ressure of the eyelid on the soft vision cornea. The result is that vertical and horizontal lines cannot be focused simultaneously. The cure for this refractive error is to use a lens which is itself asymmetric in its powers. A cylindrical lens can 7.8.3 Persistence of vision be used to correct simple astigmatism. Where it is combined with other defects the corrective lens has a more complex shape. The process of adaptation is not instantaneous. Dark adaptation takes several minutes whilst light adaptation is much faster. If an observed illuminated image is switched on and off rapidly a sensa- 7.10 Interaction of light with matter tion of flicker is produced. However if the flicker frequency is sufficiently high, the successive images merge into one another and Light may interact with matter by being reflected, refracted, ab- all flicker vanishes. The maximum flicker frequency that the eye sorbed or transmitted. Two or more of these are usually involved. can detect increases as the amplitude of the flicker increases. For square wave flicker the maximum detectable frequency is about 7.10.1 Reflection 50Hz at a level of luminance of 100 foot lamberts in the bright half of the cycle, (Kingslake, 1983). Some of the light impinging on any surface is reflected away from the surface. The reflectance varies according to the properties of the surface and the wavelength of the impinging radiation. Defects of vision Regular or specular reflection is reflection in accordance with the laws of reflection with no diffusion (surface is smooth compared to 7.9.1 Accommodation the wavelength of the impinging radiation). Diffuse reflection is diffusion by reflection in which on the micro- In a normal completely relaxed eye, the light from an object at scopic scale there is no regular reflection (surface is rough when infinity will be sharply focused on the retina and as the object moves compared to the wavelength of the impinging radiation). closer to the eye the power of the lens increases automatically to Reflectance (p) is the ra