P3 - The Eye Flashcards
What are the 4 similarities between a camera and the eye and why?
In a camera, light is refracted by a lens forming an image on the film (or CCD). The eye does the same except the image forms on the retina
The images produced by a camera and the eye see both real -
- Are both inverted
- and are smaller than the object because the object is a lot further away than the focal length of the lens
What are the 7 parts of the eye and what do they do?
The cornea is a transparent window with a convex shape and a high refractive index (it does all of the eye’s focusing)
The iris is the coloured part and is made up of muscles that control the size of the pupil to control the light intensity entering the eye
The lens changes shape to focus light from objects at varying distances. It is connected to the ciliary muscles by the suspensory ligaments
When the ciliary muscles contract, the lens takes on a flat spherical shape and when they relax, the lens takes on a thinner, flatter shape
Images are formed in the retina which is covered in light-sensitive cells that detect light and send signals to the brain
What is far point?
The furthest distance that eye can focus comfortably (infinity for normally-sighted people)
What is near point?
The closest distance the eye can focus on (usually 25cm)
What is short sight and what causes it?
Short sight is where people can’t focus on distant objects and is caused by the eyeball bring too long or the lens system being too powerful
Images of distant objects from too far in front of the retina
How do you correct short sight and how does this work?
Put a diverging lens in front of the eye so that distant light rays are diverged before they enter the eye so that the image forms on the retina
What is long sight and what causes it?
Long sight is where people can’t focus on near objects and is caused by the eyeball bring too short or the lens system being too weak
Images of near objects form behind the retina
How do you correct long sight and how does this work?
Put a converging lens in front of the eye so that near light rays are converged before they enter the eye so that the image forms on the retina
What is a laser?
A narrow, intense beam of light that produces light waves that always have the same wavelength
Why is laser surgery preferable to scalpel surgery?
Lasers burn and seal shut blood vessels as they cut through the tissue stopping blood loss and therefore infection
What are the 2 main uses of laser surgery?
Treating skin conditions
Laser eye surgery
How does laser eye surgery work?
The laser vapourises some of the cornea to change its shape in order to change its focusing ability
How do optical fibres work?
They carry visible light over long distances by bouncing light waves off the sides of a thin inner core until it reaches the end of the fibre
The waves bounce because of total internal reflection
What do you have to remember when drawing optical fibres?
The angle of the reflection from the normal is the same as the angle of incidence for the light rays in a fibre
What is total internal reflection?
When the light rays are only reflected by a material and not refracted through it at all
When will total internal reflection occur?
When the angle of incidence is larger than the critical angle
Describe how changing the angle of incidence affects reflection and refraction
If the angle of incidence is less than the critical angle, most of the light is refracted but a bit is internally reflected
If i = critical angle, the emerging ray comes out along the surface and there’s quite a lot of internal reflection
If i > critical angle, no light is refracted and total internal reflection occurs
What is the formula for critical angle?
Refractive index = 1 / sin c
c = critical angle
What effect does refractive index have on total internal reflection?
A high refractive index causes more internal reflection
What is an endoscope?
A thin tube containing optical fibres that allows surgeons examine the inside of the body
How do endoscopes work?
One optical fibres carries light to the area of interest and one carries an image back
The image can be viewed on an eyepiece or on an electronic screen if CCDs are used
What is the advantage of endoscopes?
They allow detailed viewing of the inside of the body with only a tiny hole
This is called keyhole surgery
