LIGHT
Light consists of various spectrum. On the lower end of wavelength are ultraviolet rays, in middle is visible light and on upper end are infra red rays. Only visible spectrum is visible to naked eye. Rest rays are absorbed by various ocular structures. Like cornea and lens absorb ultraviolet light. An interesting phenomenon occurs in aphakic patients in which lens of eye is removed. These patients complain that everything looks blue.
Light is measured in nanometers. It moves in the shape of a wave. Like you through a stone in water and waves are formed. A wave has amplitude and cycle, a crest and trough
The distance between one crest and trough is a complete cycle. The distance between two crests or two troughs is wavelenght. The distance between basline and peak of a crest is called amplitude.
Refractive index
speed of light in vacuum/speed of light in medium=refractive index
cornea= 1.376
air = 1.000
Total internal reflection
Total internal reflection (TIR) occurs when light travels from a high-index medium to a low-index medium and the angle of incidence exceeds a certain critical angle. Under these circumstances, the incident ray does not pass through the interface; all light is reflected back into the high-index medium
uses:
gonioscopy
fiber optic cable
Pin hole
As pupil dilates, the image on retina blurs. pin hole act as small pupil and avoid this blur and the light rays are almost parallel passing through pinhole and do not deviate practically.
if vision improves with pinhole, the patient has refractive error
if vision does not improve with pin hole then some media opacity is present
if pin hole reduces vision then macular disease should be considered although posterior polar cataract can also reduce vision
the normal size of pinhole is 1.2mm
After the best refractive correction has been determined, the pinhole can also be used with a dilated pupil
The patients less likely to need their glasses in bright light because One reason is that the bright light causes the pupil to constrict, allowing the defocused image to be less blurred on the retina. Another reason is that bright light increases contrast.
The patients with uncorrected myopia squint because to get a pinhole affect