Human Eye and Colourful World Test - 9

Least distance of distinct vision for a normal human being is 25cm, however it varies with age. As the person grows old, his ciliary muscle responsible for adjusting the eye lens gets weakened. As a result the power of accommodation reduces and hence the least distance of distinct vision for old people generally increases.

By convention,

1. The focal length (f) of concave lens is always taken as negative.

2. The object distance (u) from optical centre is always taken as negative. 

The image of an object seen persists on the retina for 1/16 second even after the removal of the object. This continuance of sensation of eye for some time is called persistence of vision.

Colour blindness is said to occur when a person cannot differentiate between colours although his vision may otherwise be normal. Causes of colour blindness are genetic and till date it has no cure.

The defect by which the person is not able to differentiate horizontal and vertical position, is called astigmatism. It is caused by an error in the shape of the cornea.  It can be corrected by using cylindrical lenses.

The angle of deviation through a triangular prism is the angle between the incident ray and the emerging ray (angle δ). However, in glass slab, the incident ray and the emergent ray are parallel to each other thus angle of incidence is equal to angle of emergence.

Dispersion is the phenomena of splitting of white light into its constituent seven colours (VIBGYOR) on passing through a glass prism.