe of the brain

UV light

• Third most energetic form of light

• Produced by the Sun

UVA

Lower energy from UVB and UVC, low risk for sunburn, can cause skin damage

UVB

Largely responsible for sunburn and eventually skin cancer

UVC

Mostly blocked by the Earth’s atmosphere, high energy, potentially most harmful to life on Earth

UV light benefits

• Breaks down DNA in microbes

• bactercidal

• helps produce vitamin D

UV light harms

• Skin cancer

  • Melanoma

  • basal cell carcinoma

  • squamous cell carcinoma

X-rays

• invisible to the human eye

• To produce picture, machine produces a concentrated beam of electrons which comes into contact with our soft body tissues, and produces an image on a metal film

Gamma rays

• Most energetic form of light

• Wavelength is less than 10-13/m

• Created by nuclear reactions or particle accelerations

• Light energy is a by-product of radioactive decay

• Can only be stopped by thick lead

• Gamma cameras and radioisotopes are used to detect abnormalities of brain, bone, kidneys and blood flow

• Gamma rays can also be used to sterilise medical equipment and food